Stock assessment of yellowfin tuna in the western and central Pacific Ocean

This paper presents the 2005 assessment of yellowfin tuna in the western and central Pacific Ocean. The assessment uses the stock assessment model and computer software known as MULTIFAN-CL. The yellowfin tuna model is age (28 age-classes) and spatially structured (6 regions) and the catch, effort, size composition and tagging data used in the model are classified by 19 fisheries and quarterly time periods from 1952 through 2004. Six independent analyses are conducted to test the impact of using different methods of standardising fishing effort in the main longline fisheries, using estimated or assumed values of natural mortality-at-age, and examining the effect of applying an incremental increase in effective fishing effort to mimic increased fishing efficiency. The analyses conducted are: SHBS-MEST Statistical habitat-based standardised effort for " main " longline fisheries, M (assumed constant across age-class) estimated. SHBS-MFIX Statistical habitat-based standardised effort for " main " longline fisheries, Mat age assumed at fixed levels. GLM-MEST General linear model standardised effort for " main " longline fisheries, M (assumed constant across age-class) estimated. GLM-MFIX General linear model standardised effort for " main " longline fisheries, Mat age assumed at fixed levels. FPOW-MEST General linear model standardised effort for " main " longline fisheries, M (assumed constant across age-class) estimated. Fishing power expansions incorporated into longline (1% per year) and purse seine (4 % per year) effort. No temporal trend in purse-seine catchability. FPOW-MFIX General linear model standardised effort for " main " longline fisheries, Mat age assumed at fixed levels. Fishing power expansions incorporated into longline (1% per year) and purse seine (4 % per year) effort. No temporal trend in purse-seine catchability. The order (from best to worst) of the models in terms of their fit to the composite data and The catch, size and tagging data used in the assessment were the same as those used last year, with the exception that additional recent fishery data (2003 and 2004 for longline, 2003 for Philippines and Indonesia, 2004 for purse seine) were included. It should be noted that 2004 data are not complete for some fisheries. The estimation of standardised effort for the main longline fisheries using the GLM and SHBS approaches involved a new method of scaling indices of abundance among regions (see Langley et al. 2005 for details). Overall, the new procedure resulted in higher relative abundance in the tropical regions (3 and 4) and lower relative abundance …

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