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Stock Status and Conservation Information
(From ISC22 Plenary Report)

Pacific Bluefin Tuna

Click here to see the ISC21 stock status and conservation information.

Stock Status

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:

  1. 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, 5 years earlier than originally anticipated by the RFMOs; and
  2. 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 (Table1), including SPR20%.
Table 1. Ratios of the estimated fishing mortalities (Fs and 1-SPRs for 2002-04, 2011-13, and 2018-2020) relative to potential fishing mortality-based reference points, terminal year SSB (t) for each reference period, and depletion ratio (SSB/SSBF=0) for the terminal year of the reference period for Pacific bluefin tuna (Thunnus orientalis) from the base-case model. Fmax: Fishing mortality (F) that maximizes equilibrium yield per recruit (Y/R). F0.1: F at which the slope of the Y/R curve is 10% of the value at its origin. Fmed: F corresponding to the inverse of the median of the observed R/SSB ratio. Fxx%SPR: F that produces a given % of the unfished spawning potential (biomass) under equilibrium conditions.

Conservation Information

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 in the time series.

Based on these findings, the following information on the conservation of the Pacific bluefin tuna stock is provided:

  1. 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;
  2. 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;
  3. The projection results assume that the CMMs are fully implemented and are based on certain biological and other assumptions. For example, these 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;
  4. 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 the management, monitoring recruitment and SSB should continue and research on a recruitment index for the stock assessment should be pursued; and
  5. The results of projections from sensitivity models with lower productivity assumptions show that this conservation information is robust to uncertainty in stock productivity.