Publication Abstract

Implications of swordfish (Xiphias gladius) stock structure and mixing on management benchmarks

E.N. Brooks and P. Apostolaki

A simulation study was performed to determine the effects on management benchmarks of various levels of movement between Atlantic swordfish stocks (Xiphias gladius). An age-structured model was used as an operating model to simulate fishery data on catches and catch rates under different movement scenarios. The simulated data were aggregated by “stock” and then used as input to the base model from the 2002 assessment (ASPIC) to determine the impact on estimates of stock status and MSY. For the movement scenarios examined, the trend observed was for the relative biomass benchmark (B/B_MSY) to be overestimated and the relative fishing mortality benchmark (F/F_MSY) to be underestimated if catches assigned to a stock exceeded the true landings. This trend was reversed in cases where attributed catches were lower than the true removals for a stock. The bias in benchmarks was driven by the mismatch between true removals and removals assigned based on the area of landing, which made a stock look more productive or less productive than it actually was. The magnitude of bias depended on the extent of mixing.

Reference:

E.N. Brooks and P. Apostolaki (2007) Implications of swordfish  (Xiphias gladius) stock structure and mixing on management benchmarks . Special issue of the ICCAT Collective Volumes of Scientific Papers