Catch At Age Models: Does Sex Matter?

Catch-at-age models that use all available information in a unified framework are a standard tool of fisheries stock assessment.  Managers use these models to obtain estimates of population abundance and to study the underlying population dynamics, including responses to management actions; however, mechanisms that cause fish populations to change are often poorly understood (model error), changeable over time (process error), and based on data measured imprecisely (sampling error).   Because of these uncertainties, justifying model choice and quantifying reliability of estimates describing stock status is complex, but critical.  Failure to address factors influencing model reliability can result in severe long-term consequences. Walleye populations are typically assessed using models that do not differentiate between sexes, despite well documented sexual dimorphism in many characteristics including growth and catchability.  Furthermore like many fisheries, harvest may not be known by sex, limiting sex-specific information to survey catch data.  An operating model approach was used to evaluate consequences of ignoring known sexual dimorphism on catch-at-age model performance.  Errors in estimates of population and fishery characteristics, such as spawner biomass and exploitation rate, for fisheries with and without highly sex and size selective harvest were evaluated.  Simulations showed catch-at-age models that incorporated sex-specific information had lower error.