Type of production system: Industrial
Climatic zone: Polar; Temperate; Tropical. Horizontal distribution: Oceanic. Vertical distribution: Pelagic.
Associated Species (Bycatch)
This document summarizes the fisheries for species covered by the IATTC Convention (tunas and other fishes caught by tuna-fishing vessels) in the eastern Pacific Ocean (EPO). The most important of these are the scombrids (Family Scombridae), which include tunas, bonitos, seerfishes, and mackerels. The principal species of tunas caught are yellowfin, skipjack, bigeye, and albacore, with lesser catches of Pacific bluefin, black skipjack, and frigate and bullet tunas; other scombrids, such as bonitos and wahoo, are also caught.
This document also covers other species caught by tuna-fishing vessels in the EPO: billfishes (swordfish, marlins, shortbill spearfish, and sailfish) carangids (yellowtail, rainbow runner, and jack mackerel), dorado, elasmobranchs (sharks, rays, and skates), and other fishes.
Most of the catches are made by the purse-seine and longline fleets; the pole-and-line fleet and various artisanal and recreational fisheries account for a small percentage of the total catches.
Detailed data are available for the purse-seine and pole-and-line fisheries; the data for the longline, artisanal, and recreational fisheries are incomplete.
The IATTC Regional Vessel Register
contains details of vessels authorized to fish for tunas in the EPO. The IATTC has detailed records of most of the purse-seine and pole-and-line vessels that fish for yellowfin, skipjack, bigeye, and/or Pacific bluefin tuna in the EPO. The Register is incomplete for small vessels. It contains records for most large (overall length >24 m) longline vessels that fish in the EPO and in other areas.
The data in this report are derived from various sources, including vessel logbooks, observer data, unloading records provided by canners and other processors, export and import records, reports from governments and other entities, and estimates derived from the species and size composition sampling program.
All weights of catches and discards are in metric tons (t). In the tables, 0 means no effort, or a catch of less than 0.5 t; - means no data collected; * means data missing or not available.
The principal species of tunas caught are: Albacore - Northern PacificBigeye tuna - Eastern Pacific (EPO)Skipjack tuna - Eastern PacificYellowfin tuna - Eastern Pacific
The principal species of tunas caught are yellowfin, skipjack, bigeye, and albacore, with lesser catches of Pacific bluefin, black skipjack, and frigate and bullet tunas; other scombrids, such as bonitos and wahoo, are also caught. This report also covers other species caught by tuna-fishing vessels in the EPO: billfishes (swordfish, marlins, shortbill spearfish, and sailfish) carangids (yellowtail, rainbow runner, and jack mackerel), dorado, elasmobranchs (sharks, rays, and skates), and other fishes.
Fleet segment THE FLEETS The purse-seine and pole-and-line fleets
The IATTC staff maintains detailed records of gear, flag, and fish-carrying capacity for most of the vessels that fish with purse-seine or pole-and-line gear for yellowfin, skipjack, bigeye, and/or Pacific bluefin tuna in the EPO. The fleet described here includes purse-seine and pole-and-line vessels that have fished all or part of the year in the EPO for any of these four species.
Historically, the owner's or builder's estimates of carrying capacities of individual vessels, in tons of fish, were used until landing records indicated that revision of these estimates was required.
Since 2000, the IATTC has used well volume, in cubic meters (m3
), instead of weight, in metric tons (t), to measure the carrying capacities of the vessels. Since a well can be loaded with different densities of fish, measuring carrying capacity in weight is subjective, as a load of fish packed into a well at a higher density weighs more than a load of fish packed at a lower density. Using volume as a measure of capacity eliminates this problem.
The IATTC staff began collecting capacity data by volume in 1999, but has not yet obtained this information for all vessels. For vessels for which reliable information on well volume is not available, the estimated capacity in metric tons was converted to cubic meters.
Until about 1960, fishing for tunas in the EPO was dominated by pole-and-line vessels operating in coastal regions and in the vicinity of offshore islands and banks. During the late 1950s and early 1960s most of the larger pole-and-line vessels were converted to purse seiners, and by 1961 the EPO fishery was dominated by these vessels. From 1961 to 2014 the number of pole-and-line vessels decreased from 93 to 2, and their total well volume from about 11 thousand to about 226 m3
. During the same period the number of purse-seine vessels increased from 125 to 217, and their total well volume from about 32 thousand to about 229 thousand m3
, an average of about 1,055 m3
per vessel. An earlier peak in numbers and total well volume of purse seiners occurred from the mid-1970s to the early 1980s, when the number of vessels reached 282 and the total well volume about 195 thousand m3
, an average of about 700 m3
per vessel (Table A-10
); (Figure A-2).
|Figure A-2: Carrying capacity, in cubic meters of well volume, of the purse-seine and pole-and-line fleets in the EPO, 1961-2014 |
The catch rates in the EPO were low during 1978-1981, due to concentration of fishing effort on small fish, and the situation was exacerbated by a major El Niño event, which began in mid-1982 and persisted until late 1983 and made the fish less vulnerable to capture. The total well volume of purse-seine and pole-and-line vessels then declined as vessels were deactivated or left the EPO to fish in other areas, primarily the western Pacific Ocean, and in 1984 it reached its lowest level since 1971, about 119 thousand m3
. In early 1990 the U.S. tuna-canning industry adopted a policy of not purchasing tunas caught during trips during which sets on tunas associated with dolphins were made. This caused many U.S.-flag vessels to leave the EPO, with a consequent reduction in the fleet to about 117 thousand m3
in 1992. With increases in participation of vessels of other nations in the fishery, the total well volume has increased steadily since 1992, and in 2014 was 229 thousand m3
The 2013 and preliminary 2014 data for numbers and total well volumes of purse-seine and pole-and-line vessels that fished for tunas in the EPO are shown (Table A-11a
) and (Table A-11b
). During 2014, the fleet was dominated by vessels operating under the Ecuadorian and Mexican flags, with about 38% and 24%, respectively, of the total well volume; they were followed by Venezula (9%), Panama (9%), Colombia (6%), European Union (Spain) (4%), Nicaragua (4% ), El Salvador (3%), and Guatemala, Peru and United States (1% each). The sum of the percentages may not add up to 100% due to rounding.
The cumulative capacity at sea during 2014 is compared to those of the previous five years in Figure 3.
|Figure A-3: Cumulative capacity of the purse-seine and pole-and-line fleet at sea, by month, 2009-2014 |
The monthly average, minimum, and maximum total well volumes at sea (VAS), in thousands of cubic meters, of purse-seine and pole-and-line vessels that fished for tunas in the EPO during 2004-2013, and the 2014 values, are shown in (Table A-12
). The monthly values are averages of the VAS estimated at weekly intervals by the IATTC staff. The fishery was regulated during some or all of the last four months of 1999-2014, so the VAS values for September-December 2014 are not comparable to the average VAS values for those months of 1999-2014. The average VAS values for 2004-2013 and 2014 were 135 thousand m3 (62% of total capacity) and 135 thousand m3 (59% of total capacity), respectively.Other fleets of the EPO
Information on other types of vessels that fish for tunas in the EPO is available in the IATTC’s Regional Vessel Register, on the IATTC website
. The Register is incomplete for small vessels. In some cases, particularly for large longline vessels, the Register contains information for vessels authorized to fish not only in the EPO, but also in other oceans, and which may not have fished in the EPO during 2014, or ever.
Estimating the total catch of a species of fish is difficult, for various reasons. Some fish are discarded at sea, and the data for some gear types are incomplete. Data for fish discarded at sea by purse-seine vessels with carrying capacities greater than 363 metric tons (t) have been collected by observers since 1993, which allows for better estimation of the total amounts of fish caught by the purse-seine fleet. Estimates of the total amount of the catch that is landed (hereafter referred to as the retained catch) are based principally on data from unloadings. Beginning with Fishery Status Report 3, which reports on the fishery in 2004, the unloading data for purse-seine and pole-and-line vessels have been adjusted, based on the species composition estimates for yellowfin, skipjack, and bigeye tunas. The current species composition sampling program, described in Purse-seine, pole-and-line, and recreational fisheries section, began in 2000, so the catch data for 2000-2014 are adjusted, based on estimates by flag for each year. The catch data for the previous years were adjusted by applying the average ratio by species from the 2000-2004 estimates, by flag, and summing over all flags. This has tended to increase the estimated catches of bigeye and decrease those of yellowfin and/or skipjack. These adjustments are all preliminary, and may be improved in the future. All of the purse-seine and pole-and-line data for 2013 and 2014 are preliminary.
Data on the retained catches of most of the larger longline vessels are obtained from the governments of the nations that fish for tunas in the EPO. Longline vessels, particularly the larger ones, direct their effort primarily at bigeye, yellowfin, albacore, or swordfish. Data from smaller longliners, artisanal vessels, and other vessels that fish for tunas, billfishes, dorado, and sharks in the EPO were gathered either directly from the governments, from logbooks, or from reports published by the governments. Data for the western and central Pacific Ocean (WCPO) were provided by the Ocean Fisheries Programme of the Secretariat of the Pacific Community (SPC). All data for catches in the EPO by longlines and other gears for 2013 and 2014 are preliminary.
The data from all of the above sources are compiled in a database by the IATTC staff and summarized in this report. In recent years, the IATTC staff has increased its effort toward compiling data on the catches of tunas, billfishes, and other species caught by other gear types, such as trollers, harpooners, gillnetters, and recreational vessels. The estimated total catches from all sources mentioned above of yellowfin, skipjack, and bigeye in the entire Pacific Ocean are shown in (Table A-1
), and are discussed further in the sections below.
Estimates of the annual retained and discarded catches of tunas and other species taken by tuna-fishing vessels in the EPO during 1985-2014 are shown in (Table A-2a
), (Table A-2a (cont.)
), (Table A-2a (cont.)
), (Table A-2b
), (Table A-2b (cont.)
) and (Table A-2c
). The catches of yellowfin, bigeye, and skipjack tunas, by gear and flag, during 1985-2014 are shown in (Table A-3a
), (Table A-3b
), (Table A-3c
), (Table A-3d
), (Table A-3e
), and the purse-seine and pole-and-line catches of tunas and bonitos during 2013-2014 are summarized by flag in (Table A-4
). There were no restrictions on fishing for tunas in the EPO during 1988-1997, but the catches of most species have been affected by restrictions on fishing during some or all of the last six months of 1998-2014. Furthermore, regulations placed on purse-seine vessels directing their effort at tunas associated with dolphins have affected the way these vessels operate, especially since the late 1980s, as discussed in “The Fleets” section.
The catches have also been affected by climate perturbations, such as the major El Niño events that occurred during 1982-1983 and 1997-1998. These events made the fish less vulnerable to capture by purse seiners due to the greater depth of the thermocline, but had no apparent effect on the longline catches. Yellowfin recruitment tends to be greater after an El Niño event.Catches by speciesYellowfin tuna
The annual catches of yellowfin during 1985-2014 are shown in (Table A-1
). The EPO totals for 1993-2014 include discards from purse-seine vessels with carrying capacities greater than 363 t. The El Niño event of 1982-1983 led to a reduction in the catches in those years, whereas the catches in the WCPO were apparently not affected. Although the El Niño episode of 1997-1998 was greater in scope, it did not have the same effect on the yellowfin catches in the EPO. In the EPO, catches increased steadily to a high of 443 thousand t in 2002; they decreased substantially in 2004, reaching their lowest level during 2006-2008, at only 44% of the highest catches of the 2001-2003 period. The 2014 catch of 234 thousand t is equal to the average for the current 5 year period. In the WCPO, the catches of yellowfin increased steadily to a high of 604 thousand t in 1998, then settled into a range of 468 to 590 thousand t during 1999-2013.
The annual retained catches of yellowfin in the EPO by purse-seine and pole-and-line vessels during 1985-2014 are shown in (Table A-2a
), (Table A-2a (cont.)
), (Table A-2a (cont.)
). The average annual retained catch during 1999-2013 was 260 thousand t (range: 167 to 413 thousand t). The preliminary estimate of the retained catch in 2014, 233 thousand t, was 7% larger than that of 2013, but 10% less than the average for 1999-2013. The average amount of yellowfin discarded at sea during 1999-2013 was about 1% of the total purse-seine catch (retained catch plus discards) of yellowfin (range: 0.1 to 2.4%) (Table A-2a).
The annual retained catches of yellowfin in the EPO by longliners during 1985-2014 are shown in Table A-2a. During 1990-2003 catches averaged about 23 thousand t (range: 12 to 35 thousand t), or about 8% of the total retained catches of yellowfin. Longline catches declined sharply beginning in 2005, averaging 10 thousand t per year (range: 8 to 13 thousand t), or about 4% of the total retained catches, through 2013. The lower longline catch total for 2014 is due to incomplete data for the year. Yellowfin are also caught by recreational vessels, as incidental catch in gillnets, and by artisanal fisheries. Estimates of these catches are shown in Table A-2a, under “Other gears” (OTR); during 1999-2013 they averaged about 1 thousand t.Skipjack tuna
The annual catches of skipjack during 1985-2014 are shown in (Table A-1
). Most of the skipjack catch in the Pacific Ocean is taken in the WCPO. Prior to 1999, WCPO skipjack catches averaged about 900 thousand t. Beginning in 1999, catches increased steadily from 1.1 million t to an all-time high of 1.8 million t in 2013. In the EPO, the greatest yearly catches occurred between 2003 and 2014, with the highest catch of 309 thousand t in 2006, and a range from 153 to 309 thousand t.
The annual retained catches of skipjack in the EPO by purse-seine and pole-and-line vessels during 1985-2014 are shown in (Table A-2a
), (Table A-2a (cont.)
), (Table A-2a (cont.)
