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Past stock status and conservation advice
(From ISC8 Plenary Report)
The advice provided by the ISC7 still holds pending the results of a new stock assessment currently scheduled for 2010. That is:
Previous scientific advice, based on the 2004 stock assessment, recommended that current fishing mortality rate (F) should not be increased. It was noted that management objectives for the IATTC and WCPFC are based on maintaining population levels which produce maximum sustainable yield. Due to updating, and improvements and refinements in data and models used in the 2006 stock assessment, it is now recognized that Fcur (0.75) is high relative to most of the F reference points (see Table 5a in Annex 5 of the ISC7 Plenary Report). On the other hand, the same analysis indicates that the current estimate of the SSB is the second highest in history but that keeping the current F would gradually reduce the SSB to the long-term average by the mid 2010s. Therefore, the recommendation of not increasing F from current level (Fcur(2002-2004)=0.75) is still valid. However, with the projection based on the continued current high F, the fishing mortality rate will have to be reduced. The degree to which, when and how reductions should occur will depend on which reference points are selected and the desired probability and practicability of success of attaining these reference points in a timeframe to be agreed. The ISC requires additional guidance on these issues from the management authorities in a timely manner to work further on these issues.
However, based on analyses conducted by the ALBWG since ISC7, the following points are highlighted:
- Estimated probabilities of the SSB remaining above the SSB reference points as calculated in the last stock assessment (2006) were modestly underestimated;
- Because the realized catch in 2007 was less than that assumed in the projections, the F in 2007 may have been less than the “current F” (0.75yr-1);
- Further guidance on the selection and application of biological reference points (BRPs) and their conditions is requested in order to facilitate response to requests for conservation advice. In particular, clarification of the timeframe (e.g. short-term versus long-term) for projections; and the specific types of reference points to be used (e.g. limit and/or target and based on which parameters) would be useful.
Pacific Bluefin Tuna
- Given the conclusions of the May-June 2008 stock assessment with regard to the current level of F relative to potential target and limit reference points, and residual uncertainties associated with key model parameters, it is important that the current level of F is not increased.
- If F remains at the current level and environmental conditions remain favorable, then recruitment should be sufficient to maintain current yield well into the future.
- A reduction in F, in combination with favorable environmental conditions, should lead to greater Y/R and SPR and, after some lag, greater sustained yield.
- Increases in F above the current level, and/or unfavourable changes in environmental conditions, may result in recruitment levels which are insufficient to sustain the current productivity of the stock.
While further guidance from the management authority is necessary,including guidance on reference points and the desirable degree of reduction, the fishing mortality rate of striped marlin (which can be converted into effort or catch in management) should be reduced from the current level (2003 or before), taking into consideration various factors associated with this species and its fishery. Until appropriate measures in this regard are taken, the fishing mortality rate should not be increased.