Community Research and Development Information Service - CORDIS

Comparative analysis of spatial dynamics of Cod (Gadus Morhua L.)

The development of geolocation methods for all ecosystems has enabled the comparison of cod migrations and distribution.

Two different movement strategies were observed: migration and residency. Migratory cod travelled different distances to different areas dependent upon their release position, and their stock identity. For example, cod tagged in the southern North Sea in spring would often move northwards to summer feeding grounds. Cod tagged in the English Channel in spring would often move west. Cod in both areas would move back towards the southern North Sea in the months prior to spawning. i.e. migrations were not random, but instead directed towards feeding or spawning grounds. Resident cod were caught close to their site of release and did not visit feeding or spawning sites at a significant distance from their point of release, even after long periods at liberty. Reconstructions of their likely horizontal movements suggested that they remained within their release locality for the period of their liberty. Examples of resident cod were observed in each CODYSSEY ecosystem except the Baltic. The proportion of resident to migratory cod varied between 30% in the Skagerrak and Kattegat, to 100% in the Faroe plateau.

Migratory cod undertook directed migrations at specific times of year that were consistent with congregation on spawning grounds and with dispersal to feeding grounds. In the northern ecosystems (Iceland plateau, Barents Sea), post-spawning migrations to offshore feeding grounds occurred in June and July, and fish began their migration back to spawning grounds as early as February, ready to spawn in April and May. In the southern ecosystems, spawning takes place between January and March, so cod migrate to feeding areas between April and June, and start their pre-spawning migrations as early as November.

In the Baltic Sea, conditions for spawning are often very limited geographically or temporally due to the unique hydrographic conditions in the deep, low salinity basins of the Bornholm Basin or the Gdansk Deep. Migrations into deeper water and below the halocline in spring therefore indicated the onset of spawning migrations. Cod tagged in the CODYSSEY project undertook directed migrations towards these basins at very similar times each year in each of three years.

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United Kingdom
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