Temporal and environmental variation in growth and maturity and effects on management reference points of Georges Bank Atlantic cod

Abstract

Temporal variation in demographic rates has been observed in various fish populations and environmental influences are likely to be a key factor. Here we show how it can be important to combine state-space models for environmental covariates and demographic rates when evaluating effects of the former on the latter. In an application to Georges Bank Atlantic cod (Gadus morhua), we show how estimates of the environmental covariate can be aliased with unknown temporal variation in growth rates and that stronger and opposite effects of the environment can incorrectly be found when the autocorrelation in the growth rate is not separately modeled. The perception of effects on maturity also depends on whether an appropriate distribution is considered for the maturity observations. Bottom temperature did not improve prediction of maturity parameters, but temporal variation driven by other sources did. Both bottom temperature and temporal variation improved performance of growth models. Finally, we found growth and maturity estimation to be important contributors to uncertainty of spawning biomass and biological reference points when incorporated into a state-space assessment model.