Biological State and Trend
Habitat and Biology
Bottom type: Unspecified. Depth zone: Abyssal ( >1000m). Horizontal distribution: Oceanic. Vertical distribution: Pelagic.
Geographical Distribution
Jurisdictional distribution: Highly migratory
Resource Structure
Considered a single stock: No
Tagging studies have shown that there is exchange of Pacific bluefin between the eastern and western Pacific Ocean. Larval, post larval, and early juvenile bluefin have been caught in the western Pacific Ocean (WPO), but not in the eastern Pacific Ocean (EPO), so it is likely that there is a single stock of bluefin in the Pacific Ocean (or possibly two stocks in the Pacific Ocean, one spawning in the vicinity of Taiwan and the Philippines and the other spawning in the Sea of Japan).
Exploitation
The catches of Pacific bluefin in the EPO during 1990-2019, by gear, are shown in Table A-2a. In recent years, purse-seine vessels accounted for over 90% of the annual catch.
The 1990-2018 average EPO retained catch is 3.6 thousand t (range: 400 t to 9.9 thousand t); the preliminary estimate for 2019 is 2.5 thousand t (
Table A-2a).
The catches of Pacific bluefin in the entire Pacific Ocean, by flag and gear, as reported by the vessels’ flag governments to the International Scientific Committee for Tuna and Tuna-like Species in the North Pacific Ocean (ISC), are shown in (
).
Catches of Pacific bluefin by recreational gear in the EPO are reported in numbers of individual fish caught, whereas all other gears report catches in weight; they are therefore converted to tons for inclusion in the EPO catch totals. The original catch data for 1990-2019, in numbers of fish, are presented in (
Table A-5a).
Most of the commercial catches of bluefin in the EPO are taken by purse seiners. Nearly all of the purse-seine catches have been made west of Baja California and California, within about 100 nautical miles of the coast, between about 23°N and 35°N. Ninety percent of the catch is estimated to have been between about 60 and 100 cm in length, representing mostly fish 1 to 3 years of age. Aquaculture facilities for bluefin were established in Mexico in 1999, and some Mexican purse seiners began to direct their effort toward bluefin during that year. During recent years, most of the catches have been transported to holding pens, where the fish are held for fattening and later sale to sashimi markets. Lesser amounts of bluefin are caught by recreational, gillnet, and longline gear. Bluefin have been caught in the EPO during every month of the year, but most of the fish are taken from May through October.
Bluefin are exploited by various gears in the WPO from Taiwan to Hokkaido, Japan. Age-0 fish, about 15 to 30 cm in length, are caught by the Japanese troll fishery during July-October south of Shikoku Island and south of Shizuoka Prefecture. During November-April, age-0 fish about 35 to 60 cm in length are taken in troll fisheries south and west of Kyushu Island. Age-1 and older fish are caught by purse seining, mostly during May-September, between about 30°-42°N and 140°-152°E. Bluefin of various sizes are also caught by traps, gillnets, and other gear, especially in the Sea of Japan. Additionally, small amounts of bluefin are caught near the southeastern coast of Japan by longlining. The Chinese Taipei small-scale longline fishery, which has expanded since 1996, takes bluefin tuna more than 180 cm in length from late April to June, when they are aggregated for spawning in the waters east of the northern Philippines and Taiwan.
The high-seas longline fisheries are directed mainly at tropical tunas, albacore, and billfishes, but small amounts of Pacific bluefin are caught by these fisheries. Small amounts of bluefin are also caught by Japanese pole-and-line vessels on the high seas.
Tagging studies, conducted with conventional and archival tags, have revealed a great deal of information about the life history of bluefin. Some fish apparently remain their entire lives in the WPO, while others migrate to the EPO. These migrations begin mostly during the first and second years of life. The first- and second-year migrants are exposed to various fisheries before beginning their journey to the EPO. Then, after crossing the ocean, they are exposed to commercial and recreational fisheries off California and Baja California. Eventually, the survivors return to the WPO.
Bluefin more than about 50 cm in length are most often found in waters where the sea-surface temperatures (SSTs) are between 17° and 23°C. Fish 15 to 31 cm in length are found in the WPO in waters where the SSTs are between 24° and 29°C. The survival of larval and early juvenile bluefin is undoubtedly strongly influenced by the environment. Conditions in the WPO probably influence recruitment, and thus the portions of the juvenile fish there that migrate to the EPO, as well as the timing of these migrations. Likewise, conditions in the EPO probably influence the timing of the return of the juvenile fish to the WPO.