). During 1999-2013 the annual retained catch averaged 234 thousand t (range 144 to 297 thousand t). The preliminary estimate of the retained catch in 2014, 262 thousand t, is 12% greater than the average for 1999-2013, and 12% less than the record-high retained catch of 2008. Discards of skipjack at sea decreased each year during the period, from 11% in 2000 to a low of less than 1% in 2013. During the period about 5% of the total catch of the species was discarded at sea (Table A-2a).
Small amounts of EPO skipjack are caught with longlines and other gears (Table A-2a).Bigeye tuna
The annual catches of bigeye during 1985-2014 are shown in (Table A-1
). Overall, the catches in both the EPO and WCPO have increased, but with considerable fluctuations. In the EPO, the average catch for the period was 103 thousand t, with a low of 72 thousand t in 1985 and a high of 149 thousand t in 2000. In the WCPO the catches of bigeye increased to more than 77 thousand t during the late 1970s, decreased during the early 1980s, and then increased steadily to 111 thousand t in 1996. In 1997 the total jumped to 154 thousand t, and reached a high of 180 thousand t in 2004. Since 2004 the catch has fluctuated between 128 and 153 thousand t.
The annual retained catches of bigeye in the EPO by purse-seine and pole-and-line vessels during 1985-2014 are shown in Table A-2a. During 1993-1994 the use of fish-aggregating devices (FADs), placed in the water by fishermen to aggregate tunas, nearly doubled, and continued to increase in the following years. This resulted in greater catches of bigeye by purse-seine vessels. Before this increase, the annual retained catch of bigeye taken by purse-seine vessels in the EPO was about 5 thousand t (Table A-2a
), (Table A-2a (cont.)
), (Table A-2a (cont.)
). As a result of the development of the FAD fishery, it increased from 35 thousand t in 1994 to between 44 and 95 thousand t during 1995-2013. The preliminary estimate of the retained catch in the EPO in 2014 is 60 thousand t.
During 1999-2013 the purse-seine catch of the species discarded at sea has steadily decreased, from 9% in 1999 to less than 1% in 2013, for an average discard rate of about 2.5%. No bigeye catch has been reported by pole-and-line vessels in recent years.
From 1985 to 1993, before the increase in the use of FADs, longliners caught an average of 95% of the bigeye in the EPO (average 86 thousand t; range; 66 to 104 thousand t). During 1999-2013 this average dropped to 39%, with a low of 25% in 2008 (average: 42 thousand t; range: 26 to 74 thousand t) (Table A-2a). The preliminary estimate of the longline catch in the EPO in 2014 is 35 thousand t (Table A-2a).
Small amounts of bigeye are caught by other gears, as shown in Table A-2a.Bluefin tuna
The catches of Pacific bluefin in the EPO during 1985-2014, by gear, are shown in (Table A-2a
), (Table A-2a (cont.)
), (Table A-2a (cont.)
). Purse-seine and pole-and-line vessels accounted for over 94% of the total EPO retained catch during 1999-2013. During this period the annual retained catch of bluefin in the EPO by purse-seine vessels averaged 4.6 thousand t (range 1.2 to 9.9 thousand t). The preliminary estimate of the retained purse-seine catch of bluefin in 2014, 4.9 thousand t, is slightly greater than the average for 1999-2013 (Table A-2a).
The catches of Pacific Bluefin in the entire Pacific Ocean, by flag and gear, are shown in (Table A-5a
). The data, which were obtained from the International Scientific Committee for Tuna and Tuna-like Species in the North Pacific Ocean (ISC), are reported by fishing nation or entity, regardless of the area of the Pacific Ocean in which the fish were caught.
Catches of Pacific bluefin by recreational gear in the EPO are reported in numbers of individual tuna caught, whereas all other gears report catch in weight (metric tons). These numbers are then converted to metric tons for inclusion in the EPO catch totals for all gears. The original catch data for 1985-2014, in numbers of fish, are presented in (Table A-5b
The catches of albacore in the entire Pacific Ocean, by gear and area (north and south of the equator) are shown in (Table A-6a
) and (Table A-6b
). The catches of albacore in the EPO, by gear, are shown in (Table A-2a
), (Table A-2a (cont.)
), (Table A-2a (cont.)
). A significant portion of the albacore catch is taken by troll gear, included under “Other gears” (OTR) in Table A-2a. The catch data were obtained from IATTC data for the EPO and from data compiled by the SPC for the WCPO.Other tunas and tuna-like species
While yellowfin, skipjack, and bigeye tunas comprise the most significant portion of the retained catches of the purse-seine and pole-and-line fleets in the EPO, other tunas and tuna-like species, such as black skipjack, bonito, wahoo, and frigate and bullet tunas, contribute to the overall harvest in this area. The estimated annual retained and discarded catches of these species during 1985-2014 are presented in (Table A-2a
), (Table A-2a (cont.)
), (Table A-2a (cont.)
). The catches reported in the “unidentified tunas” category (TUN) in Table A-2a contain some catches reported by species (frigate or bullet tunas) along with the unidentified tunas. The total retained catch of these other species by these fisheries was 7.0 thousand t in 2014, which is greater than the 1999-2013 average retained catch of 6.4 thousand t (range: 500 t to 19 thousand t).
Black skipjack are also caught by other gears in the EPO, mostly by coastal artisanal fisheries. Bonitos are also caught by artisanal fisheries, and have been reported as catch by longline vessels in some years.Billfishes
Catch data for billfishes (swordfish, blue marlin, black marlin, striped marlin, shortbill spearfish, and sailfish) are shown in (Table A-2b
), (Table A-2b (cont.)
In general, dolphins, sea turtles, whale sharks, and small fish are the only animals captured in the purse-seine fishery that are released alive. In previous versions of this report, all billfishes caught in that fishery were classified as discarded dead. When most of the individuals of species caught incidentally are discarded, the difference between catches and discards is not significant for those species, but as the rate of retention of species formerly discarded increases, part of the bycatch becomes catch, and the distinction becomes important. As a result of a review in 2010, this has been clarified in Table A-2b with the addition of a column for retained catch next to the column for discards.
Swordfish are caught in the EPO with large-scale and artisanal longline gear, gillnets, harpoons, and occasionally with recreational gear. During 1999-2008 the longline catch of swordfish averaged 12 thousand t, but during 2011-2013 the average almost doubled to over 22 thousand t. It is not clear whether this is due to increased abundance of swordfish or increased effort directed toward that species.
Other billfishes are caught with large-scale and artisanal longline gear and recreational gear. The average annual longline catches of blue marlin and striped marlin during 1999-2013 were about 4.1 thousand and 1.9 thousand t, respectively. Smaller amounts of other billfishes are taken by longline.
Unfortunately, little information is available on the recreational catches of billfishes, but they are believed to be substantially less than the commercial catches for all species.
Small amounts of billfishes are caught by purse seiners, some are retained, and others are considered to be discarded although some may be landed but not reported. These data are also included in Table A-2b. During 1999-2013 purse seiners accounted about 1% of the total catch of billfishes in the EPO.Other species
Data on the catches and discards of carangids (yellowtail, rainbow runner, and jack mackerel), dorado, elasmobranchs (sharks, rays, and skates), and other fishes caught in the EPO are shown in (Table A-2c
Bycatches in the purse-seine fishery are reported in Table A-2c as either retained or discarded. A revision was made to the allocation of catches into those categories as a result of a review in 2010.
Dorado are unloaded mainly in ports in Central and South America. Although the reported catches have been as high as 71 thousand t in recent years, the fishing gears used are often not reported.Distributions of the catches of tunasPurse-seine catches
The average annual distributions of the purse-seine catches of yellowfin, skipjack, and bigeye, by set type, in the EPO during 2009-2013, are shown in Figures A-1a, A-2a, and A-3a,
|Figure A-1a: Average annual distributions of the purse-seine catches of yellowfin, by set type, 2009-2013. The sizes of the circles are proportional to the amounts of yellowfin caught in those 5° by 5° areas. |
|Figure A-2a: Average annual distributions of the purse-seine catches of skipjack, by set type, 2009-2013. The sizes of the circles are proportional to the amounts of skipjack caught in those 5° by 5° areas. |
|Figure A-3a: Average annual distributions of the purse-seine catches of bigeye, by set type, 2009-2013. The sizes of the circles are proportional to the amounts of bigeye caught in those 5° by 5° areas. |
and preliminary estimates for 2014 are shown in Figures A-1b, A-2b, and A-3b.
|Figure A-1b: Annual distributions of the purse-seine catches of yellowfin, by set type, 2014. The sizes of the circles are proportional to the amounts of yellowfin caught in those 5° by 5° areas. |
|Figure A-2b: Annual distributions of the purse-seine catches of skipjack, by set type, 2014. The sizes of the circles are proportional to the amounts of skipjack caught in those 5° by 5° areas. |
|Figure A-3b: Annual distributions of the purse-seine catches of bigeye, by set type, 2014. The sizes of the circles are proportional to the amounts of bigeye caught in those 5° by 5° areas. |
The majority of the yellowfin catches in 2014 were taken in sets associated with dolphins from 3 general areas: between 5°N and 15°N west of 115°W longitude, north of 15°N and east of 115°W longitude, and between 5°N and 15°N east of 105°W longitude. Offshore catches of yellowfin in association with dolphins were found further south than in the previous year.
Yellowfin catches on unassociated schools in 2014 decreased by 27% from the previous year, mainly due to a substantial decrease in catch in the inshore areas off southern Mexico. Inshore catches around the equator were lower than the 2009-2013 average. Smaller amounts of yellowfin were caught south of the equator throughout the EPO, mostly in association with floating objects.
Inshore skipjack catches in 2014 were similar to those of previous years, though the percentage of catch in association with floating objects increased. Offshore catches of skipjack were almost exclusively in association with floating objects, and the overall 2014 offshore catches decreased from the previous year.
Bigeye are not often caught north of about 7°N, and the catches of bigeye have decreased in the inshore areas off South America for several years. With the development of the fishery for tunas associated with FADs, the relative importance of the inshore areas has decreased, while that of the offshore areas has increased. Most of the bigeye catches are taken in sets on FADs between 5°N and 5°S.Longline catches
Data on the spatial and temporal distributions of the catches in the EPO by the distant-water longline fleets of China, French Polynesia, Japan, the Republic of Korea, Spain, Chinese Taipei, the United States, and Vanuatu are maintained in databases of the IATTC. Bigeye and yellowfin tunas make up the majority of the catches by most of these vessels. The distributions of the catches of bigeye and yellowfin tunas in the Pacific Ocean by Chinese, Japanese, Korean, and Chinese Taipei longline vessels during 2009-2013 are shown in Figure A-4.
|Figure A-4: Distributions of the average annual catches of bigeye and yellowfin tunas in the Pacific Ocean, in metric tons, by Chinese, Japanese, Korean and Chinese Taipei longline vessels, 2009-2013. The sizes of the circles are proportional to the amounts of bigeye and yellowfin caught in those 5° by 5° areas. |
Data for the Japanese longline fishery in the EPO during 1956-2007 are available in IATTC Bulletins describing that fishery.Size compositions of the catches of tunasPurse-seine, pole-and-line, and recreational fisheries
Length-frequency samples are the basic source of data used for estimating the size and age compositions of the various species of fish in the landings. This information is necessary to obtain age-structured estimates of the populations for various purposes, including the integrated modeling that the staff has employed during the last several years. The results of such studies have been described in several IATTC Bulletins, in its Annual Reports for 1954-2002, and in its Stock Assessment Reports.
Length-frequency samples of yellowfin, skipjack, bigeye, Pacific bluefin, and, occasionally, black skipjack from the catches of purse-seine, pole-and-line, and recreational vessels in the EPO are collected by IATTC personnel at ports of landing in Ecuador, Mexico, Panama, the USA, and Venezuela. The catches of yellowfin and skipjack were first sampled in 1954, bluefin in 1973, and bigeye in 1975. Sampling has continued to the present.
The methods for sampling the catches of tunas are described in the IATTC Annual Report for 2000
and in IATTC Stock Assessment Reports 2
. Briefly, the fish in a well of a purse-seine or pole-and-line vessel are selected for sampling only if all the fish in the well were caught during the same calendar month, in the same type of set (floating-object, unassociated school, or dolphin), and in the same sampling area. These data are then categorized by fishery (Figure A-5),
|Figure A-5: The fisheries defined by the IATTC staff for stock assessment of yellowfin, skipjack, and bigeye in the EPO. The thin lines indicate the boundaries of the 13 length-frequency sampling areas, and the bold lines the boundaries of the fisheries. |
based on the staff’s most recent stock assessments.
Data for fish caught during the 2009-2014 period are presented in this report. Two sets of length-frequency histograms are presented for each species, except bluefin and black skipjack; the first shows the data by stratum (gear type, set type, and area) for 2014, and the second shows the combined data for each year of the 2009-2014 period. For bluefin, the histograms show the 2007-2012 catches by commercial and recreational gear combined. For black skipjack, the histograms show the 2009-2014 catches by commercial gear. Only a small amount of catch was taken by pole-and-line vessels in 2013 and 2014, and no samples were obtained from these vessels.
For stock assessments of yellowfin, nine purse-seine fisheries (four associated with floating objects, three associated with dolphins, and two unassociated) and one pole-and-line fishery are defined (Figure A-5). ). The last fishery includes all 13 sampling areas. Of the 815 wells sampled during 2014, 625 contained yellowfin. The estimated size compositions of the fish caught are shown in Figure A-6a.
|Figure A-6a: Estimated size compositions of the yellowfin caught in the EPO during 2014 for each fishery designated in Figure A-5. The average weights of the fish in the samples are given at the tops of the panels. |
The majority of the yellowfin catch was taken in sets associated with dolphins in the Northern and Inshore dolphin fisheries throughout the year. Most of the larger yellowfin (>100 cm) were caught in the Northern dolphin fishery in all four quarters, and in the Inshore dolphin fishery in the first and second quarters. Smaller yellowfin (<100 cm) were caught primarily in the Inshore floating object fishery during the first quarter.
