Return migrations of Atlantic cod (Gadus morhua L) to the North Sea evidenced by archival tagging of cod off the eastern Skagerrak coast

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Here we show that cod (>40 cm) tagged off the Skagerrak coast migrated towards the North Sea, in particular, during the spawning period. Furthermore, onboard and retrospective positioning from retained archival tags by astronomical derivations of time-logged ambient light measurements depicted a migratory route to the eastern North Sea during the spawning season, and back again to the Skagerrak later in spring. The light intensity based geolocations were cross-validated with tidal location methods and experienced temperature of the tagged fish, supporting the view of a North Sea-bound migration route during the spawning period. These findings gives a strong behavioural signal with bearings on population separation and divergence in marine fish species.