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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:

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

 

 

Summary of the Latest Assessment

The 2017 assessment suggested that the stock remains at a low state, estimated to be 13% of the initial spawning stock biomass, and below the level to produce maximum sustainable yield. There has been improvement since previous stock assessments which indicated the stock was at 5.5% of original biomass in 2011 and 9% in 2014. The total biomass of SBT aged 10 and over relative to initial is estimated to be 11%, which is an increase from the estimate of 5% in 2011 and 7% in 2014. The current fishing mortality rate is below the level associated with maximum sustainable yield.

Summary of indicators

The 2017 meeting of the Extended Scientific Committee (ESC) considered updated fishery indicators. A summary of the results is as follows:

  • Two indicators of juvenile (age 1–4) SBT abundance (i.e. scientific aerial survey index and the trolling index) were available for 2017. Both the scientific aerial survey and trolling index decreased compared to 2016.
  • Indicators of age 4+ SBT CPUE from the New Zealand domestic longline fishery increased in 2016.
  • Recent Japanese longline CPUE indicators suggest that the current stock levels for the 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 age 8-11 group have increased steadily since 2011. The indices for age class 12+ have declined gradually since 2011.
  • The Taiwanese standardised CPUE for the central-eastern and the western areas reveal quite different trends. For the central-eastern area, this CPUEs increased gradually before 2007, showed a decreasing trend from 2007 to 2011, increased substantially in 2012 before decreasing gradually and then increased again in 2016. For the western area, the standardised CPUE series indicates a generally decreasing trend with some fluctuation after 2002.
  • The Korean standardised CPUE series has shown an increasing trend in recent years.

The review of indicators did not suggest any need for change to the conclusions drawn from the 2017 assessment. Overall there are signs of higher recruitment in recent years and there are some consistent positive trends in the age-based longline CPUE estimates. This suggests that there may be some relatively strong cohorts moving through the fishery, though they have yet to contribute to the spawning stock. The ESC noted that increased recruitment is of itself 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

In 2017 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 aerial survey data), the question of unaccounted mortality, reported catch, length and age of Indonesian catches on the spawning ground and the results of reconditioning of the CCSBT Operating Models. The ESC concluded there was no reason to take action to modify the 2018 TAC recommendation in relation to its review of exceptional circumstances.