The estimated size compositions of the yellowfin caught by all fisheries combined during 2009-2014 are shown in Figure A-6b.
|Figure A-6b: Estimated size compositions of the yellowfin caught by purse-seine and pole-and-line vessels in the EPO during 2009-2014. The average weights of the fish in the samples are given at the tops of the panels. |
The average weight of the yellowfin caught in 2014 (9.8 kg) was about the same as the previous year, but much less than the high for the 6 year period of 14.4 kg in 2009.
For stock assessments of skipjack, seven purse-seine fisheries (four associated with floating objects, two unassociated, one associated with dolphins) and one pole-and-line fishery are defined (Figure A-5). The last two fisheries include all 13 sampling areas. Of the 815 wells sampled, 479 contained skipjack. The estimated size compositions of the fish caught during 2014 are shown in Figure A-7a.
|Figure A-7a: Estimated size compositions of the skipjack caught in the EPO during 2014 for each fishery designated in Figure A-5. The average weights of the fish in the samples are given at the tops of the panels. |
Large amounts of skipjack in the 40- to 60-cm size range were caught in the Equatorial floating-object fishery in the third and fourth quarters, in the Inshore floating-object fishery in the first, second and third quarters, in the Southern floating-object fishery throughout the year, and in the Southern unassociated fishery during the first quarter. Larger skipjack in the 60- to 70-cm size range were taken in the Equatorial floating-object fishery during the third and fourth quarters, in the Southern floating-object fishery during the fourth quarter, and in the Southern unassociated fishery during the second and fourth quarters.
The estimated size compositions of the skipjack caught by all fisheries combined during 2009-2014 are shown in Figure A-7b.
|Figure A-7b: Estimated size compositions of the skipjack caught by purse-seine and pole-and-line vessels in the EPO during 2009-2014. The average weights of the fish in the samples are given at the tops of the panels. |
The average weight of skipjack in 2014 (2.2 kg) was equal to the average of the previous five years (range: 2.0 to 2.5 kg).
For stock assessments of bigeye, six purse-seine fisheries (four associated with floating objects, one unassociated, one associated with dolphins) and one pole-and-line fishery are defined (Figure A-5). The last three fisheries include all 13 sampling areas. Of the 815 wells sampled, 159 contained bigeye. The estimated size compositions of the fish caught during 2014 are shown in Figure A-8a.
|Figure A-8a: Estimated size compositions of the bigeye caught in the EPO during 2014 for each fishery designated in Figure A-5. The average weights of the fish in the samples are given at the tops of the panels. |
Bigeye in the 40- to 100- size range was taken primarily in the Northern floating-object fishery during the second and fourth quarters, and in the Southern floating-object fishery in the third and fourth quarters. Larger bigeye (>100 cm) were caught primarily in the Equatorial floating-object fishery in the second and fourth quarters, and in the Southern floating-object fishery in the third and fourth quarters.
The estimated size compositions of bigeye caught by all fisheries combined during 2009-2014 are shown in Figure A-8b.
|Figure A-8b: Estimated size compositions of the bigeye caught by purse-seine vessels in the EPO during 2009-2014. The average weights of the fish in the samples are given at the tops of the panels. |
The average weight of bigeye in 2014 (5.7 kg) was slightly higher than the previous year, but lower than the 8.0 and 6.7 kg recorded in 2011 and 2012, respectively.
Pacific bluefin are caught by purse-seine and recreational gear off California and Baja California from about 23°N to 35°N, with most of the catch being taken during May through October. During 2012 bluefin were caught between 28°N and 32°N from June through August. The majority of the catches of bluefin by both commercial and recreational vessels were taken during July and August. Prior to 2004, the sizes of the fish in the commercial and recreational catches have been reported separately. During 2004-2012, however, small sample sizes made it infeasible to estimate the size compositions separately. Therefore, the sizes of the fish in the commercial and recreational catches of bluefin were combined for each year of the 2004-2012 period. The average weight of the fish caught during 2012 (14.2 kg) was less than that of 2011 (15.4 kg), but very close to the average weights in 2009 and 2010. The estimated size compositions are shown in Figure A-9.
|Figure A-9: Estimated catches of Pacific bluefin by purse-seine and recreational gear in the EPO during 2007-2012. The values at the tops of the panels are the average weights. |
Prior to 2013, IATTC staff collected length-frequency samples from recreational vessels with landings in San Diego and from purse seiners. Beginning in 2013, sampling of recreational vessels was taken over by the U.S. National Marine Fisheries Service (NMFS). Very few samples were collected from commercial purse-seiners in 2013 and 2014. The size composition estimates for bluefin will be updated after development of a methodology that will incorporate the changes in sampling.
Black skipjack are caught incidentally by fishermen who direct their effort toward yellowfin, skipjack, and bigeye tuna. The demand for this species is low, so most of the catches are discarded at sea, but small amounts, mixed with the more desirable species, are sometimes retained. The estimated size compositions for each year of the 2009-2014 period are shown in Figure A-10.
|Figure A-10: Preliminary size compositions of the catches of black skipjack by purse-seine vessels in the EPO during 2009-2014. The values at the tops of the panels are the average weights. |
The estimated size compositions of the catches of yellowfin and bigeye by the Japanese longline fishery in the EPO during 2008-2012 are shown in Figures A-11
|Figure A-11: Estimated size compositions of the catches of yellowfin tuna by the Japanese longline fishery in the EPO, 2009-2013. |
and Figure A-12.
|Figure A-12: Estimated size compositions of the catches of bigeye tuna by the Japanese longline fishery in the EPO, 2009-2013. |
The average weight of yellowfin in 2013 (61.3 kg) was considerably greater than the previous 4 years (44.7 to 56.3 kg). The average weight of bigeye in 2013 was consistent with the previous 4 years at 46.3 kg. Information on the size compositions of fish caught by the Japanese longline fishery in the EPO during 1958-2008 is available in IATTC Bulletins describing that fishery.Catches of tunas and bonitos, by flag and gear
The annual retained catches of tunas and bonitos in the EPO during 1985-2014, by flag and gear, are shown in (Table A-3a
), (Table A-3b
), (Table A-3c
), (Table A-3d
), (Table A-3e
). These tables include all of the known catches of tunas and bonitos compiled from various sources, including vessel logbooks, observer data, unloading records provided by canners and other processors, export and import records, estimates derived from the species and size composition sampling program, reports from governments and other entities, and estimates derived from the species- and size-composition sampling program. Similar information on tunas and bonitos prior to 2001, and historical data for tunas, billfishes, sharks, carangids, dorado, and miscellaneous fishes are available on the IATTC website
. The purse-seine catches of tunas and bonitos in 2013 and 2014, by flag, are summarized in (Table A-4
) Of the 566 thousand t of tunas and bonitos caught in 2014, 45% were caught by Ecuadorian vessels, and 24% by Mexican vessels. Other countries with significant catches of tunas and bonitos in the EPO included Panama (9 %), Venezuela (7%), and Colombia (8%).
Estimates of the numbers of purse-seine sets of each type (associated with dolphins, associated with floating objects, and unassociated) in the EPO during the 1999-2014 period, and the retained catches of these sets, are shown in (Table A-7
) , (Table A-7 (cont.)
) and in Figure 1.
|Figure 1: Purse-seine catches of tunas, by species and set type, 1999-2014 |
The estimates for vessels ≤363 t carrying capacity were calculated from logbook data in the IATTC statistical data base, and those for vessels >363 t carrying capacity were calculated from the observer data bases of the IATTC, Colombia, Ecuador, the European Union, Mexico, Nicaragua, Panama, the United States, and Venezuela. The greatest numbers of sets associated with floating objects and unassociated sets were made from the mid-1970s to the early 1980s. Despite opposition to fishing for tunas associated with dolphins and the refusal of U.S. canners to accept tunas caught during trips during which sets were made on dolphin-associated fish, the numbers of sets associated with dolphins decreased only moderately during the mid-1990s, and in 2003 were the greatest recorded.
There are two types of floating objects, flotsam and fish-aggregating devices (FADs). The occurrence of the former is unplanned from the point of view of the fishermen, whereas the latter are constructed by fishermen specifically for the purpose of attracting fish. The use of FADs increased sharply in 1994, with the percentage of FADs almost doubling from the previous year, to almost 60% of all floating-object sets. Their relative importance has continued to increase since then, reaching 97% of all floating-object sets by vessels with >363 t carrying capacity in recent years, as shown in (Table A-8
The reported nominal fishing effort (in thousands of hooks) by longline vessels in the EPO, and their catches of the predominant tuna species, are shown in (Table A-9
The 1995 FAO Code of Conduct for Responsible Fisheries stipulates that States and users of living aquatic resources should conserve aquatic ecosystems and it provides that management of fisheries should ensure the conservation not only of target species, but also of species belonging to the same ecosystem or associated with or dependent upon the target species (The Code also provides that management measures should ensure that biodiversity of aquatic habitats and ecosystems
is conserved and endangered species are protected and that States should assess the impacts of environmental factors
on target stocks and species belonging to the same ecosystem or associated with or dependent upon the target stocks, and assess the relationship among the populations in the ecosystem
). In 2001, the Reykjavik Declaration on Responsible Fisheries in the Marine Ecosystem elaborated these principles with a commitment to incorporate an ecosystem approach into fisheries management.
Consistent with these instruments, one of the functions of the IATTC under the 2003 Antigua Convention is to “adopt, as necessary, conservation and management measures and recommendations for species belonging to the same ecosystem and that are affected by fishing for, or dependent on or associated with, the fish stocks covered by this Convention, with a view to maintaining or restoring populations of such species above levels at which their reproduction may become seriously threatened”.
Consequently, the IATTC has taken account of ecosystem issues in many of its decisions, and this report on the offshore pelagic ecosystem of the tropical and subtropical Pacific Ocean, which is the habitat of tunas and billfishes, has been available since 2003 to assist in making its management decisions. This section provides a coherent view, summarizing what is known about the direct impact of the fisheries upon various species and species groups of the ecosystem, and reviews what is known about the environment and about other species that are not directly impacted by the fisheries but may be indirectly impacted by means of predator-prey interactions in the food web.
This review does not suggest objectives for the incorporation of ecosystem considerations into the management of tuna or billfish fisheries, nor any new management measures. Rather, its prime purpose is to offer the Commission the opportunity to ensure that ecosystem considerations are part of its agenda.
It is important to remember that the view that we have of the ecosystem is based on the recent past; we have almost no information about the ecosystem before exploitation began. Also, the environment is subject to change on a variety of time scales, including the well-known El Niño fluctuations and more recently recognized longer-term changes, such as the Pacific Decadal Oscillation and other climate changes.
In addition to reporting the catches of the principal species of tunas and billfishes, the staff has reported the bycatches of non-target species that are either retained or discarded. In this section, data on these bycatches are presented in the context of the effect of the fishery on the ecosystem. Unfortunately, while relatively good information is available for the tunas and billfishes, information for the entire fishery is not available. The information is comprehensive for large (carrying capacity greater than 363 metric tons) purse seiners that carry observers under the Agreement on the International Dolphin Conservation Program (AIDCP), and information on retained catches is also reported for other purse seiners, pole-and-line vessels, and much of the longline fleet. Some information is available on sharks that are retained by parts of the longline fleet. Information on retained and discarded non-target species is reported for large purse-seiners, and is available for very few trips of smaller ones. There is little information available on the bycatches and discards for other fishing vessels.IMPACT OF CATCHESSingle-species assessments
Current information on the effects of the tuna fisheries on the stocks of individual species in the eastern Pacific Ocean (EPO) and the detailed assessments are found in this document. An ecosystem perspective requires a focus on how the fishery may have altered various components of the ecosystem. Sections 2.2 and 2.3 of this report refer to information on the current biomass of each stock considered, compared to estimates of what it might have been in the absence of a fishery. Furthermore, section 2.2 includes a summary of some recent research conducted on drifting fish aggregating device- (FAD) associated aggregations, including methods which may lead to solutions on how to reduce the fishing mortality on undesirable-sizes of bigeye and yellowfin tunas. There are no direct measurements of the stock size before the fishery began, and, in any case, the stocks would have varied from year to year. In addition, the unexploited stock size may be influenced by predator and prey abundance, which is not included in the single-species analyses.Tunas
Information on the effects of the fisheries on bigeye, yellowfin, and skipjack tunas is found in Documents SAC-06-05
, and 07
, respectively, and Pacific bluefin tuna is addressed in the report of the ISC Working Group
. Albacore tuna will be addressed at this meeting. The ISC Northern Albacore Working Group completed its stock assessment
IATTC staff recently published two studies that focused on the potential reduction of fishing mortality by purse seine on undesirable sizes of bigeye and yellowfin tunas and other species of concern, while still capturing associated schools of skipjack tuna. The first of these studies evaluated the simultaneous behaviors of skipjack, bigeye, and yellowfin tunas within large multi-species aggregations associated with FADs. The researchers documented spatial and temporal differences in the schooling behavior of the three species of tunas, including depth distributions, and found that the differences did not appear sufficient such that modifications in purse seine fishing practices could effectively avoid the capture of small bigeye and yellowfin, while optimizing the capture of skipjack. The second study assessed a fishing captain’s ability to predict species composition, sizes, and quantities of tunas associated with drifting FADs, before encirclement with a purse-seine. The captain’s predictions were significantly related to the actual total catch and catch by species, but not to size categories by species. Predictions of species composition were most accurate when estimates of bigeye and yellowfin tuna were combined, indicating the captain was overestimating one species while underestimating the other.Billfishes
Information on the effects of the tuna fisheries on swordfish, blue marlin, striped marlin, and sailfish is presented in the IATTC Fishery Status Report 13
), and shortbill spearfish
No recent stock assessments have been made for these species, although there are some data published jointly by scientists of the National Research Institute of Far Seas Fisheries (NRIFSF) of Japan and the IATTC in the IATTC Bulletin series that show trends in catches, effort, and catches per unit of effort (CPUEs).Summary
Preliminary estimates of the catches (including purse-seine discards), in metric tons, of tunas, bonitos, and billfishes during 2014 in the EPO are found in (Table A-2a
), (Table A-2a (cont.)
