The Roles of Environmental Change and Fishing in Norwegian Coastal Cod Stock Dynamics

Disentangling the roles of fishing and environmental factors in fish stock dynamics is fundamental for sustainable fisheries management. The stock biomass of Norwegian coastal cod north of 67°N has varied between periods of increase and decline in recent decades, with declining biomass since 2014. Here we developed a population model for the dynamics of this stock. The model was statistically fitted to survey indices of abundance- and length-at-age and environmental covariates from 1995 to 2022. The final model included a negative effect of saithe biomass on survival to age 1, positive temperature effects on growth to ages 2 and 3, density dependence in survival and growth, size-dependent maturation, and negative fishery effects on adult survival. Hindcast scenario simulations, where environmental influences or mortality from fishing were fixed, were used to disentangle the roles of recruitment, growth and adult survival for stock fluctuations in recent decades. The changes in stock biomass appear mainly to be driven by adult survival and hence fishing. The fishery effects on stock biomass were mostly the direct effects of biomass removal, with no indication of fishery-caused recruitment failure at the spatial scale represented by the model.