Warm, windy winters drive cod north and homing of spawners keeps them there

Abstract

Summary Climatic and anthropogenic effects often interact leading to unexpected results. For example, climate may lead to a change in the spatial distribution of a fish stock and thereby its vulnerability to exploitation. The North Sea cod stock is currently under pressure from both environmental change and human exploitation. This stock has experienced a series of poor recruitments since the late 1990s and, concomitant with the decrease in abundance, the distribution of cod has changed. While it has been suggested that the change in distribution can be linked to increasing temperatures and fishing pressure, there is little evidence for this hypothesis. Using winter and summer survey catches, we investigated whether a directional shift in the distribution of cod has taken place over the years 1983–2003. We then examined whether the change could be linked to climatic conditions, fishing mortality, stock size or limited directional movement of cod. Using the derived models, we investigated whether fishing has increased the sensitivity of the cod population to climate‐induced distribution changes. A series of winters characterized by high temperatures and southerly winds during the egg and larval phases of cod led to a northward shift in the distribution of juvenile North Sea cod the following year. A concomitant northern shift of mature fish around the time of spawning was linked directly to a tendency for northerly distributed juveniles to remain northerly throughout their life. This shift of the spawners further augmented that of the new recruits. Although fishing mortality on a North Sea scale was not directly correlated with the displacement of any of the age groups, fishing has severely decreased the number of fish in older age groups. This increased the sensitivity of the distribution of the cod stock to climatic changes. Synthesis and applications. The centre of gravity of North Sea cod has moved north as a result of the effect of a series of warm, windy winters on the distribution of recently settled cod. The shift was followed by a northwards shift in the distribution of older age groups. Unless a series of cold and calm years combined with a reduced mortality in the southern areas allows a southern spawning population to rebuild, the cod stock is unlikely to return to its previous area of distribution. Furthermore, protecting adult cod mainly in northern areas is unlikely to result in improved recruitment to the southern North Sea.