), (Table A-2a (cont.)
) and (Table A-2b
), (Table A-2b (cont.)
) of this document.Marine mammals
Marine mammals, especially spotted dolphins (Stenella attenuata), spinner dolphins (S. longirostris), and common dolphins (Delphinus delphis), are frequently found associated with yellowfin tuna in the size range of about 10 to 40 kg in the EPO. Purse-seine fishermen have found that their catches of yellowfin in the EPO can be maximized by setting their nets around herds of dolphins and the associated schools of tunas, and then releasing the dolphins while retaining the tunas. The estimated incidental mortality of dolphins in this operation was high during the early years of the fishery, and the populations of dolphins were reduced from their unexploited levels during the 1960s and 1970s. After the late 1980s the incidental mortality decreased precipitously, and there is now evidence that the populations are recovering. Preliminary mortality estimates of dolphins in the fishery in 2014 are shown in (Table 1
The IATTC staff is responsible for the assessment of dolphin populations associated with the purse-seine fishery for tunas, as a basis for the dolphin mortality limits established by the Agreement on the International Dolphin Conservation Program (AIDCP).
Studies of the association of tunas with dolphins have been an important component of the staff’s long-term approach to understanding key interactions in the ecosystem. The extent to which yellowfin tuna and dolphins compete for resources, whether either or both of them benefits from the interaction, why the tuna are most often found with spotted dolphins versus other dolphins, and why the species associate most strongly in the eastern tropical Pacific, remain critical pieces of information, given the large biomasses of both groups and their high rates of prey consumption. Three studies were conducted to address these hypotheses: a simultaneous tracking study of spotted dolphins and yellowfin tuna, a trophic interactions study comparing their prey and daily foraging patterns, and a spatial study of oceanographic features correlated with the tuna dolphin association. These studies demonstrated that the association is neither permanent nor obligatory, and that the benefits of the association are not based on feeding advantages. The studies support the hypothesis that one or both species reduce the risk of predation by forming large, mixed-species groups. The association is most prevalent where the habitat of the tuna is compressed to the warm, shallow, surface waters of the mixed layer by the oxygen minimum zone, a thick layer of oxygen-poor waters underlying the mixed layer. The association has been observed in areas with similar oceanographic conditions in other oceans, but it is most prevalent and consistent in the eastern tropical Pacific, where the oxygen minimum zone is the most hypoxic and extensive in the world.
During August-December 2006, scientists of the U.S. National Marine Fisheries Service (NMFS) conducted the latest in a series of research cruises under the Stenella Abundance Research (STAR) project. The primary objective of the multi-year study is to investigate trends in population size of the dolphins that have been taken as incidental catch by the purse-seine fishery in the EPO. Data on cetacean distribution, herd size, and herd composition were collected from the large-scale line-transect surveys to estimate dolphin abundance. Oceanographic data are collected to characterize habitat and its variation over time. Data on distribution and abundance of prey fishes and squids, seabirds, and sea turtles further characterize the ecosystem in which these dolphins live. The 2006 survey covered the same areas and used the same methods as past surveys. Data from the 2006 survey produced new abundance estimates, and previous data were re-analyzed to produce revised estimates for 10 dolphin species and/or stocks in the EPO between 1986 and 2006. The 2006 estimates for northeastern offshore spotted dolphins were somewhat greater, and for eastern spinner dolphins substantially greater, than the estimates for 1998-2000. Estimates of population growth for these two depleted stocks and the depleted coastal spotted dolphin stock may indicate they are recovering, but the western-southern offshore spotted dolphin stock may be declining. The 1998-2006 abundance estimates for coastal spotted, whitebelly spinner, and rough-toothed (Steno bredanensis
) dolphins showed an increasing trend, while those for the striped (S. coeruleoalba
), short-beaked common (Delphinus delphis
), bottlenose (Tursiops truncatus
), and Risso’s (Grampus griseus
) dolphins were generally similar to previous estimates obtained with the same methods. Because there have been no NMFS surveys since 2006, new modelling was conducted over the past year on trends in dolphin relative abundance using purse-seine observer data. That research concluded that indices of relative abundance from purse-seine observer data for species such as dolphins in the EPO that are directly associated with the fishing process are unlikely to be reliable indicators. Not only are such indices susceptible to the usual problems of changes in fishing behavior, but there is not a clear distinction between indexing the dolphin-tuna association and indexing dolphin abundance. This research, as well as alternative means of monitoring dolphin stocks, are discussed in Documents SAC-05-11d
Scientists of the NMFS have made estimates of the abundances of several other species of marine mammals based on data from research cruises made between 1986 and 2000 in the EPO. Of the species not significantly affected by the tuna fishery, short-finned pilot whales (Globicephala macrorhynchus
) and three stocks of common dolphins showed increasing trends in abundance during that 15-year period. The apparent increased abundance of these mammals may have caused a decrease in the carrying capacity of the EPO for other predators that overlap in diet, including spotted dolphins. Bryde’s whales (Balaenoptera edeni
also increased in estimated abundance, but there is very little diet overlap between these baleen whales and the upper-level predators impacted by the fisheries. The abundance estimates for sperm whales (Physeter macrocephalus
) tended to decrease during 1986-2000.
Some marine mammals are adversely affected by reduced food availability during El Niño events, especially in coastal ecosystems. Examples that have been documented include dolphins, pinnipeds, and Bryde’s whales off Peru, and pinnipeds around the Galapagos Islands. Large whales are able to move in response to changes in prey productivity and distribution.Sea turtles
Sea turtles are caught on longlines when they take the bait on hooks, are snagged accidentally by hooks, or are entangled in the lines. Estimates of incidental mortality of turtles due to longline and gillnet fishing are few. At the 4th meeting of the IATTC Working Group on Bycatch
in January 2004, it was reported that 166 leatherback (Dermochelys coriacea
) and 6,000 other turtle species, mostly olive Ridley (Lepidochelys olivacea
), were incidentally caught by Japan’s longline fishery in the EPO during 2000, and that, of these, 25 and 3,000, respectively, were dead. At the 6th meeting of the Working Group
in February 2007, it was reported that the Spanish longline fleet targeting swordfish in the EPO averaged 65 interactions and 8 mortalities per million hooks during 1990-2005. The mortality rates due to longlining in the EPO are likely to be similar for other fleets targeting bigeye tuna, and possibly greater for those that set their lines at shallower depths for albacore and swordfish. About 23 million of the 200 million hooks set each year in the EPO by distant-water longline vessels target swordfish with shallow longlines.
In addition, there is a sizeable fleet of artisanal longline vessels that fish for tunas, billfishes, sharks, and dorado (Coryphaena
spp.) in the EPO. Since 2005, staff members of the IATTC and some other organizations, together with the governments of several coastal Latin American nations, have been engaged in a program to reduce the hooking rates and mortalities of sea turtles in these fisheries. Additional information on this program can be found in the section “Actions by the IATTC and the AIDCP addressing Ecosystem considerations under Sea turtles.”
Sea turtles are occasionally caught in purse seines in the EPO tuna fishery. Most interactions occur when the turtles associate with floating objects, and are captured when the object is encircled. In other cases, nets set around unassociated schools of tunas or schools associated with dolphins may capture sea turtles that happen to be at those locations. The olive Ridley turtle is, by far, the species of sea turtle taken most often by purse seiners. It is followed by green sea turtles (Chelonia mydas
), and, very occasionally, by loggerhead (Caretta caretta
) and hawksbill (Eretmochelys imbricata
) turtles. From 1990, when IATTC observers began recording this information, through 2014, only three mortalities of leatherback turtles have been recorded. Some of the turtles are unidentified because they were too far from the vessel or it was too dark for the observer to identify them. Sea turtles, at times, become entangled in the webbing under fish-aggregating devices (FADs) and drown. In some cases, they are entangled by the fishing gear and may be injured or killed. Preliminary estimates of the mortalities (in numbers) of turtles caused by large purse-seine vessels during 2014, by set type (on floating objects (OBJ), unassociated schools (NOA), and dolphins (DEL)), are shown in (Table 2
The mortalities of sea turtles due to purse seining for tunas are probably less than those due to other types of human activity, which include exploitation of eggs and adults, beach development, pollution, entanglement in and ingestion of marine debris, and impacts of other fisheries.
The populations of olive Ridley turtles are designated as vulnerable, those of green and loggerhead turtles are designated as endangered, and those of hawksbill and leatherback turtles as critically endangered, by the International Union for the Conservation of Nature (IUCN).Sharks and other large fishes
Sharks and other large fishes are taken by both purse-seine and longline vessels. Silky sharks (Carcharhinus falciformis
) are the most commonly-caught species of shark in the purse-seine fishery, followed by oceanic whitetip sharks (C. longimanus
). The longline fisheries also take silky sharks. An analysis of longline and purse-seine fishing is necessary to estimate the impact of fishing on the stock(s). Estimated indices of relative abundance of silky sharks, based on data for purse-seine sets on floating objects, showed decreasing trends for large (>150 cm total length) and medium-sized sharks (90-150 cm total length) during 1994-2004, and remained relatively constant for large sharks and increased slightly for medium sharks between 2005 and 2009. The trends in unstandardized bycatch per set were similar for the other two types of purse-seine sets (standardized trends are not yet available). The unstandardized average bycatches per set of oceanic whitetip sharks also showed decreasing trends for all three set types during the same period. It is not known whether these decreasing trends were due to incidental capture by the fisheries, changes in the environment (perhaps associated with the 1997-1998 El Niño event), or other factors. The decreasing trends do not appear to be due to changes in the density of floating objects.
Apart from blue and silky sharks, there are no stock assessments available for shark species in the EPO, and hence the impacts of the bycatches on the stocks are unknown. A stock assessment for silky sharks covering the 1993-2010 period was attempted using the Stock Synthesis model. Unfortunately, the model was unable to fit the main index of abundance adequately, and therefore the results were not reliable since relative trends and absolute scale are compromised in the assessment. Results are presented in Document SAC-05 INF-F
. The majority of the catches of silky sharks in the EPO is estimated to be taken by longliners, some of them targeting sharks. As an alternative to conventional stock assessment models, a suite of possible stock status (or stability) indicators (SSIs), which could be considered for managing the northern and southern stocks of silky sharks in the EPO, are provided in Document SAC-05-11a
A new stock assessment
of blue sharks (Prionace glauca
) in the North Pacific Ocean was conducted by scientists of the ISC Shark Working Group in 2014. The report
states, “Results of the reference case model showed that the stock biomass was near a time-series high in 1971, fell to its lowest level between the late 1980s and early 1990s, and subsequently increased gradually and has leveled off at a biomass similar to that at the beginning of the time-series.”
A project was conducted during May 2007-June 2008 by scientists of the IATTC and the NMFS to collect and archive tissue samples of sharks, rays, and other large fishes for genetics analysis. Data from the archived samples are being used in studies of large-scale stock structure of these taxa in the EPO, information that is vital for stock assessments and is generally lacking throughout the Pacific Ocean. The preliminary results of an analysis for silky sharks showed that for management purposes, silky sharks in the EPO should be divided into two stocks, one north and one south of the equator. In addition, the results of a mitochondrial-DNA study from 2013 show a slight genetic divergence between silky sharks in the western and eastern Pacific, which supports assessing and managing these two populations separately.
Preliminary estimates of the catches (including purse-seine discards), in metric tons, of sharks and other large fishes in the EPO during 2014, other than those mentioned above, by large purse-seine vessels are shown in (Table 3
). Complete data are not available for small purse-seine, longline, and other types of vessels.
The catch rates of species other than tunas in the purse-seine fishery are different for each type of set. With a few exceptions, the bycatch rates are greatest in sets on floating objects, followed by unassociated sets and, at a much lower level, dolphin sets. Dolphin bycatch rates are greatest for dolphin sets, followed by unassociated sets and, at a much lower level, floating-object sets. In general, the bycatch rates of manta rays (Mobulidae
), and stingrays (Dasyatidae
) are greatest in unassociated sets, followed by dolphin sets, and lowest in floating-object sets, although 2014 is an exception. Because of these differences, it is necessary to follow the changes in frequency of the different types of sets to interpret the changes in bycatch data. The estimated numbers of purse-seine sets of each type in the EPO during 1999-2014 are shown in (Table A-7
) , (Table A-7 (cont.)
) of this document.
The reduction of bycatches is a goal of ecosystem-based fisheries management. A recently-published study analyzed the ratio of bycatch to target catch across a range of set size-classes (in tons). The study demonstrated that the ratios of total bycatch to tuna catch and silky shark bycatch to tuna catch decreased as set size increased. The greatest bycatch ratios occurred in sets catching <20 t.
