Application of the sex-specific age-structured assessment method for swordfish, Xiphias gladius, in the North Pacific Ocean

Abstract

Swordfish are known to be sexually dimorphic. However, previous assessments of the status of swordfish in the North Pacific Ocean have ignored this. A sex-specific age-structured assessment model was therefore constructed and fitted to catch, catch-rate and length–frequency data for the swordfish fisheries that operate in the North Pacific Ocean. Except if natural mortality is lower than its “best” estimate, the results indicate that the spawning stock biomass in 2002 was at a high fraction of its unfished level and that the fishing intensity in 2002 was less than F MSY. Therefore, the swordfish stock in the North Pacific Ocean appears to be relatively stable at the current level of exploitation. However, the results of the assessment model are sensitive to the values for natural mortality and the steepness of the stock-recruitment relationship. Forward projections based on samples from a Bayesian posterior distribution indicate that there is negligible risk of the stock dropping below 40% of the unfished spawning stock biomass if fleet-aggregated fishing intensity remains at the current level. However, the risk of population depletion is higher if natural mortality is lower than the “best” estimate.