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Latest Stock Assessment

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

An update of the stock status is provided in the report of the Twentieth 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 will be conducted during 2017.

 

 

Summary of the Latest Assessment

The 2014 assessment suggested that the SBT spawning biomass is at a very low fraction (9%) of its original biomass as well as below the level that could produce maximum sustainable yield. However, there has been some improvement since the 2011 stock assessment and fishing mortality is below the level associated with MSY. The current TAC has been set using the management procedure adopted in 2011, which has a 70% probability of rebuilding to the interim target biomass level by 2035.

Summary of indicators

The 2016 meeting of the Extended Scientific Committee (ESC) considered updated fishery indicators. A summary of the results is as follows:

  • In terms of recruitment indicators, the fact that there was no information on recruitment collected in 2015 was noted. The 2016 aerial survey (an index of age 2-4 relative abundance) was the highest on record, following the high 2014 index. The 2016 trolling survey index was higher than the 2014 index and slightly above the average median value 2006-2016. Preliminary analysis of 2016 CDS data from NZ showed a very strong mode of fish around 20kgs (processed weight), which has not been seen in previous years, and possibly reflects strong recruitment consistent with the 2016 aerial survey.
  • Recent longline CPUE index values for the Japanese fleet for ages 5 to 7 were well above the historically lowest levels observed in the mid-2000s. The index for these ages showed an increasing trend in recent years. The CPUE index for ages 8-11 has increased since 2011. The index for age 12+ has fluctuated around a low level. The Korean standardised CPUE series also showed an increasing trend over recent years. The time-series of direct ageing distribution data available from the New Zealand foreign charter fishery indicated relatively strong cohorts now about to enter the spawning component of the stock.

Overall the ESC observed that there are signs of higher recruitment in recent years and there are some consistent positive trends in the longline CPUE. This suggests that some relatively strong cohorts are moving through the fishery, though 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.

Sensitivity analysis around all sources of unaccounted catch mortality

The 2014 assessment included sensitivity analysis around all sources of unaccounted catch mortality. The ESC noted that it appears that significant levels of unaccounted mortality may have occurred which were not considered in the design of the Management Procedure (MP) and that if these levels are indeed true, they would amount to exceptional circumstances because the probability of rebuilding under the MP will be well below what was intended by the Extended Commission (EC).

The ESC also noted that continuing to follow the MP as proposed does lead to continued rebuilding in the short term even if the circumstances of the hypothesised additional unaccounted mortality are true. Hence, the ESC advised the EC to continue to follow the MP as formulated but, as a matter of urgency, to take steps to quantify all sources of unaccounted SBT mortality. If substantial levels of unaccounted mortality are confirmed, the ESC noted that there will be a need to retune the MP to achieve the EC’s stated rebuilding objective. In addition, the ESC advised that the EC should take steps to ensure adherence to its TACs.

The 2016 ESC reaffirmed these views and considered additional information that had become available, including:

  • Estimates of potential non-cooperating Non-Member (NCNM) longline catches in the Pacific, Indian and Atlantic oceans. The estimated mean total catch by NCNMs ranged between 59 to 306 t annually (2011-2014), depending on the assumption about whether the reported effort was bycatch or targeted at SBT. The estimates are uncertain. If current levels of NCNM effort remain and stock size increases, levels of NCNM catch will likely increase.
  • A draft report on a study of SBT in China’s market, which confirmed the ongoing presence of SBT within the Chinese sashimi market (particularly in Shanghai). Additional information is required to assess the scale and proportion of reported catches in this market.

Given the decisions of the Extended Commission to account for NCNM catch by deducting this from the 2018-20 TAC, and that Members have agreed to account for all their SBT fishing mortality (including commercial catch, discards, recreational catch and artisanal fishing etc.) from within their allocation of the TAC by 2018, the ESC concluded there was no reason to take action to modify the MP recommended TAC for 2018-20 in relation to this exceptional circumstance.