A Bayesian assessment of the South Atlantic population of albacore which explicitly models changes in targeting

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International Commission for the Conservation of Atlantic Tuna (ICCAT) Secretariat


University of Cape Town


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The primary assessment method applied to South Atlantic albacore during the October 2000 ICCAT SCRS assessment session was an age-structured production model which assumed a fleet-aggregated selectivity pattern with a single change only (in 1969), and estimated model parameters using only CPUE data. A particular problem of this approach is that the Japan CPUE series is not considered comparable over the complete period for which it is available because of changed targeting practices, so that this series needs to be split into three separate segments when estimating model parameters. This paper refines that 2000 assessment by treating the Japanese, Taiwanese and Brazilian fisheries all as initially primarily albacore directed fisheries which have shifted over time to bigeye tuna targeted fisheries, taking albacore as a decreasing bycatch. The transitions over time are explicitly estimated, as are separate selectivities for the two components of each national fishery by also including catch-at-age information in the data fitted by the model. Compared to the high levels of uncertainty in past assessments, the refined model provides a reasonable to good fit to all available information, and suggests that the productivity of the resource is somewhat higher than estimated previously.