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Detailed analysis of the catch-at-length and age composition data to check results for recent year recruitment estimates for eastern Atlantic bluefin tuna

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posted on 17.09.2021, 13:29 by Lisa E Ailloud, Tristan Rouyer, Ai Kimoto, Rishi Sharma, Doug ButterworthDoug Butterworth
A model-free exercise was carried out to determine the plausibility of the high 2004-2007 recruitment estimates that resulted from the VPA base run for Eastern Atlantic bluefin tuna. Information on year class strength from the base run was compared to information on year class strength present in the catch at length data for the Japanese Northeast Atlantic longline fishery and the trap fishery. The analysis revealed inconsistencies: while the VPA results suggest the 2004-2007 cohorts are of near equal strength to the 2003 cohort, the catch at length data suggest that the 2003 cohort was certainly stronger. In addition, the catch at length information was somewhat more compatible with alternative VPA runs that indicate lower recruitment and lower biomass in recent years than the VPA base run does. This analysis suggests applying some assessment approach which will result in a reduction in the size of the 2004-2007 cohorts relative to the 2003 cohort. However, the analysis did not allow a quantification of the extent of change in VPA specifications that would best reflect the available data as a whole.

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Department of Mathematics and Applied Mathematics, University of Cape Town