In October 2006, the NMFS hosted a workshop on bycatch reduction in the EPO purse-seine fishery. The attendees supported a proposal for research on methods to reduce bycatches of sharks by attracting them away from floating objects prior to setting the purse seine. They also supported a suite of field experiments on bycatch reduction devices and techniques; these would include FAD modifications and manipulations, assessing behavioral and physiological indicators of stress, and removing living animals from the seine and deck (e.g.
sorting grids, bubble gates, and vacuum pumps). A third idea was to use IATTC data to determine if spatial, temporal, and environmental factors can be used to predict bycatches in FAD sets and to determine to what extent time/area closures would be effective in reducing bycatches.
Scientists at the University of Washington have conducted an analysis of the temporal frequency of areas of high bycatches of silky sharks in purse-seine sets on floating objects, which will be useful for determining the effectiveness of area-time closures as a means of reducing shark bycatch. Results show that both model predictions and observed data tend to indicate that these bycatches occurred most frequently north of 4°N and west of 100-105°W. However, due to large tuna catches south of 5°N, the greatest reduction in bycatch from sets on floating objects with the least loss of tuna catch would be achieved north of approximately 6°N.
Dorado (Coryphaena hippurus
) is one of the most important species caught in the artisanal fisheries of the coastal nations in the EPO. Dorado are also caught incidentally in the purse-seine tuna fishery in the EPO. Under the Antigua Convention and its ecosystem approach to fisheries, it is therefore appropriate that the IATTC staff study the species, with a view to determining the impact of fishing, and to recommend appropriate conservation measures of this important resource if required. In this context, some Members of the IATTC with coastlines in the region have requested that collaborative research on dorado be carried out with the IATTC staff so that solid scientific information is available for this purpose.
The IATTC held its first technical meeting on dorado
in 2014. That meeting had three objectives: 1) to promote synergy among the Members of the IATTC for a regional investigation of dorado in the EPO; 2) to review the current state of knowledge of dorado and identify available data sets across fisheries/regions in the EPO); and 3) to plan a future collaborative research plan. This collaborative effort thus far includes: analysis of available catch statistics and trade records, improvement of field data collection programs, investigation of seasonal trends, and identification of fishery units. In addition, available fishery data on dorado from IATTC Members and other nations are being analyzed to develop stock status indicators (SSIs) which could potentially provide a basis for advice for managing the species in the EPO (see SAC-05-11b
). The work will be continued in 2015.OTHER ECOSYSTEM COMPONENTS Seabirds
There are approximately 100 species of seabirds in the tropical EPO. Some seabirds associate with epipelagic predators near the sea surface, such as fishes (especially tunas) and marine mammals. Subsurface predators often drive prey to the surface to trap them against the air-water interface, where the prey becomes available to the birds. Most species of seabirds take prey within a half meter of the sea surface or in the air (flyingfishes (Exocoetidae
) and squids (primarily Ommastrephidae
)). In addition to driving the prey to the surface, subsurface predators make prey available to the birds by injuring or disorienting the prey, and by leaving scraps after feeding on large prey. Feeding opportunities for some seabird species are dependent on the presence of tuna schools feeding near the surface.
Seabirds are affected by the variability of the ocean environment. During the 1982-1983 El Niño event, seabird populations throughout the tropical and northeastern Pacific Ocean experienced breeding failures and mass mortalities, or migrated elsewhere in search of food. Some species, however, are apparently not affected by El Niño episodes. In general, seabirds that forage in upwelling areas of the tropical EPO and Peru Current suffer reproductive failures and mortalities due to food shortage during El Niño events, while seabirds that forage in areas less affected by El Niño episodes may be relatively unaffected.
According to the Report of the Scientific Research Program under the U.S. International Dolphin Conservation Program Act
, prepared by the NMFS in September 2002, there were no significant temporal trends in abundance estimates over the 1986-2000 period for any species of seabird, except for a downward trend for the Tahiti petrel (Pseudobulweria rostrata
), in the tropical EPO. Population status and trends are currently under review for waved (Phoebastria irrorata
), black-footed (P. nigripes
), and Laysan (P. immutabilis
Some seabirds, especially albatrosses and petrels, are susceptible to being caught on baited hooks in pelagic longline fisheries. Satellite tracking and at-sea observation data have identified the importance of the IATTC area for waved, black-footed, Laysan, and black-browed (Thalassarche melanophrys
) albatrosses, plus several other species that breed in New Zealand, yet forage off the coast of South America. There is particular concern for the waved albatross because it is endemic to the EPO and nests only in the Galapagos Islands. Observer data from artisanal vessels show no interactions with waved albatross during these vessels’ fishing operations. Data from the US pelagic longline fishery in the northeastern Pacific Ocean indicate that bycatches of black-footed and Laysan albatrosses occur. Few comparable data for the longline fisheries in the central and southeastern Pacific Ocean are available. At the 6th meeting of the IATTC Working Group on Bycatch in February 2007, it was reported that the Spanish surface longline fleet targeting swordfish in the EPO averaged 40 seabird interactions per million hooks, virtually all resulting in mortality, during 1990-2005. In 2007, the IATTC Stock Assessment Working Group identified areas of vulnerability to industrial longline fishing for several species of albatross and proposed mitigation measures. Additional information can be found in the section “Actions by the IATTC and the AIDCP addressing Ecosystem considerations under Seabirds.”Forage
The forage taxa occupying the middle trophic levels in the EPO are obviously important components of the ecosystem, providing a link between primary producers at the base of the food web and the upper-trophic-level predators, such as tunas and billfishes. Indirect effects on those predators caused by environmental variability are transmitted to the upper trophic levels through the forage taxa. Little is known, however, about fluctuations in abundance of the large variety of prey species in the EPO. Scientists from the NMFS have recorded data on the distributions and abundances of common prey groups, including lantern fishes (Myctophidae
), flyingfishes, and some squids, in the tropical EPO during 1986-1990 and 1998-2000. Mean abundance estimates for all fish taxa and, to a lesser extent, for squids increased from 1986 through 1990. The estimates were low again in 1998, and then increased through 2000. Their interpretation of this pattern was that El Niño events in 1986-1987 and 1997-1998 had negative effects on these prey populations. More data on these taxa were collected during the NMFS STAR 2003 and 2006 cruises.
Cephalopods, especially squids, play a central role in many, if not most, marine pelagic food webs by linking the massive biomasses of micronekton, particularly myctophid fishes, to many oceanic predators. Given the high trophic flux passing through the squid community, a concerted research effort on squids is thought to be important for understanding their role as key prey and predators. In 2013, a special volume of the journal Deep Sea Research II, Topical Studies in Oceanography (Vol. 5) was focused on The Role of Squids in Pelagic Ecosystems. The volume covers six main research areas: squids as prey, squids as predators, the role of squids in marine ecosystems, physiology, climate change, and the Humboldt or jumbo squid
) as a recent example of ecological plasticity in a cephalopod species.
Humboldt squid populations in the EPO have increased in size and geographic range in recent years. For example, the Humboldt squid expanded its range to the north into waters off central California, USA from 2002 to mid-2010. In addition, in 2002 observers on tuna purse-seine vessels reported increased incidental catches of Humboldt squid taken with tunas, primarily skipjack, off Peru. Juvenile stages of these squid are common prey for yellowfin and bigeye tunas, and other predatory fishes, and Humboldt squid are also voracious predators of small fishes and cephalopods throughout their range. Large Humboldt squid have been observed attacking skipjack and yellowfin inside a purse seine. Not only have these squid impacted the ecosystems that they have expanded into, but they are also thought to have the capacity to affect the trophic structure in pelagic regions. Changes in the abundance and geographic range of Humboldt squid could affect the foraging behavior of the tunas and other predators, perhaps changing their vulnerability to capture.
Some small fishes, many of which are forage for the larger predators, are incidentally caught by purse-seine vessels in the EPO. Frigate and bullet tunas (Auxis
spp.), for example, are a common prey of many of the animals that occupy the upper trophic levels in the tropical EPO. In the tropical EPO ecosystem model (Ecosystem Modeling Section), frigate and bullet tunas comprise 10% or more of the diet of eight predator species or groups. Small quantities of frigate and bullet tunas are captured by purse-seine vessels on the high seas and by artisanal fisheries in some coastal regions of Central and South America. The vast majority of frigate and bullet tunas captured by tuna purse-seine vessels is discarded at sea. Preliminary estimates of the catches (including purse-seine discards), in metric tons, of small fishes by large purse-seine vessels with observers aboard in the EPO during 2014 are shown in (Table 4
).Larval fishes and plankton
Larval fishes have been collected by manta (surface) net tows in the EPO for many years by personnel of the NMFS Southwest Fisheries Science Center. Of the 314 taxonomic categories identified, 17 were found to be most likely to show the effects of environmental change. The occurrence, abundance, and distribution of these key taxa revealed no consistent temporal trends. Recent research has shown a longitudinal gradient in community structure of the ichthyoplankton assemblages in the eastern Pacific warm pool, with abundance, species richness, and species diversity high in the east (where the thermocline is shallow and primary productivity is high) and low but variable in the west (where the thermocline is deep and primary productivity is low).
The phytoplankton and zooplankton populations in the tropical EPO are variable. For example, chlorophyll concentrations on the sea surface (an indicator of phytoplankton blooms) and the abundance of copepods were markedly reduced during the El Niño event of 1982-1983, especially west of 120°W. Similarly, surface concentrations of chlorophyll decreased during the 1986-1987 El Niño episode and increased during the 1988 La Niña event due to changes in nutrient availability.
The species and size composition of zooplankton is often more variable than the zooplankton biomass. When the water temperatures increase, warm-water species often replace cold-water species at particular locations. The relative abundance of small copepods off northern Chile, for example, increased during the 1997-1998 El Nino event, while the zooplankton biomass did not change.
Copepods often comprise the dominant component of secondary production in marine ecosystems. An analysis of the trophic structure among the community of pelagic copepods in the EPO was conducted by a student of the Centro Interdisciplinario de Ciencias Marinas, Instituto Politécnico Nacional, La Paz, Mexico, using samples collected by scientists of the NMFS STAR project. The stable nitrogen isotope values of omnivorous copepods were used in a separate analysis of the trophic position of yellowfin tuna, by treating the copepods as a proxy for the isotopic variability at the base of the food web (see next section).TROPHIC INTERACTIONS
Tunas and billfishes are wide-ranging, generalist predators with high energy requirements, and, as such, are key components of pelagic ecosystems. The ecological relationships among large pelagic predators, and between them and animals at lower trophic levels, are not well understood. Given the need to evaluate the implications of fishing activities on the underlying ecosystems, it is essential to acquire accurate information on the trophic links and biomass flows through the food web in open-ocean ecosystems, and a basic understanding of the natural variability forced by the environment.
Knowledge of the trophic ecology of predatory fishes has historically been derived from stomach contents analysis, and more recently from chemical indicators. Large pelagic predators are considered efficient biological samplers of micronekton organisms, which are poorly sampled by nets and trawls. Diet studies have revealed many of the key trophic connections in the pelagic EPO, and have formed the basis for representing food-web interactions in an ecosystem model (IATTC Bulletin, Vol. 22, No. 3
) to explore indirect ecosystem effects of fishing. For example, studies in the 1990s and 2000s revealed that the most common prey items of yellowfin tuna caught by purse seines offshore were frigate and bullet tunas, red crabs (Pleuroncodes planipes
), Humboldt squid, a mesopelagic fish (Vinciguerria lucetia
), and several epipelagic fishes. Bigeye tuna feed at greater depths than do yellowfin and skipjack, and consume primarily cephalopods and mesopelagic fishes. The most important prey of skipjack overall were reported to be euphausiid crustaceans during the late 1950s, whereas the small mesopelagic fish V. lucetia
appeared dominant in the diet during the early 1990s. Tunas that feed inshore often utilize different prey than those caught offshore.
Historical studies of tuna diets in the EPO were based on qualitative data from few samples, with little or no indication of relative prey importance. Contemporary studies, however, have used diet indices, typically volume or weight importance, numeric importance, and frequency of occurrence of prey items to quantify diet composition, often in conjunction with chemical indicators, such as stable-isotope and fatty-acid analyses. Recently, information about tuna bioenergetics, diets, niche separation, daily ration, chemical indicators of diet, and inter-annual variability and potential effects of climate change on the trophic ecology of tunas in all oceans was summarized by species in a book chapter entitled “Bioenergetics, trophic ecology, and niche separation of tunas.” The chapter will be published in 2015 in a book entitled “Tunas and their Fisheries: Safeguarding Sustainability in the 21st
Century.” Each species of tuna appears to have a generalized feeding strategy, in the sense that their diets were characterized by high prey diversity and overall low abundance of individual prey types.
New statistical methods for analyzing complex, multivariate stomach-contents data have been developed through an international collaboration, Climate Impacts on Oceanic Top Predators-Integrated Marine Biogeochemistry and Ecosystem Research (CLIOTOP-IMBER), Working Group 3
(Trophic pathways in open-ocean ecosystems), to assess the trophodynamics of marine top predators. This methodology shows promise for analyzing broad-scale spatial, temporal, environmental, and biological relationships in a classification-tree modeling framework that predicts the prey compositions of predators. Two recent studies of yellowfin tuna and silky sharks in the EPO, discussed below, used the approach to infer changes in prey populations over space (yellowfin and silky sharks) and time (yellowfin) based on stomach contents data.
