A PRELIMINARY INVESTIGATION OF THE ALBACORE TUNA (THUNNUS ALALUNGA) STOCK IN THE SOUTHERN ATLANTIC OCEAN USING A NON-EQUILIBRIUM PRODUCTION MODEL, INCLUDING COMPARISON WITH AN AGE-STRUCTURED PRODUCTION MODEL

SUMMARY A non-equilibrium surplus-production model was attempted for the Albacore stock in the southern Atlantic Ocean, using the software package ASPIC ver. 5.34. The model utilised the same catch and effort data incorporated in the 2007 base case age-structured production model (ASPM) developed during the 2007 ICCAT albacore stock assessment working group. Several sensitivity runs were conducted, changing the input parameters and model assumptions, and although the overall population trends were similar to those for the ASPM, the ASPIC models generally produced results that were more optimistic than the age-structured model. In general, all the models predicted that at some stage in the recent past, the southern Albacore stock had been subjected to overfishing and had been overfished. In all cases, the fishing pressure appears to have eased in recent years, with a subsequent recovery in biomass. The incorporation of the additional age-structured information into a stock assessment model is generally considered to be beneficial. However, the surplus production model allows the investigation of structural uncertainty in the stock assessment process. In addition, alternate studies have indicated that although more complex models generally perform better in some cases, their benefits may be relatively small and are also dependant on the quality of agespecific information available. It is thus recommended that ASPIC (along with other stock assessment models) be used in the southern albacore stock assessment process.