Stock Status and Conservation Information
(From ISC23 Plenary Report)
Pacific Bluefin Tuna
Click here to see the ISC22 stock status and conservation information.
PBF spawning stock biomass (SSB) has gradually increased in the last 10 years, and the rate of increase is accelerating. These biomass increases coincide with a decline in fishing mortality, particularly for fish aged 0 to 3, over the last decade. The latest (2020) SSB is estimated to be 10.2% of SSB0.
Based on these findings, the following information on the status of the Pacific bluefin tuna stock is provided by the ISC23 Plenary:
- No biomass-based limit or target reference points have been adopted for PBF, but the PBF stock is overfished relative to the potential biomass-based reference points (20%SSB0) adopted for other tuna species by the IATTC and WCPFC. On the other hand, SSB reached its initial rebuilding target (SSBMED = 6.3%SSB0) in 2019, five years earlier than originally anticipated by the RFMOs; and
- No fishing mortality-based reference points have been adopted for PBF by the IATTC and WCPFC. The recent (2018-2020) F%SPR is estimated to produce a fishing intensity of 30.7%SPR and is below the level corresponding to overfishing for many F-based reference points proposed for tuna species, including SPR20%.
After the steady decline in SSB from 1996 to the historically low level in 2010, the PBF stock has started recovering, and recovery has been more rapid in recent years, consistent with the implementation of stringent management measures. The 2020 SSB was above the initial rebuilding target but remains below the second rebuilding target adopted by the WCPFC and IATTC. However, stock recovery is occurring at a faster rate than anticipated by managers when the Harvest Strategy to foster rebuilding (WCPFC HS 2017-02) was implemented in 2014. The fishing mortality (F%SPR) in 2018-2020 has been reduced to a level producing 30.7%SPR, the lowest observed fishing mortality in the time series.
Based on these findings, the following information on the conservation of the PBF stock is provided by the ISC23 Plenary:
- The PBF stock is recovering from the historically low biomass in 2010 and has exceeded the initial rebuilding target (SSBMED1952-2014) five years earlier than expected. The rate of recovery is increasing and under all projection scenarios evaluated, it is very likely the second rebuilding target (20%SSB0 with 60% probability) will be achieved (probabilities > 90%) by 2029. The risk of SSB falling below the historical lowest observed SSB at least once in 10 years is negligible;
- The projection results show that increases in catches are possible without affecting the attainment of the second rebuilding objective. Increases in catch should consider both the rebuilding rate and the distribution of catch between small and large fish;
- The projection results assume that the CMMs are fully implemented and are based on certain biological and other assumptions. For example, the future projection results do not contain assumptions about discard mortality. Although the impact of discards on SSB is small compared to other fisheries, discards should be considered in future harvest scenarios;
- Given the uncertainty in future recruitment and the influence of recruitment on stock biomass as well as the impact of changes in fishing operations due to management, monitoring recruitment and SSB should continue and research on a recruitment index for the stock assessment should be pursued; and
- The results of projections from sensitivity models with lower productivity assumptions show that this conservation information is robust to uncertainty in stock productivity.
Stock Status Update
To confirm whether any unexpected behavior in the stock occurred, the PBFWG checked the most recent catch and abundance indices for spawners and recruitment in March 2023 (Figure 1).
The ISC23 Plenary concluded that there is no new information that necessitates revising the existing stock status or conservation information for PBF.