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Latest Stock Assessment

Details of the latest full stock assessment of southern bluefin tuna (SBT), including updated fishery indicators, are available from the report of the Twenty Fifth meeting of the CCSBT Scientific Committee:

A brief report on the Report on Biology, Stock Status and Management of Southern Bluefin Tuna is available at:

The next full stock assessment for southern bluefin tuna is scheduled to be conducted during 2023.

 

Summary of the Latest Assessment

All the key stock status statistics from the 2020 stock assessment are more optimistic than when the last assessment was completed (2017) and the results are consistent with projections made at that time. The relative Total Reproductive Output (TRO) is estimated to be 20% (16-24 80% P.I.). The stock remains below the level estimated to produce maximum sustainable yield (MSY). There has been improvement since previous stock assessments conducted in 2017 which indicated the stock was at 13% (11-17% 80% PI) of initial biomass. The fishing mortality rate is below the level associated with MSY. The results of sensitivity tests did not show any unusual or unexpected impacts on stock status (median relative TRO is 19-20% across the tests).

The current estimated trends indicate that the stock has been rebuilding by approximately 5% per year since the low point in 2009, and the Management Procedure based rebuilding plan for SBT appears to be on track to achieving the Extended Commission’s objective. Comparison with earlier assessments shows that this trend is consistent with past results. The current TAC was set in 2016 (for the 2018-2020 quota block) following the recommendation obtained from the Bali Management Procedure adopted in 2011.

Summary of indicators

The review of indicators by the ESC in 2020 provides mixed messages on recruitment with: (i) the gene tagging absolute abundance estimate showing a slight decrease; and (ii) the trolling survey index (piston-line index of age 1) increasing from the previous two estimates which were both zero.

There are some consistently positive recent trends in the age-based longline CPUE estimates for a number of Members, including the Japanese (core vessels) and Korean fleets but overall the most recent estimates are near to recent estimates or have decreased slightly. The ESC noted an increase from 2010 to 2014 in the close-kin mark recapture empirical index of spawning stock abundance; However, in 2015 this decreased slightly.

Annual Review of implementation of the CCSBT Management Procedure

The 2020 ESC meeting evaluated whether there are events, or observations, that are outside the range for which the Cape Town Procedure (CTP) was tested, and the implications of this for TAC setting. The scope of this evaluation covered: (i) all input data (gene-tagging, CPUE, and Parent-Offspring-Pairs and Half-Sibling-Pairs) used by the CTP to calculate a recommended global TAC; (ii) changes in estimates of the population’s dynamics and productivity incorporated into the 2020 stock assessment; (iii) the shift in size distribution towards small fish in the Indonesian spawning ground fishery since 2013; and (iv) the potential for fishing mortality (from Members and non-Members) to be greater than that used to calculate the TAC recommended by the Management Procedure. Following the review of exceptional circumstances, the ESC concluded there was no reason to declare exceptional circumstances, and hence to perhaps modify the TAC recommended by the CTP.

Application of the CTP adopted by CCSBT 26 resulted in an annual TAC of 17,647t for the period 2021-2023. Based on the review of the exceptional circumstances, the ESC therefore recommended that the annual TAC for the 2021-2023 quota block should remain as 17,647t.