Size-selectivity for British Columbia Sablefish (Anoplopoma fimbria) estimated from a long-term tagging study

Abstract

The underlying size-distribution of commercial fish stocks is usually unknown, so fishery size-selectivity must be estimated as a latent process embedded within age-structured stock assessments. However, dome-shaped fishery size-selectivity, in particular, is often inestimable because decreasing selectivity is confounded with mortality at older ages. In this paper, we test for dome-shaped selectivity in British Columbia's Sablefish fishery using a long-term tagging data set. We incorporate alternative fishery size-selectivity assumptions within a mark-recapture framework based on an asymptotic logistic model and dome-shaped models using gamma and normal probability density functions. We also fit each model using both time-invariant and time-varying parameterizations. Our results strongly suggest dome-shaped size-selectivity for tagged-Sablefish in longline trap, longline hook, and bottom trawl fisheries. Time-varying models were generally favored over time-invariant models, although alternative time-varying models often produced similar statistical fits. Dome-shaped selectivity in longline fisheries could be a function of fishery targeting, fish movement, or by lower reporting rates for large size-classes.