A spatially disaggregated, length-based, age-structured population model of yellowfin tuna (Thunnus albacares) in the western and central Pacific Ocean

Abstract

A spatially disaggregated, length-based, age-structured model for yellowfin tuna (Thunnus albacares) in the western and central Pacific Ocean is described. Catch, effort, length-frequency and tagging data stratified by quarter (for the period 1962–99), seven model regions and 16 fisheries are used in the analysis. The model structure includes quarterly recruitment in each region, 20 quarterly age classes, independent growth patterns for juveniles and adults, structural time-series variation in catchability for all non-longline fisheries, age-specific natural mortality, and age-specific movement among the model regions. Acceptable fits to each component data set comprising the log-likelihood function were obtained. The model results suggest that declines in recruitment, and as a consequence, population biomass, have occurred in recent years. Although not obviously related to over-exploitation, the recruitment decline suggests that the productivity of the yellowfin tuna stock may currently be lower than it has been previously. Recent catch levels appear to have been maintained by increases in fishing mortality, possibly related to increased use of fish aggregation devices in the purse-seine fishery. A yield analysis indicates that average catches over the past three years may have slightly exceeded the maximum sustainable yield. The model results also reveal strong regional differences in the impact of fishing. Such heterogeneity in the fisheries and the impacts on them will need to be considered when future management measures are designed.