Modelling the Distribution of Norwegian Coastal Cod (Gadus morhua)

Abstract

Knowledge about a species' distribution, and the factors driving the distribution, is important in both fishery management and conservation. It can help explain distributional shifts and variations in abundance as a response to environmental changes. But spatial dynamics are complex and not easy to analyse, and a flexible model is therefore necessary. Atlantic cod (Gadus morhua) is a commercially important species and includes two stocks present on the northern coast of Norway, one of which is the Norwegian coastal cod. The distribution of coastal cod has not yet been analysed using spatial modelling, which is the focus of this thesis. Using Generalized Additive Models (GAM), we investigated if the distribution of coastal cod in northern Norway changed during the period 2003-2021, and if the distribution could be explained by both density-dependent and density-independent factors. We also investigated if there were any differences in the environmental conditions experienced by different-sized cod by modelling the density of three selected length groups: 25cm, 55cm, and 85cm. The results showed that the distribution has not changed significantly during the time period, although the local densities have varied. Temperature and spawning stock biomass had both an effect on the spatial variations in density and an increase in these factors could result in an expanded distribution. The temperature could have a negative effect on the abundance of the local populations in some areas and a positive effect in other areas. Increased spawning stock biomass could result in a wider distribution, but some local populations could also contribute to the increased spawning stock biomass. When studying different-sized cod, the results showed that temperature and depth alone could not explain the spatial variations observed between the length groups, although the cod seem to experience different habitats during ontogeny.