Exploring cohort-based density-dependent effects on maturity in Norwegian spring-spawning herring (<i>Clupea harengus</i>)

Abstract The influence of density-dependent effects on fish maturity is rarely considered when evaluating different harvest strategies, nor when formulating short-term catch advice. In cases where these effects are included, the spawning stock size is commonly used as the density variable. However, this approach is inadequate for a stock where juvenile and mature individuals have limited interaction. In such instances, using the abundance of the recruits of a cohort as a density variable is a more appropriate alternative. In this study, we have used the Norwegian spring-spawning herring (Clupea harengus) as a test stock to investigate this concept and develop a model for predicting future trends in maturity-at-age. This stock is an optimal candidate since previous publications have highlighted a spatial separation between juveniles and adults, and changes in maturity in response to historical stock dynamics have been observed and documented. Our approach provides increased accuracy for predicting maturity-at-age when compared to an assumption of density independence. Furthermore, a further expansion of this approach, i.e. applying a relationship with somatic growth, can contribute to more realistic simulations for predicting future stock dynamics, and more appropriate catch advice.