Vertical distribution of herring and blue whiting in the Norwegian Sea

The Norwegian Sea harbours some of the largest pelagic fish stocks in the world including the Norwegian spring-spawning (NSS) herring and blue whiting, and it has been hypothesized that there are foraging interactions between these stocks. Here we address the vertical distribution and interactions of these stocks based on 12 years of survey data. The blue whiting mainly had a unimodal vertical distribution, while herring were often distributed in separate layers. The median depth of herring was 35 m both during day and night, while the blue whiting had a consistent vertical migration and a median depth of 325 m during day and 265 m at night. There was an increase in the depth of herring from east to west, whereas the blue whiting had its shallowest distribution in the frontal areas. The herring depth varied inter-annually and was negatively correlated with fish (herring + blue whiting) biomass. This suggests that there are interactions between NSS herring and blue whiting seen as a shift in vertical distribution of herring when the fish biomass in the Norwegian Sea is large and the potential for exploitative competition is at its highest.