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Pacific Bluefin Tuna Working Group
The Pacific Bluefin Tuna Working Group (PBFWG) was established since the foundation of ISC. First formal stock assessment was conducted in May 2008. The working group is focusing the preparation of the most recent full stock assessment was completed in 2012. In late 2013 by the collaboration of Albacore WG, PBF WG scheduled to hold a workshop for age and growth of tunas in the north Pacific, targetting north Pacifc albacore and Pacific bluefin tuna.
Pacific Bluefin Tuna (Thunnus orientalis) --- North Pacific
◊ Biological Profile
This species is widely distributed in the North Pacific Ocean and occured in the Southern Hemisphere with limited amounts. The juveniles have trans-Pacific migration analyzed by archival tags. Migration after spawning to the Southern Hemisphere has been also reported by pop-up tags.
◊ Fisheries for Pacific Bluefin Tuna in the North Pacific Ocean
◊ Stock Status (From ISC13)
Overfishing is occurring and the stock is heavily overfished. Current spawning stock biomass is near their lowest level (3.6%) and has been declining for over a decade. Although no target or limit reference points have been established for PBF, the current F (average 2007-2009) is above all reasonable target and limit biological reference points (BRPs). Since reference points have not been identified for PBF by the WCPFC-NC, two Kobe plots based on alternative choices for reference points are provided. Caveats accompany the interpretation of these plots, which is delineated in the full stock assessment report.
Citation of these plots without appropriate interpretation may be misleading since our understanding of PBF population dynamics is incomplete. Plot A has a stock trajectory that is consistent with expectations that the stock was in a healthy condition early in the time series. However, plot B indicates overexploitation throughout the assessment (1952-2010) period. There is no plausible explanation why there was overexploitation at the beginning of the time-series (1952). If PBF productivity was strongly environmentally driven, the state of the stock may be hard to represent via a Kobe plot.
Fishery impact analysis (1970-2010) suggests that historically, the Japan coastal fishery group has had the greatest impact (i.e., expected spawning stock biomass) on the PBF stock; since about 1999 the impact of the WPO purse seine fleet has increased, and the effect of this fleet is currently greater than any of the other fishery groups. The impact of EPO fisheries was large before the mid-1980s, decreasing after the 1990s. The WPO longline fleet has had a limited effect on the stock throughout the analysis period.
Based on 2012 data, (1) reported catches of juveniles and adults in WPO fisheries declined, (2) decreased catch in the troll fishery were observed and may suggest declining recruitment, and (3) Japanese longline CPUE continued to decline. Potential risk to the spawning stock may be higher than previously thought. Under low recruitment levels, the risk of SSB falling below the historically lowest SSB level will increase under current (F2007-2009) harvesting levels.
Alternative Kobe plots for Pacific bluefin tuna (Thunnus orientalis).
A. SSBMED and FMED; B. SSB20% and SPR20%.
◊ Conservation Advice (From ISC13)
The current (2010) PBF biomass level is near historically low levels and experiencing high exploitation rates above all biological reference points (BRPs) commonly used by fisheries managers. Based on projection results, extending the status quo (2007-2009) fishing levels is unlikely to improve stock status. Continued monitoring of abundance indices is recommended to track SSB.
Preliminary WPO data indicate an unusually low catch of age-0 PBF in 2012; this may imply low recruitment, which would adversely affect projected stock rebuilding and increase the risk of SSB falling below its historical lowest level observed. Further reduction of fishing mortality, especially for juvenile fish, is needed to reduce the risk of SSB falling below its historically lowest level.
Strengthening the monitoring of recruitment is highly recommended to comprehend the trend of recruitment in a timely manner.
Latest Stock Assessment
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Working Group Reports and Working Papers
⇒ Shizuoka, Japan (November, 2000)
Working Group Chair