Project
Stock assessments and differences in sex
Many commercially exploited fish species show differences in growth between the sexes. Fisheries stock assessments are generally based on age structured population models that do not distinguish between the sexes. When strong shifts in the sex ratio occur, fisheries assessments can be inaccurate for species with sexual dimorphism in growth.
Summary
Sexual dimorphism in growth can lead to inaccurate predictions of fisheries assessment models when the population sex ratio shifts over time. This occurs because due to differences in size-at-age, catchability of gear and surveys and natural processes such as maturation and natural mortality differ per age class between the sexes. The European eel sex ratio in commercial catches from Dutch lakes has shifted strongly over the last decades, probably due to changes in population density. Therefore, a sex-structured fisheries model is necessary to estimate fisheries mortalities and stock biomasses for this stock. The aim of this project is to adjust a standard fisheries assessment model (the state-spaced assessment model, SAM) such that it takes differences in catches and natural processes between the sexes into account. It can then be used to test the effect of sexual dimorphism and changes in sex ratios on stock assessment estimates such as fisheries mortalities and stock biomasses for the European eel population in the Dutch Lakes.