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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 Second meeting of the CCSBT Scientific Committee:

An update of the stock status is provided in the report of the Twenty Third 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 2020.

 

Summary of the Latest Assessment

The 2017 assessment suggested that the stock remains at a low state, estimated to be 13% (11-17 80% P.I.) of the initial spawning stock biomass, and below the level to produce maximum sustainable yield (MSY). There has been improvement since previous stock assessments which indicated the stock was at 5% (3-8%) of original biomass in 2011 and 9% (7- 12%) in 2014. There are positive estimates of earlier rebuilding of the stock than anticipated in 2011, because fishing mortality rate estimates are below the level associated with MSY.

Summary of indicators

The 2018 meeting of the Extended Scientific Committee (ESC) considered updated fishery indicators. A summary of the results is as follows:

  • The standardised Korean CPUEs for both areas described have shown an increasing trend since the mid-2000s.
  • For the Taiwanese CPUE standardisation, the standardised CPUEs for Area E (eastern area of 60 deg E) have been increasing since 2015.
  • For the Taiwanese CPUE standardisation, the standardised CPUEs for Area W (Western area of 60 degrees East) generally revealed a decreasing trend with a fluctuation since 2002. It should be noted that the Taiwanese SBT fishery in Area W is a small component of a very mixed fishery where SBT are primarily bycatch and as such any trends should be interpreted with caution at this time.
  • One indicator of age 1 SBT abundance (i.e. trolling index of piston line) was undertaken in 2018; the trolling index of piston line decreased since the last update.
  • The first data point in the gene tagging project indicated that in 2016 there were 2,417,786 age 2 SBT in the stock, which was similar to median estimates from the 2017 stock assessment.
  • Indicators of aggregated CPUE from the New Zealand domestic longline fishery increased in 2017.
  • The Japanese longline CPUE indicators suggest that the current stock levels for 4, 5 and 6 - 7 age groups are well above the historically lowest levels observed in the late 1980s or the mid-2000s. The CPUE indices for the age 8-11 group show some decreases in recent years. The indices for age class 12+ have declined gradually since 2011. This decline may relate to very low cohorts of 1999 to 2001.

The review of indicators did not suggest a need to change conclusions from those drawn in 2017. Overall, there is a low trolling (piston-line) index of age 1 in 2018, mixed signals of higher recruitment in recent years, and there are some consistent positive trends in the age-based longline CPUE estimates. There may be several relatively strong cohorts moving through the fishery, although these have yet to contribute to the spawning stock. The ESC noted that increased recruitment is not necessarily indicative of increased spawning stock biomass. The ESC noted that it will take a few more years before there is sufficient data to confirm the recent apparent strong recruitments evident in the aerial survey.

Annual Review of implementation of the CCSBT Management Procedure

The ESC evaluated whether there are events, or observations, that are outside the range for which the management procedure was tested and the implications of this for TAC setting. The scope of this evaluation covered input data to the Management Procedure (CPUE and the absence of aerial survey data for 2018), the question of unaccounted mortality, reported catch and length and age of Indonesian catches on the spawning ground, the higher productivity of the stock noted in 2017, and the update to historical CPUE data in 2018. Following the meta-rule review of exceptional circumstances, the ESC concluded there was no reason to take action to modify the 2019 TAC recommendation.

Based on the annual review of the exceptional circumstances and fishery indicators, the ESC recommended that there is no need to revise the Extended Commission’s 2016 decision regarding the TAC for 2018-20. Therefore, the recommended TAC for 2019 and the 2018-20 quota block remains 17,647t.