Stomach samples of ubiquitous generalist predators, such as the tunas, can be used to infer changes in prey populations by identifying changes in foraging habits over time. Prey populations that support upper-level predators vary over time (see Forage section), and some prey impart considerable predation pressure on animals that occupy the lower trophic levels (including the early life stages of large fishes). A comprehensive analysis of predation by yellowfin tuna on a decadal scale in the EPO was completed in 2013. Samples from 6,810 fish were taken from 433 purse-seine sets during two 2-year periods separated by a decade. Simultaneously, widespread reductions in biological production, changes in phytoplankton community composition, and a vertical expansion and intensification of the oxygen minimum zone appeared to alter the food webs in tropical and subtropical oceans (see Physical environment section). A modified classification tree approach, mentioned above, was used to analyze spatial, temporal, environmental, and biological covariates explaining the predation patterns of the yellowfin during 1992-1994 and 2003-2005. For the majority of the yellowfin stock in the EPO, a major diet shift was apparent during the decade. Fishes were more abundant (by weight) during the early 1990s, while cephalopods and crustaceans predominated a decade later. As a group, epipelagic fishes declined from 82% to 31% of the diet, while mesopelagic species increased from 9% to 29% over the decade. Spatial partial dependence plots revealed range expansions by Vinciguerria lucetia
, Humboldt squid (Dosidicus gigas
), and Pleuroncodes planipes
, range contractions by Auxis
spp. and a boxfish (Lactoria diaphana
), and a near disappearance of driftfish (Cubiceps
spp.) from the diet. Evidence from predation rates suggests that biomasses of V. lucetia
and D. gigas
have increased in the first half of the 2000s and that the distribution of D. gigas
apparently expanded offshore as well as poleward (see Forage section).
The food-web representations that form the basis of ecosystem models are usually highly generalized, and do not account for variability in space and time. To gain insight into the role of the silky shark in the ecosystem, in 2014 an analysis of spatial variability was carried out, based on the stomach contents of 289 silky sharks captured as bycatch in sets on floating objects, primarily drifting fish-aggregating devices (FADs), by the tuna purse-seine fishery of the EPO. The dataset is novel because biological data for open-ocean carcharhinid sharks are difficult to collect, and it includes data for silky sharks caught over a broad region of the tropical EPO. Results from classification tree and quantile regression methodologies suggest that the silky shark is an opportunistic predator that forages on a variety of prey. Broad-scale spatial and shark size covariates explained the feeding habits of the silky sharks. A strong spatial shift in diet was revealed, with different foraging patterns in the eastern (inshore) and western (offshore) regions. Greater proportions of FAD-associated prey than non-FAD-associated prey were observed in the diet throughout the EPO, but especially in the offshore region. Yellowfin tuna and silky sharks shared some of the same prey resources during these same two 2-year periods separated by a decade, e.g., Humboldt squid, flyingfishes, jacks and pompanos, and Tetraodontiformes. As was the case for yellowfin tuna, spatial and temporal factors likely both have a role in determining silky shark predation habits, but the samples were inadequate to test whether the diet of the sharks had changed over time. The analysis provided a comprehensive description of silky shark predation in the EPO, while demonstrating the need for increased sampling coverage over space and time, and presents important information on the dynamic component of trophic interactions of silky sharks. This information can be used to improve future ecosystem models.
Trophic-ecology studies have become focused on understanding entire food webs, initially by describing the inter-specific connections among the predator communities, comprising tunas, sharks, billfishes, dorado, wahoo, rainbow runner, and others. In general, considerable resource partitioning is evident among the components of these communities, and researchers seek to understand the spatial scale of the observable trophic patterns, and also the role of climate variability in influencing the patterns. In 2012, an analysis of predation by a suite of apex predators (including sharks, billfishes, tunas, and other fishes and mammals) on yellowfin and skipjack tunas in the EPO was published. Predation rates on yellowfin and skipjack were high for sharks and billfishes, and those animals consumed a wide size range of tunas, including subadults capable of making a notable contribution to the reproductive output of tuna populations. The tropical tunas in the EPO act as mesopredators more than apex predators.
While diet studies have yielded many insights, stable isotope analysis is a useful complement to stomach contents for delineating the complex structure of marine food webs. Stomach contents represent a sample of only the most-recent several hours of feeding at the time of day an animal is captured, and under the conditions required for its capture. Stable carbon and nitrogen isotopes, however, integrate information on all components of the entire diet into the animal’s tissues, providing a recent history of trophic interactions and information on the structure and dynamics of ecological communities. More insight is provided by compound-specific isotope analysis of amino acids (AA-CSIA). In samples of consumer tissues, “source” amino acids (e.g.
phenylalanine, glycine) retained the isotopic values at the base of the food web, and “trophic” amino acids (e.g.
glutamic acid) became enriched in 15
N by about 7.6‰ relative to the baseline. In AA-CSIA, predator tissues alone are adequate for trophic-position estimates, and separate analysis of the isotopic composition of organisms at the base of the food web is not necessary. An analysis of the spatial distribution of stable isotope values of yellowfin tuna in relation to those of copepods showed that the trophic position of yellowfin tuna increased from inshore to offshore in the EPO, a characteristic of the food web never detected in diet data. This is likely a result of differences in food-chain length due to phytoplankton species composition (species with small cell size) in offshore oligotrophic waters versus larger diatom species in the more productive eastern waters.
CSIA was recently utilized in the EPO and other regions through a research grant from the Comparative Analysis of Marine Ecosystem Organization (CAMEO) program, which is implemented as a partnership between the NMFS and the U.S. National Science Foundation, Division of Ocean Sciences. The research collaboration among the IATTC, the University of Hawaii, Scripps Institution of Oceanography, and the Oceanic Institute, Hawaii, seeks to develop amino acid compound-specific isotopic analysis as a tool that can provide an unbiased evaluation of trophic position for a wide variety of marine organisms and to use this information to validate output from trophic mass-balance ecosystem models. To accomplish this goal, the research combines laboratory experiments and field collections in contrasting ecosystems that have important fisheries. The field component was undertaken in varying biogeochemical environments, including the equatorial EPO, to examine trophic position of a range of individual species, from macrozooplankton to large fishes, and to compare trophic position estimates derived from AA-CSIA for these species with ecosystem model output. The project began in 2010 and was extended into 2014.
Most of the samples for the EPO portion of the study were collected and stored frozen by personnel of the NMFS, Protected Resources Division, Southwest Fisheries Science Center (SWFSC), aboard the research vessels David Starr Jordan
and McArthur II
during the Stenella
Abundance Research Project (STAR) in 2006. The samples for the study nearly span the food web in the EPO, and all were taken along an east-to-southwest transect that appeared to span a productivity gradient. The components include macroplankton (two euphausiid crustaceans, Euphausia distinguenda
and E. tenera
), mesopelagic-micronekton (two myctophid fishes, Myctophum nitidulum
and Symbolophorus reversus
), cephalopods (two species of pelagic squids, Dosidicus gigas
and Sthenoteuthis oualaniensis
), and small and large micronektonivores and nektonivores (skipjack, yellowfin, and bigeye tunas collected aboard commercial purse-seine vessels fishing in the EPO during 2003-2005).
Stable isotope analyses of bulk tissues and amino acids were conducted on several specimens each of the species listed above. Bulk δ15
N values varied markedly across the longitude and latitude gradients. There were no distinct longitudinal trends, but the δ15
N values increased consistently with increasing latitude. Trophic position estimates based on the amino-acid δ15
N values, however, varied little intra-specifically across the sample transect. These two results suggest that the isotopic variability in the food web was likely due to biogeochemical variability at the base of the food web rather than differences in diets within the food web. Increasing δ15
N values with latitude correspond to high rates of denitrification associated with the large oxygen minimum zone in the ETP. Among-species comparisons of absolute trophic positions based on AA-CSIA estimates with estimates based on diet from the EPO ecosystem model (IATTC Bulletin, Vol. 22, No. 3
) showed underestimates for the predators occupying higher trophic levels, i.e
. the three tunas and two squids. These underestimates are likely because the previously-accepted trophic enrichment factor of 7.6 ‰ for phenylalanine and glutamic acid, which was derived from laboratory experiments with primary producers and invertebrate consumers, is inadequate for higher-level predators. This issue is also being addressed by collaborators on the CAMEO project.PHYSICAL ENVIRONMENT*
Environmental conditions affect marine ecosystems, the dynamics and catchability of tunas and billfishes, and the activities of fishermen. Tunas and billfishes are pelagic during all stages of their lives, and the physical factors that affect the tropical and sub-tropical Pacific Ocean can have important effects on their distribution and abundance. Environmental conditions are thought to cause considerable variability in the recruitment of tunas and billfishes. Stock assessments by the IATTC have often incorporated the assumption that oceanographic conditions might influence recruitment in the EPO.
Different types of climate perturbations may impact fisheries differently. It is thought that a shallow thermocline in the EPO contributes to the success of purse-seine fishing for tunas, perhaps by acting as a thermal barrier to schools of small tunas, keeping them near the sea surface. When the thermocline is deep, as during an El Niño event, tunas seem to be less vulnerable to capture, and the catch rates have declined. Warmer- or cooler-than-average sea-surface temperatures (SSTs) can also cause these mobile fishes to move to more favorable habitats.
The ocean environment varies on a variety of time scales, from seasonal to inter-annual, decadal, and longer (e.g.
climate phases or regimes). The dominant source of variability in the upper layers of the EPO is known as the El Niño-Southern Oscillation (ENSO). The ENSO is an irregular fluctuation involving the entire tropical Pacific Ocean and global atmosphere. It results in variations of the winds, rainfall, thermocline depth, circulation, biological productivity, and the feeding and reproduction of fishes, birds, and marine mammals. El Niño events occur at 2- to 7-year intervals, and are characterized by weaker trade winds, deeper thermoclines, and abnormally-high SSTs in the equatorial EPO. El Niño’s opposite phase, often called La Niña (or anti-El Niño), is characterized by stronger trade winds, shallower thermoclines, and lower SSTs. Research has documented a connection between the ENSO and the rate of primary production, phytoplankton biomass, and phytoplankton species composition. Upwelling of nutrient-rich subsurface water is reduced during El Niño episodes, leading to a marked reduction in primary and secondary production. ENSO also directly affects animals at middle and upper trophic levels. Researchers have concluded that the 1982-1983 El Niño event, for example, deepened the thermocline and nutricline, decreased primary production, reduced zooplankton abundance, and ultimately reduced the growth rates, reproductive successes, and survival of various birds, mammals, and fishes in the EPO. In general, however, the ocean inhabitants recover within short periods because their life histories are adapted to respond to a variable habitat.
The IATTC staff issues quarterly reports of the monthly average oceanographic and meteorological data for the EPO, including a summary of current ENSO conditions. In January 2014, the SSTs were very close to normal throughout the entire tropical EPO. In February, a band of cool water appeared, extending along the equator from the coast of South America to about 145°W, but by April it had disappeared. There were also patches of cool water along the coasts of Ecuador and Peru from March to May. Patches of warm water that appeared off Mexico and Central America in February were still present in June, and in May and June there was a band of warm water along the equator from the coast of South America to west of 180°. In May, June, and July there was a band of cool water along 10°S from the coast of South America to about 125°W, but it was less pronounced during August and September. Meanwhile, extensive areas of warm water appeared north of about 10°S, apparently the early onset of the El Niño event that had been predicted by the U.S. National Weather Service. However, the warm water was confined mostly to the area north of the equator through December, and a small area of cool water that appeared well south of the equator grew larger in December. The SSTs were mostly below normal from October 2013 through March 2014, but during April-December 2014 they were almost all above normal. According to the Climate Diagnostics Bulletin of the U.S. National Weather Service for December 2014, “Most models predict the SST anomalies to remain weak El Niño levels (3-month values of the Niño-3.4 index between 0.5°C and 0.9°C) during December-February 2014-15, and lasting into the Northern Hemisphere spring 2015. If El Niño were to emerge, the forecaster consensus favors a weak event that ends in early Northern Hemisphere spring. In summary, there is an approximately 50-60 percent chance of El Niño conditions during the next two months, with ENSO-neutral favored thereafter.”
Variability on a decadal scale (i.e.
10 to 30 years) also affects the EPO. During the late 1970s there was a major shift in physical and biological states in the North Pacific Ocean. This climate shift was also detected in the tropical EPO by small increases in SSTs, weakening of the trade winds, and a moderate change in surface chlorophyll levels. Some researchers have reported another major shift in the North Pacific in 1989. Climate-induced variability in the ocean has often been described in terms of “regimes,” characterized by relatively stable means and patterns in the physical and biological variables. Analyses by the IATTC staff have indicated that yellowfin tuna in the EPO have experienced regimes of lower (1975-1982) and higher (1983-2001) recruitment, and possibly intermediate (2002-2012) recruitment. The increased recruitment during 1983-2001 is thought to be due to a shift to a higher productivity regime in the Pacific Ocean. Decadal fluctuations in upwelling and water transport are simultaneous to the higher-frequency ENSO pattern, and have basin-wide effects on the SSTs and thermocline slope that are similar to those caused by ENSO, but on longer time scales.