See also fishery fact sheet:
EPO Tunas and billfishes fishery |
Retained catches of Pacific bluefin tuna, by gear, 1952-2018. GN: gillnet; LL: longline; LX: hook and line; OTR: other; PS: purse seine.  |
Assessment
Overall Assessment Results
The total catches of bluefin have fluctuated considerably during the last 50 years (Figure E-1).
 |
| Retained catches of Pacific bluefin tuna, by gear, 1952-2018. GN: gillnet; LL: longline; LX: hook and line; OTR: other; PS: purse seine. |
The consecutive years of above-average catches (mid-1950s to mid-1960s) and below-average catches (early 1980s to early 1990s) could be due to consecutive years of above-average and below-average recruitments. The estimated impact of the fisheries on the bluefin population for the entire time period modeled (1952-2016) is substantial. The WPO fisheries have had a greater impact than the EPO fisheries, and their impact increased starting in 1980s only leveling off in 2000s (Figure E-2).
 |
| Figure E-2: Estimates of the impact on the Pacific bluefin tuna population of fisheries in the EPO and in the WPO (upper panel). The dashed line represents the estimated hypothetical unfished spawning biomass, and the solid line the estimated actual spawning biomass. The shaded areas indicate the impact attributed to each fishery. The lower panel presents the proportion of impact attributed to the EPO and WPO. (Figure from the draft Executive Summary of ISC 2020 stock assessment; subject to change and approval by the ISC Plenary.) |
An update stock assessment was carried out by the Pacific Bluefin Working Group of the International Scientific Committee for Tuna and Tuna-like Species in the North Pacific Ocean (ISC) in 2020. The assessment was conducted with Stock Synthesis 3, an integrated statistical age-structured stock assessment model. The base-case model results show that: (1) spawning stock biomass (SSB) fluctuated throughout the assessment period (fishing years 1952-2018); (2) the SSB steadily declined from 1996 to 2010; (3) the slow increase of the stock biomass continues since 2011; (4) total biomass in 2018 exceeded the historical median with an increase in immature fish; and (5) fishing mortality (F%SPR) declined from a level producing about 1% of SPR in 2004-2009 to a level producing 14% of SPR in 2016-2018. Historical recruitment estimates have fluctuated since 1952 without an apparent trend. Relatively low recruitment levels estimated in 2010-2014 were of concern in the 2016 assessment. The 2015 recruitment estimate is lower than the historical average while the 2016 recruitment estimate is higher than the historical average. The recruitment estimates for 2017 and 2018, which are based on fewer observations and more uncertain, are below the historical average. A substantial decrease in estimated F is observed in ages 0-2 in 2016-2018 relative to the previous years. Note that stricter management measures in WCPFC and IATTC have been in place since 2015.
The point estimate of the 2018 SSB was 4.5% of the SSB in the absence of fishing (4.5%SSBF=0), and the recent (2016-2018) fishing mortality (F) corresponds to F14%SPR. Because the harvest strategy contains catch limits, fishing mortality is expected to decline, i.e., Fx%SPR will increase as biomass increases. No biomass-based limit or target reference points have been adopted to evaluate whether Pacific bluefin is overfished. However, the stock is overfished relative to common target reference points and to the IATTC limit reference point used for tropical tunas. Also, no fishing intensity-based limit or target reference points have been adopted to evaluate whether overfishing of Pacific bluefin is occurring, but the stock is subject to overfishing relative to most common fishing intensity-based reference points.
Resolution
C-16-08 states that the Commission recognizes that the management objective of the IATTC is to maintain or restore fish stocks at levels capable of producing MSY, and shall implement a provisional rebuilding plan in part by adopting an initial (first) rebuilding target of SSBmed, 1952-2014 (the median point estimate for 1952-2014) to be achieved by 2024 with at least 60% probability. The IATTC has adopted resolutions to restrict the catch of bluefin tuna in the EPO. Resolution
C-16-08 limits the commercial catches in the IATTC Convention Area by all CPCs to a combined total of 6,600 metric tons during 2017-2018, respectively. No CPC shall exceed 3,500 metric tons in 2017. In the event that the total actual catch in 2017 is either above or below 3,300 metric tons, the catch limit for 2018 shall be adjusted accordingly to ensure that the total catch for both years does not exceed 6,600 metric tons. Resolution
C-16-08 requires that in 2018, and taking into account the outcomes of the 2nd IATTC-WCPFC NC Joint Working Group Meeting, the Commission shall adopt a second rebuilding target, to be achieved by 2030. Resolution
C-16-08 also requires that no later than the IATTC meeting in 2018, taking into account the outcomes of the Joint IATTC-WCPFC NC Working Group, the Commission shall consider and develop reference points and harvest control rules for the long-term management of Pacific bluefin tuna, which should be comparable to those adopted by the WCPFC.
The harvest strategy proposed at the Joint WCPFC NC-IATTC WG meeting guided projections conducted by the ISC to provide catch reduction options if the projection results show that the initial rebuilding target will not be achieved with at least 60% probability by 2024 or to provide relevant information for a potential increase in catch if the probability of achieving the initial rebuilding target exceeds 75% by 2024. The projection based on the base-case model mimicking the current management measures by the WCPFC (CMM 2017-08) and IATTC (C-16-08) under the low recruitment scenario resulted in an estimated 100% probability of achieving the initial rebuilding target by 2024. This estimated probability is above the threshold (75% or above in 2024) prescribed by the harvest strategy. The low recruitment scenario is more precautionary than the recent 10 years recruitment scenario. In the harvest strategy, the recruitment scenario is switched from the low recruitment to the average recruitment scenario beginning in the year after achieving the initial rebuilding target. The estimated probability of achieving the second rebuilding target 10 years after the achievement of the initial rebuilding target or by 2034, whichever is earlier, is 99%. This estimate is above the threshold (60% or above in 2034) prescribed by the harvest strategy.
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