Factors influencing size-structured models’ ability to estimate natural mortality

Abstract

Natural mortality (M) is crucially important for stock assessment since it strongly influences estimates of spawning stock biomass, MSY and fishing mortality. Variation in M can occur between sexes, maturity stages and temporally. Estimates of M are confounded with those of catchability, recruitment, and growth. Previous studies, which focused on age-structured population dynamics models, suggest that it would be better to estimate M rather than pre-specify it. Size-structured population dynamics models, used for species that are hard to age such as crabs, have unique characteristics such as a size-transition matrix and the possibility of a terminal molt, which could impact the ability to estimate M. Therefore, this study explores the ability of size-structured stock assessment models to estimate M for several scenarios. These scenarios include whether the operating model and estimation method have a terminal molt, whether M is constant, time-varying, or sex- and maturity stage-specific, the quality/sample size of the biomass index and the size-composition data and whether growth is estimated simultaneously with M. The results show that size-structured models can estimate time-varying, sex- and maturity stage-specific M when the estimation method mirrors the operating model. Surprisingly, terminal molt does not affect the ability to estimate M. However, estimating growth simultaneously with M has a negative impact on the ability to estimate M but a positive effect on the quality of the estimates of spawning stock biomass.