Recent peer-reviewed literature provides strong evidence that large-scale changes in biological production and habitat have resulted from physical forcing in the subtropical and tropical Pacific Ocean. These changes are thought to be capable of affecting prey communities. Primary production has declined over vast oceanic regions in the recent decade(s). A study published in 2008, using “Sea-viewing Wide Field-of-view Sensor” (SeaWiFS) remote-sensed ocean color data, showed that, in the North and South Pacific, the most oligotrophic surface waters have increased in area by 2.2 and 1.4 % per year, respectively, between 1998 and 2006. These statistically-significant increases in the oligotrophic gyres occurred concurrently with significant increases in mean SSTs. In the North Pacific, the direction of expansion was northeast, reaching well into the eastern Pacific to about 120°W and as far south as about 15°N. Net primary productivity also has declined in the tropical and subtropical oceans since 1999. The mechanism is recognized as increased upper-ocean temperature and vertical stratification, influencing the availability of nutrients for phytoplankton growth. Evidence is also strong that primary producers have changed in community composition and size structure in recent decades. Phytoplankton cell size is relevant to predation dynamics of tunas because food webs that have small picophytoplankton at their base require more trophic steps to reach predators of a given size than do food webs that begin with larger nanophytoplankton (e.g.
diatoms). Energy transfer efficiency is lower for picophytoplankton-based food webs than for nanophytoplankton-based food webs, i.e. for a given amount of primary production less energy will reach a yellowfin of a given size in the former than in the latter because mean annual trophic transfer efficiency at each step is relatively constant. A study published in 2012 used satellite remotely-sensed SSTs and chlorophyll-a concentrations to estimate the monthly size composition of phytoplankton communities during 1998-2007. With the seasonal component removed, the median phytoplankton cell size estimated for the subtropical 10°-30°N and 10°-30°S Pacific declined by 2.2% and 2.3%, respectively, over the 9-year period. Expansion of the oxygen minimum zone (OMZ) is a third factor that demonstrates ecosystem change on a scale capable of affecting prey communities. The OMZ is a thick low-oxygen layer at intermediate depths, which is largely suboxic (<~10 μmol kg-1
) in the tropical EPO. Time series of dissolved oxygen concentration at depth from 1960 to 2008 revealed a vertical expansion and intensification of the OMZ in the central and eastern tropical Pacific and Atlantic Oceans, and in other regions of the world’s oceans. Potential biological consequences of an expanding OMZ are numerous, but for the epipelagic tunas habitat compression can have profound implications. Shoaling of the OMZ restricts the depth distribution of tunas and other pelagic fishes into a narrower surface layer, compressing their foraging habitat and altering forage communities. Enhanced foraging opportunities for all epipelagic predators could alter trophic pathways and affect prey species composition. In addition, with a shoaled OMZ, mesopelagic vertically-migrating prey, such as the phosichthyid fish Vinciguerria lucetia
, myctophid fishes, and ommastrephid squids, would likely occur at shallower daytime depths and become more vulnerable to epipelagic predators. These are some of the taxa that increased most in the yellowfin diet in the tropical EPO between 1992-1994 and 2003-2005 (see Trophic interactions section).*Some of the information in this section is from Fiedler, P.C. 2002. Environmental change in the eastern tropical Pacific Ocean: review of ENSO and decadal variability. Mar. Ecol. Prog. Ser. 244: 265-283AGGREGATE INDICATORS
Recognition of the consequences of fishing for marine ecosystems has stimulated considerable research in recent years. Numerous objectives have been proposed to evaluate fishery impacts on ecosystems and to define over-fishing from an ecosystem perspective. Whereas reference points have been used primarily for single-species management of target species, applying performance measures and reference points to non-target species is believed to be a tractable first step. Current examples include incidental mortality limits for dolphins in the EPO purse-seine fishery under the AIDCP. Another area of interest is whether useful performance indicators based on ecosystem-level properties might be developed. Several ecosystem metrics or indicators, including community size structure, diversity indices, species richness and evenness, overlap indices, trophic spectra of catches, relative abundance of an indicator species or group, and numerous environmental indicators, have been proposed. Whereas there is general agreement that multiple system-level indicators should be used, there is concern over whether there is sufficient practical knowledge of the dynamics of such metrics and whether a theoretical basis for identifying precautionary or limit reference points based on ecosystem properties exists. Ecosystem-level metrics are not yet commonly used for managing fisheries.Ecological Metrics
. Relationships between indices of species associations in the catch and environmental characteristics are viewed as potentially valuable information for bycatch mitigation. Preliminary work in 2007-2008, based on novel methods of ordination developed by scientists at the Institute of Statistical Mathematics in Tokyo, Japan, showed clear large-scale spatial patterns in different groupings of target and bycatch species for floating-object sets in the EPO purse-seine fishery and relationships to environmental variables, such as SST, chlorophyll-a density, and mixed layer depth. More work is needed on this or similar approaches.
A variety of ecological metrics were employed in a study published in 2012**
to evaluate the ecological effects of purse-seine fishing in the EPO during 1993-2008. Comparisons of the catch of target and non-target (bycatch) species, both retained and discarded, by types of purse-seine set (on dolphins, floating objects, and unassociated tunas) were made on the basis of replacement time, diversity, biomass (weight), number of individuals, and trophic level. Previous comparisons considered only numbers of individuals and only discarded animals, without regard to body size, life-history characteristics, or position in the food web. During 1993-2008, the mean biomass removed was 17.0, 41.1 and 12.8 t/set for dolphin sets, floating-object sets, and unassociated sets, respectively. Of these amounts, bycatch was 0.3% for dolphin sets, 3.8% for floating-object sets, 1.4% for unassociated sets, and 2.1% for all methods combined. The discard rate was 0.7% for dolphin sets, 10.5% for floating-object sets, 2.2% for unassociated sets, and 5.4% for all methods combined. With the addition of 0.7% estimated for smaller vessels, the overall discard rate was 4.8%. This rate is low compared with global estimates of 7.5% for tuna longlines, 30.0% for tuna mid-water trawls, and 8.0% for all fisheries combined.Replacement time
is a measure of the length of time required for replacement of biomass removed by the fishery. Unsustainable levels of harvest may lead to greater decreases in probabilities of persistence of long-lived animals with low fecundity and late age of maturity than of fast-growing, highly fecund species. In contrast to trophic-level metrics, replacement-time metrics were sensitive to categories of animals with relatively high biomass to production-of-biomass (B/P) ratios, such as bigeye tunas, sharks, and cetaceans. Mean replacement time for total removals averaged over years was lowest for dolphin sets (mean 0.48 years), intermediate for unassociated sets (0.57 years), and highest for floating-object sets (0.74 years). There were no temporal trends in mean replacement time for landings, and mean replacement times for discards were more variable than those for landings. Mean replacement times for dolphin-set discards were approximately 7 times the mean replacement times for floating-object or unassociated-set discards because dolphins have a low reproductive rate.Diversity
. Fishing alters diversity by selectively removing target species. The relationship between diversity of species removed and effects on the diversity and stability of the ecosystem from which they were removed may be complex. Higher diversity of catch may be associated with fewer undesirable effects on the ecosystem, although the complexity of competitive and trophic interactions among species makes the relationship between diversity of catch and diversity and stability of the ecosystem difficult to determine. The Shannon diversity index for total removals was lowest for dolphin sets (mean 0.62), intermediate for unassociated sets (1.22), and highest for floating-object sets (1.38). The diversity of dolphin-set landings increased by 0.023/year, on average, from 0.45 to 0.79, due primarily to an increase of the percentage of skipjack tuna in the catch from <1% to >7% and a concurrent decrease in the percentage of yellowfin tuna. The diversity of unassociated-set landings and discards both decreased, and diversity of total removals decreased by a mean of 0.024/year, from 1.40 to 1.04.Biomass.
The relative amounts and characteristics of the biomass removed by each of the fishing methods varied as a function of how removal was measured. Landings from floating-object sets were greatest by all four measures of removal, but were particularly high when removal was measured on the basis of number of individuals or replacement time. The amount and composition of discards varied among the fishing methods. Discards of the target tuna species were the greatest proportion of removed animals whether measured in biomass, number of individuals, or trophic-level units. Discards of cetaceans in dolphin sets and sharks in floating-object and unassociated sets were greater when measured in replacement-time units than when measured in other units because of the low reproductive rates of these animals.Trophic structure and trophic levels of catches
. Ecologically-based approaches to fisheries management place renewed emphasis on achieving accurate depictions of trophic links and biomass flows through the food web in exploited systems. The structure of the food web and the interactions among its components have a demonstrable role in determining the dynamics and productivity of ecosystems. Trophic levels (TLs) are used in food-web ecology to characterize the functional role of organisms, to facilitate estimates of energy or mass flow through communities, and for elucidating trophodynamics aspects of ecosystem functioning. A simplified food-web diagram, with approximate TLs, of the pelagic tropical EPO, is shown in Figure L-1.
|Figure L-1: Simplified food-web diagram of the pelagic ecosystem in the tropical EPO. The numbers inside the boxes indicate the approximate trophic level of each group. |
Toothed whales (Odontoceti, average TL 5.2), large squid predators (large bigeye tuna and swordfish, average TL 5.2), and sharks (average TL 5.0) are top-level predators. Other tunas, large piscivores, dolphins (average TL 4.8), and seabirds (average TL 4.5) occupy slightly lower TLs. Smaller epipelagic fishes (e.g
spp. and flyingfishes, average TL 3.2), cephalopods (average TL 4.4), and mesopelagic fishes (average TL 3.4) are the principal forage of many of the upper-level predators in the ecosystem. Small fishes and crustaceans prey on two zooplankton groups, and the herbivorous micro-zooplankton (TL 2) feed on the producers, phytoplankton and bacteria (TL 1).
In exploited pelagic ecosystems, fisheries that target large piscivorous fishes act as the system’s apex predators. Over time, fishing can cause the overall size composition of the catch to decrease, and, in general, the TLs of smaller organisms are lower than those of larger organisms. The mean TL of the organisms taken by a fishery is a useful metric of ecosystem change and sustainability because it integrates an array of biological information about the components of the system. There has been increasing attention to analyzing the mean TL of fisheries catches since a study demonstrated that, according to FAO landings statistics, the mean TL of the fishes and invertebrates landed globally had declined between 1950 and 1994, which was hypothesized by the authors of that study to be detrimental to the ecosystems. Some ecosystems, however, have changed in the other direction, from lower to higher TL communities. Given the potential utility of this approach, mean TLs were estimated for a time series of annual catches and discards by species from 1993 to 2013 for three purse-seine fishing modes and the pole-and-line fishery in the EPO. The estimates were made by applying the TL values from the EPO ecosystem model (see Ecosystem Modeling Section), weighted by the catch data by fishery and year for all model groups from the IATTC tuna, bycatch, and discard data bases. The TLs from the ecosystem model were based on diet data for all species groups and mass balance among groups. The weighted mean TLs of the summed catches of all purse-seine and pole-and-line fisheries were similar and fairly constant from year to year (Figure L-2: Average PS+LP).
|Figure L-2: Yearly mean trophic level estimates of the catches (retained and discarded) by the purse-seine and pole-and-line fisheries in the tropical EPO, 1993-2013. |
A slight downward trend for the unassociated sets, amounting to 0.05 TL over the 20-year period, resulted from increasing proportions of skipjack and decreasing proportions of yellowfin tuna in the catch, not from increasing catches of low trophic-level species. It is not, therefore, considered an ecologically-detrimental decline. In general, the TLs of the unassociated sets and the pole-and-line fishery were below average and those of the dolphin sets were above average for most years (Figure L-2). The TLs of the floating-object sets varied more than those of the other set types and fisheries, primarily due to the inter-annual variability in the amounts of bigeye and skipjack caught in those sets. The TLs of floating-object sets were positively related to the percentage of the total catch comprised of large bigeye and negatively related to the percentage of the catch comprised of skipjack.
Mean TLs were also estimated separately for the time series of retained and discarded catches of the purse-seine fishery each year from 1993 to 2013 (Figure L-3).
|Figure L-3: Trophic level estimates of the retained catches and discarded catches by purse-seine fisheries in the tropical EPO, 1993-2013. |
The discarded catches were much less than the retained catches, and thus the TL patterns of the total (retained plus discarded) catches (Figure L-2) were determined primarily by the TLs of the retained catches (Figure L-3). The TLs of the discarded catches varied more year-to-year than those of the retained catches, due to the species diversity of the incidental catches. The considerable reduction in the mean TLs of the dolphin-set discards over the 20-year period (Figure L-3), was largely due to an increase in the proportions of discarded prey fishes (bullet and frigate tunas (Auxis
spp.) and miscellaneous epipelagic fishes) and rays (Rajiformes, mostly manta rays, Mobulidae) with lower trophic levels. For unassociated sets, marked inter-annual reductions in TL were due to increased bycatches of rays (TL 3.68), which feed on plankton and other small animals that occupy low TLs, a reduction in the catches of large sharks (TL 4.93-5.23), and an increase in prey fishes such as Auxis
spp. (TL 3.86) in the bycatch. For floating-object sets, the discards of bigeye were related to higher mean TLs of the discarded catches.**Gerrodette, T., R. Olson, S. Reilly, G. Watters, and W. Perrin. 2012. Ecological metrics of biomass removed by three methods of purse-seine fishing for tunas in the eastern tropical Pacific Ocean. Conservation Biology. 26 (2): 248-256ECOLOGICAL RISK ASSESSMENT
Long-term ecological sustainability is a requirement of ecosystem-based fisheries management. Fishing directly impacts the populations of not only target species, but also the species incidentally caught as bycatch. The vulnerability to overfishing of many of the stocks incidentally caught in the EPO tuna fisheries is unknown, and biological and fisheries data are severely limited for most of those stocks. Many fisheries managers and scientists are turning to risk assessments to evaluate vulnerability to fishing. Vulnerability is defined here as the potential for the productivity of a stock to be diminished by direct and indirect fishing pressure. The IATTC staff has applied a version of productivity and susceptibility analysis (PSA***
), used to evaluate fisheries in other ocean regions in recent years, to estimate the vulnerability of data-poor, non-target species caught by the purse-seine fishery in the EPO. PSA considers a stock’s vulnerability as a combination of its productivity and its susceptibility to the fishery. Stock productivity is the capacity of a stock to recover if it is depleted, and is a function of the species’ life history traits. Stock susceptibility is the degree to which a fishery can negatively impact a stock, i.e. the propensity of a species to be captured by, and incur mortality from, a fishery. Productivity and susceptibility indices of a stock are determined by deriving a score ranging from 1 (low) to 3 (high) for a standardized set of attributes related to each index. The individual attribute scores are then averaged for each factor and graphically displayed on an x-y scatter plot. The scale of the x-axis on the scatter plot is reversed because species/stocks with a high productivity score and a low susceptibility score (i.e
. at the origin of the plots) are considered to be the least vulnerable. When scoring the attributes, the data quality associated with each attribute score is assessed, and the attributes are weighted by the data-quality score. Stocks that receive a low productivity score (p
) and high susceptibility score (s
) are considered to be at a high risk of becoming depleted, while stocks with a high productivity score and low susceptibility score are considered to be at low risk. Vulnerability scores (v
) are calculated from the p
scores as the Euclidean distance from the origin of the x-y scatter plot and the datum point:
To examine the utility of productivity and susceptibility indices for assessing the vulnerability of incidentally-caught fishes, mammals, and turtles to overfishing in the EPO, a preliminary evaluation of three purse-seine “fisheries” in the EPO was made in 2010, using 26 species that comprise the majority of the biomass removed by Class-6 purse-seine vessels (carrying capacity greater than 363 metric tons) during 2005-2009. Nine productivity and eight susceptibility attributes, based on established PSA methodology***
, were used in the preliminary PSA, and some were modified for greater consistency with data from the tuna fisheries in the EPO. Information corresponding to the productivity attributes for each species was compiled from a variety of published and unpublished sources and EPO fisheries data (i.e.
