Evaluation of a sex-specific age-structured assessment method for the 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 not been based on population models that distinguish the sexes. A sex-specific age-structured assessment model was therefore constructed for the stock of swordfish in the North Pacific Ocean. However, prior to the application of this method to the data for the stock in question, its performance needs to be evaluated. Monte Carlo simulation is used to evaluate the impact of observation errors and errors in assumed parameters. The impact of observation error associated with catch-rate indices has a larger impact on estimation ability than errors in total catch and its length–frequency. Improved performance, both in accuracy and precision of the estimates of quantities of management interest, can be obtained by increasing the length–frequency sample size. The estimates of spawning stock biomass, MSY, and (to a lesser extent) fishing intensity are substantially more biased when the assessment does not consider sexual dimorphism. The ratios of current to unfished spawning stock biomass and to the spawning stock biomass corresponding to MSY were found to be the quantities estimated most robustly of those considered.