Habitat and Biology
Bottom type: Unspecified. Depth zone: Abyssal ( >1000m). Horizontal distribution: Oceanic. Vertical distribution: Pelagic.
Jurisdictional distribution: Highly migratory
Considered a single stock: No
Tagging studies have shown that there is exchange of Pacific bluefin between the eastern and western Pacific Ocean. Larval, postlarval, 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).
The catches of Pacific bluefin in the entire Pacific Ocean, by flag and gear, are shown in (Table A-5
). 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.
The catches of Pacific bluefin in the EPO during 1982-2011, by gear, are shown in Table A-2a. During 1996-2010 the annual retained catch of bluefin from the EPO by purse-seine and pole-and-line vessels averaged 4.6 thousand t (range 1.2 to 9.9 thousand t). The preliminary estimate of the retained catch of bluefin in 2011, 2.7 thousand t, is 1.9 thousand t less than the average for 1996-2010. Small amounts of bluefin are discarded at sea by purse-seine vessels (Table A-2a
Most of the 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 during every month of the year, but most of the fish are taken during May through October.
Bluefin are exploited by various gears in the WPO from Taiwan to Hokkaido. Age-0 fish about 15 to 30 cm in length are caught by trolling 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 by trolling 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. 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. The migrants, after crossing the ocean, 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 the portions of the juvenile fish there that migrate to the EPO, and also 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. |
Overall Assessment Results
An index of abundance for the predominantly young bluefin in the EPO has been calculated, based on standardization of catch per vessel day using a generalized linear model, and including the variables latitude, longitude, SST, SST2
, month, and vessel identification number. The index is highly variable, but shows a peak in the early 1960s, very low levels for a period in the early 1980s, and some increase since that time.
A full 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 2008. The assessment results were highly sensitive to the assumptions made about biological parameters, particularly natural mortality. Regardless of these uncertainties, the following trends were robust to different assumptions on natural mortality:
- Recruitment has fluctuated without trend over the assessment period (1952-2006), and does not appear to have been adversely affected by fishery exploitation;
- Recent (2000-2006) levels of spawning biomass (mature females) are above the median historic level;
- The bluefin catch (in weight and numbers) is dominated by recruits (0 years) and juveniles (1-3 years). Fishing mortality (F) on recruits has gradually increased and remained above median historic exploitation levels for more than a decade (since the early 1990s). Fishing mortality on 1-2 year old fish has also increased since the early 1990s, but these levels have fluctuated around median historic levels.
The Pacific Bluefin Working Group of the ISC has subsequently conducted workshops in 2009 and 2010, mainly to deal with data updates and modelling improvements.. A full stock assessment meeting is scheduled for May-June 2012.
The total catches of bluefin have fluctuated considerably during the last 50 years (Figure E-1).
|Figure E-1: Retained catches of Pacific bluefin tuna. |
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.
Biological State and Trend
Exploitation state: Fully exploitedExploitation rate: Moderate fishing mortality
Abundance level: Intermediate abundance