Migratory behaviour and range in Atlantic cod: inference from a century of tagging

A century of tagging experiments on 174 Atlantic cod (Gadus morhua) groups is reviewed and the frequency and spatial distribution of four migratory behaviours documented. Of all cod groups, 41% were categorized as sedentary, 18% as accurate homers. 20% as inaccurate homers and 20% as dispersers (ranging over large areas without recognizable return migrations). All behaviours occurred over the full spatial range of cod. Coastal groups did not differ significantly from offshore (shelf) groups in the relative frequency of migratory behaviours. However, the north-east Atlantic had more sedentary and accurate homing groups than did the north-west Atlantic. which had more dispersing groups. Overall, sedentary cod groups had lower maximum historical biomass than did other groups, confirming that migration/dispersal begets abundance. Maximum historical biomass was strongly related to the area occupied [log (biomass. tonnes)] = 1.58 log (range, km(2)) + 1.529; r(2) = 0.9). irrespective of migratory type (for 23 major groups a total of 22 million tonnes over 3.3 million km(2) with an average density of 7 tonnes km(-2)). Historical densities were not related to area occupied. although all large groups exhibited high densities (10-12 tonnes km(-2) : smaller groups had a wide range of density). The four migratory strategies in cod enable entrainment by diverse physical, oceanographic and biological ecosystem properties, and is key to cod success in the North Atlantic.