Stock Status and Conservation Information
(From ISC21 Plenary Report)
Pacific Bluefin Tuna
The Plenary reviewed and agreed to forward the stock status and conservation information adopted at ISC20 unchanged, except for the omission of accompanying figures and tables and clarifying modifications.
Click here to see the ISC20 stock status and conservation information.
The WCPFC and IATTC adopted an initial rebuilding biomass target (the median SSB estimated for the period from 1952 through 2014) and a second rebuilding biomass target (20%SSBF=0 under average recruitment), without specifying a fishing mortality reference level. The 2020 assessment estimated the initial rebuilding biomass target (SSBMED1952-2014) to be 6.4%SSBF=0 and the corresponding fishing mortality expressed as F6.4%SPR. The Kobe plot shows that the point estimate of the SSB2018 was 4.5%SSBF=0 and the recent (2016-2018) fishing mortality corresponds to F14%SPR. Although no reference points have been adopted to evaluate the status of PBF, an evaluation of stock status against some common reference points shows that the stock is overfished relative to biomass-based limit reference points adopted for other species in WCPFC (20%SSBF=0) and fishing mortality has declined but not reached the level corresponding to that reference point (F20%SPR).
The PBF spawning stock biomass (SSB) has gradually increased in the last 8 years (2011-2018). Young fish (age 0-2) shows a more rapid increase in recent years. These changes in biomass coincide with a decline in fishing mortality over the last decade. Based on these findings, the following information on the status of the Pacific Bluefin tuna stock is provided:
- The latest (2018) SSB is estimated to be 4.5% of SSBF=0, which is an increase from 4.0% estimated for 2016 (the terminal year in the previous assessment). No biomass-based limit or target reference points have been adopted for PBF. However, the PBF stock is overfished relative to the potential biomass-based reference points (SSBMED and 20%SSBF=0) adopted for other tuna species by the IATTC and WCPFC.
- The recent (2016-2018) F%SPR is estimated to produce 14%SPR. Although no fishing mortality-based limit or target reference points have been adopted for PBF by the IATTC and WCPFC, recent fishing mortality is above the level producing 20%SPR. However, the stock is subject to rebuilding measures including catch limits and the capacity of the stock to rebuild is not compromised by this fishing mortality, as shown by the projection results.
After the steady decline in SSB from 1995 to the historically low level in 2010, the PBF stock has started recovering slowly, consistent with the management measures implemented in 2014-2015. The spawning stock biomass in 2018 was below the two biomass rebuilding targets adopted by the WCPFC while the 2016-18 fishing mortality (F%SPR) has reduced to a level producing 14%SPR.
The projection results based on the base-case model under several harvest and recruitment scenarios and time schedules requested by the RFMOs are in the ISC20 Plenary Report. The projection results show that PBF SSB recovers to the biomass-based rebuilding targets due to reduced fishing mortality by applying catch limits as the stock increases. In most of the scenarios, the SSB biomass is projected to recover to the initial rebuilding target (SSBMED) in the 2020 fishing year (April of 2021) with a probability above the 60% level prescribed in the WCPFC CMM 2019-02.
A Kobe chart and impacts by fleets estimated from future projections under the current management scheme are provided for information. Because the projections include catch limits, fishing mortality (Fx%) is expected to decline, i.e., SPR will increase, as biomass increases. Further stratification of future impacts is possible if the allocation of increased catch limits among fleets/countries is specified.
Based on these comments, the following conservation information is provided:
- Under all examined scenarios the initial goal of WCPFC and IATTC, rebuilding to SSBMED by 2024 with at least 60% probability, is reached and the risk of SSB falling below historical lowest observed SSB at least once in 10 years is negligible.
- 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 the harvest scenarios.
- Given the low SSB, the uncertainty in future recruitment, and the influence recruitment has on stock biomass, monitoring recruitment and SSB should continue so that the recruitment level can be understood in a timely manner.