not adopted from previous PSAs) to better approximate the distribution of life history characteristics observed in the species found in the EPO. Scoring thresholds for productivity attributes were derived by dividing the compiled data into equal thirds. Scoring criteria for the susceptibility attributes were taken from the example PSA***
and modified where appropriate to better fit the EPO fisheries. However, problems arose when trying to compare susceptibility estimates for species across the different fisheries (Fishery Status Report 8
). In 2012, the PSA was revised to include seven additional species, based on data from 2005-2011 (Fishery Status Report 10
The staff of the Biology and Ecosystem Program had planned to finalize and publish the PSA analysis during 2014, but the retirement of one staff member and budget constraints have prevented the work from being finished. However, three modifications of the analysis have been made since it was reviewed at the SAC meeting in May 2014: 1) the procedures for determining which species to include in the analysis were modified; 2) the susceptibility values for each fishery were combined to produce one overall susceptibility value for each species; and 3) the use of bycatch and catch information in the formulation of s
was modified. The list of productivity attributes remains unchanged (Table L-1
) while the list of susceptibility attributes has been revised due to this 3rd
modification (Table L-2
) These three modifications are described briefly below. For the remainder of this section, the term “catch” will be used to refer to bycatch for non-tuna species and catch for tuna species.
The first modification was to establish a two-step procedure to identify and exclude rare species, based on the biomass caught per fishery. However, as a precautionary measure, rare species classified as “vulnerable,” “endangered,” or “near threatened” on the IUCN Red List were retained, or are now included, in the analysis. Currently, the PSA includes 32 species (Table L-3a
) an additional eight sensitive species, two rays and six sharks, will be included in the future.
The second modification was to combine the susceptibility values for each species across fisheries to produce one overall species-specific purse-seine susceptibility. A preliminary combined susceptibility score for a species, sj
¹, was calculated as the weighted sum of the individual fishery susceptibility values for that species (Table L-3), with weights equal to the proportion of sets in each fishery:
¹ is the combined susceptibility for species j sjk
is the susceptibility for species j
in set type k,
computed using only the attributes in Table L-2, sjk
ranges from 1 (lowest) to 3 (highest). For a species with catches < 5% in set type k
≡ 1, unless a sjk
was computed for one of the previous PSAs (Fishery Status Reports 8
), in which case this sjk
was used; otherwise it was assumed that if catches were less than 5% in a fishery, the species was only minimally susceptible to that fishery. A previous PSA (Fishery Status Report 10
) used catch trend information as an additional attribute to calculate the sjk
, however, the catch trend information was removed from the sjk
here because, following the established PSA***
methodology, the other susceptibility attributes are time-invariant (but see below).
is the total number of sets (class-6) of set type k
¹ takes into account fishing effort by set type, even for set types with little or no catch of a species. A preliminary PSA plot using sj
¹ is shown in Figure L-4a,
|Figure L-4a: Productivity and susceptibility x-y plot for target and bycatch species caught by the purse-seine fishery of the EPO during 2005-2013, based on s(j)¹. The pie charts show the proportion of bycatch (non-tuna species) or proportion of catch (tuna species), by set type, for those set types with bycatch or catch ≥ 5% for the species. The 3-alpha species codes next to each pie chart are defined in Table L-3a. |
and the values of sjk, sj¹ and v1 are shown in Table L-3a. A concern with regard to sj¹ for some species is that the variation in the sjk computed from the attributes in Table L-2 does not correlate well with differences observed among catch rates by set type, suggesting the attributes in Table L-2 do not capture the full susceptibility of species j; in general it is assumed that higher catch rates should reflect higher overall susceptibility. In addition, the sjk do not account for long-term trends.
The third modification, the use of catch information in the formulation of s
, was made to try to account for differences in observed catch rates among set types, by species, and to account for long-term trends in abundance. Two preliminary alternate susceptibility formulations were computed as “proof of concept” for these ideas. The first, sj²
, modifies sj
to take into consideration current catch rates, which are assumed to be an alternate proxy for susceptibility and to reflect the actual integrated effects of the susceptibility attributes in Table L-2:
sj² is the combined susceptibility for species j, adjusted for recent catch rates
s*jk is the average of sjk and of the catch rate susceptibility:
sjk is as defined for sj¹
Scps_jk is the catch rate susceptibility and takes a value of 1, 2 or 3, assigned as follows. If the species is not a target tuna species, catch-per set, in number of animals per set, is used to assign a value to Scps_jk
If the species is a target tuna species, then the following values are assigned to Scps_jk :
is the catch-per-set for species j
in set type k
(= class-6 catch (in numbers of animals) divided by number of class-6 sets), for the most recent year (2013). Catch-per-set was used instead of total catch in order to control for differences in effort among set types.pk
is as defined for sj1
A preliminary PSA plot using sj2
is shown in Figure L-4b
|Figure L-4b: Productivity and susceptibility x-y plot for target and bycatch species caught by the purse-seine fishery of the EPO during 2005-2013, based on s(j)². The pie charts show the proportion of bycatch (non-tuna species) or proportion of catch (tuna species), by set type, for those set types with bycatch or catch ≥ 5% for the species. The 3-alpha species codes next to each pie chart are defined in Table L-3b. |
and the values of s*jk, sj2 and v2 are shown in (Table L-3b
). sj2 could be affected by differences in abundance among species because catch-per-set is affected by abundance. Ranking cpsjk may help to minimize this problem. The present rules for ranking cpsjk for non-target tuna species were based on the idea that no catch equates to minimal susceptibility, catch that increases at a rate of less than one animal per set equates to moderate susceptibility, and catch that increases at an effort rate of one or more animals per set equates to high susceptibility. However, these rules are a “proof of concept” and could be modified.
The second alternate susceptibility formulation, computed for species other than target tunas and dolphins, sj³, adjusts for long-term trends:
sj3 is the combined susceptibility for species j, adjusted for long-term trends
S**jk is the average of sjk and the trend susceptibility:
sjk is as defined for sj1
Strend_jk is the trend susceptibility for species j in set type k, obtained as follows: trendjk
is the slope of the regression of cpsjk,y
and year y, from the start of the data collection (which may vary by species). trendjk
was computed for species for which full assessments (or management indicators) do not exist and for which the fishery data have not been determined to be unsuitable for trend estimation; i
., for species other than the three target tuna species and the dolphin species (but see below). A significant trend was any slope with a p
-value < 0.05. cps,jk,y
is the catch-per-set of species j
of set type k
in year y
A preliminary PSA plot using sj3
for species other than the three target tuna species and dolphin species is shown in Figure L-4c,
|Figure L-4c: Productivity and susceptibility x-y plot for bycatch species caught by the purse-seine fishery of the EPO during 2005-2013, based on s(j)3. s(j)3 was not computed for species for which full assessments (or management indicators) exist or for which the fishery data have been determined to be unsuitable for trend estimation; i.e., for the three target tuna species and the dolphin species. The pie charts show the proportion of bycatch (non-tuna species), by set type, for those set types with bycatch ≥ 5% for the species. The 3-alpha species codes next to each pie chart are defined in Table L-3c. |
and the values of S*jk
, and v3
are shown in (Table L-3c
). For the future, s3j
could be expanded to include the three target tuna species by estimating trends from spawning biomass, and could be expanded to dolphin species by using trends estimated from historical line-transect abundance estimates. A concern with regards to s3j
is that trends estimated from catch-per-set may not reliably track changes in abundance (as was shown for dolphins in Document SAC-05-11d
The three susceptibility measures, s1j,
, and s3j
, are considered preliminary and represent “proof of concept” ideas to illustrate several options for computing susceptibility tailored to the EPO purse-seine fishery. The IATTC staff will continue working to improve and refine the productivity and susceptibility analysis during 2015. Future work will focus on evaluation of which of the three susceptibility measures is preferable, and whether further modifications should be made. In addition, a full literature review is in progress to determine if susceptibility attributes in Table L-2 and corresponding scores and productivity scores should be updated as a result of new research.***Patrick, W.S., P. Spencer, J. Link, J. Cope, J. Field, D. Kobayashi, P. Lawson, T. Gedamke, E. Cortés, O. Ormseth, K. Bigelow, and W. Overholtz. 2010. Using productivity and susceptibility indices to assess the vulnerability of United States fish stocks to overfishing. Fish. Bull. U.S. 108: 305-322.ECOSYSTEM MODELING
It is clear that the different components of an ecosystem interact. Ecosystem-based fisheries management is facilitated through the development of multi-species ecosystem models that represent ecological interactions among species or guilds. Our understanding of the complex maze of connections in open-ocean ecosystems is at an early stage, and, consequently, the current ecosystem models are most useful as descriptive devices for exploring the effects of a mix of hypotheses and established connections among the ecosystem components. Ecosystem models must be compromises between simplistic representations on the one hand and unmanageable complexity on the other.
The IATTC staff has developed a model of the pelagic ecosystem in the tropical EPO (IATTC Bulletin, Vol. 22, No. 3
) to explore how fishing and climate variation might affect the animals at middle and upper trophic levels. The ecosystem model has 38 components, including the principal exploited species (e.g.
tunas), functional groups (e.g
. sharks and flyingfishes), and sensitive species (e.g
. sea turtles). Some taxa are further separated into size categories (e.g
. large and small marlins). The model has finer taxonomic resolution at the upper trophic levels, but most of the system’s biomass is contained in the middle and lower trophic levels. Fisheries landings and discards were estimated for five fishing “gears”: pole-and-line, longline, and purse-seine sets on tunas associated with dolphins, with floating objects, and in unassociated schools. The model focuses on the pelagic regions; localized, coastal ecosystems are not adequately described by the model.
Most of the information describing inter-specific interactions in the model came from a joint IATTC-NMFS project, which included studies of the food habits of co-occurring yellowfin, skipjack, and bigeye tuna, dolphins, pelagic sharks, billfishes, dorado, wahoo, rainbow runner, and others. The impetus of the project was to contribute to the understanding of the tuna-dolphin association, and a community-level sampling design was adopted.
The ecosystem model has been used to evaluate the possible effects of variability in bottom-up forcing by the environment on the middle and upper trophic levels of the pelagic ecosystem. Predetermined time series of producer biomasses were put into the model as proxies for changes in primary production that have been documented during El Niño and La Niña events, and the dynamics of the remaining components of the ecosystem were simulated. The model was also used to evaluate the relative contributions of fishing and the environment in shaping ecosystem structure in the tropical pelagic EPO. This was done by using the model to predict which components of the ecosystem might be susceptible to top-down effects of fishing, given the apparent importance of environmental variability in structuring the ecosystem. In general, animals with relatively low turnover rates were influenced more by fishing than by the environment, and animals with relatively high turnover rates more by the environment than by fishing.
The structure of marine ecosystems is generally thought to be controlled by one of two mechanisms: ‘bottom-up’ control (resource-driven) where the dynamics of primary producers (e.g.
phytoplankton) controls the production and biomass at higher trophic levels, or ‘top-down’ control (consumer-driven) where predation by high trophic-level predators controls the abundance and composition of prey at lower trophic levels. In relatively recent years, ‘wasp-waist’ control of marine ecosystems has also been recognized. ‘Wasp-waist’ control is a combination of bottom-up and top-down forcing by a small number of abundant, highly productive, and short-lived species at intermediate trophic levels (e.g.
sardines and anchovies) that form a narrow ‘waist’ through which energy flow in the system is regulated. These species exert top-down predatory control of energy flows from zooplankton, but also have bottom-up control by providing energy for high trophic-level predators. It has been assumed that wasp-waist control occurs primarily in highly productive and species-poor coastal systems (e.g
. upwelling regions), which can be highly unstable and undergo rapid natural regime shifts in short periods of time. The ecosystem model for the tropical EPO was used in conjunction with a model for a region off the east coast of Australia where tunas and billfishes are caught to examine possible forcing dynamics of these systems. These two large species-rich pelagic ecosystems also showed wasp-waist-like structure, in that short-lived and fast-growing cephalopods and fishes in intermediate trophic levels comprise the vast majority of the biomass. The largest forcing effects were seen when altering the biomasses of mid trophic-level epipelagic and mesopelagic fishes in the models, whereby dramatic trophic cascades occurred both upward and downward in the system. These tropical pelagic ecosystems appear to possess a complex structure whereby several waist groups and alternate trophic pathways from primary producers to apex predators can cause unpredictable effects when the biomasses of particular functional groups are altered. Such models highlight the possible structuring mechanisms in pelagic systems, which have implications for fisheries that exploit these groups, such as squid fisheries, as well as for fisheries of top predators such as tunas and billfishes that prey upon wasp-waist species.