Evaluation of the sensitivity of biological reference points to the spatio-temporal distribution of fishing effort when seasonal migrations are sex-specific

Offshore lingcod (Ophiodon elongatus) in British Columbia (BC) make seasonal migrations that differ by sex: in winter most adult males aggregate in shallow water and most adult females aggregate in deep water, while in summer both sexes distribute equally in deep and shallow water. We developed a semi-age structured delay-difference model that accommodates sex-, and age/depth-specific seasonal migration and examined the sensitivity of biological reference points for BC offshore lingcod to migration pattern and the allocation of fishing effort across seasons and depths. When a migration pattern with a homogeneous distribution of males and females in shallow and deep water throughout the year was assumed, the estimated maximum sustainable yield (MSY), biomass at MSY (BMSY), and percent spawning per recruitment at MSY (%SPRMSY) were robust to changes in effort allocation. However, when a migration pattern that assumed a winter distribution of 95% of adult males in shallow water and 95% of adult females in deep water was used, the estimated MSY, BMSY, and %SPRMSY were markedly sensitive to changes in effort allocation. These results suggest that incorporating sex-, age/depth-, and season-specific structuring in stock assessment models could enable improved evaluations of management options for BC lingcod and other species with sex- and season-specific migrations.