Movements and distribution of cod (Gadus morhua) in the southern North Sea and English Channel: results from conventional and electronic tagging experiments

Abstract

The sub-structure of Atlantic cod (Gadus morhua) stocks in the North Sea has important consequences for fisheries management as the Common Fisheries Policy moves towards a more regional approach. We investigated the movements, distribution and behaviour of cod in the southern North Sea (ICES IVc) and English Channel (ICES VIId) by re-analysing historic data from conventional tagging experiments, and by conducting new experiments with electronic tags. Cod tagged and released in IVc showed a northwards shift in distribution during the feeding season consistent with a homing migration away from spawning grounds along the coasts of the UK and the Netherlands. In contrast, cod tagged and released in VIId did not exhibit a consistent pattern of seasonal movement. Many cod released in VIId were subsequently recaptured close to their release position, although some moved out of the Channel and into the southern North Sea. Overlap between the recapture areas of cod released in the different management areas was no more than 25% in either the spawning or feeding season. Behavioural data from electronic tags suggest that cod in IVc make use of tidal streams to migrate northwards and eastwards in spring, whereas selective tidal stream transport was rarely exhibited by cod tagged and released in VIId. Overall, the evidence suggests that there are behavioural differences between cod in IVc and VIId that limit the mixing of cod from these two areas during the feeding and spawning seasons.