Feeding behaviour of cod,Gadus morhua: activity rhythm and chemically mediated food search

Abstract

I studied the feeding behaviour of cod in late spring in a fjord in northern Norway by means of a stationary positioning system. The activity rhythm of eight cod tagged with acoustic transmitters was observed continuously for 3–8 days in terms of swimming speed and size of area occupied. I studied responses to food odour by setting a line with mackerel baits in the experimental area. The cod had a diurnal rhythm of swimming activity with a slightly lower swimming speed during a 5-h period at night, indicating that they searched more actively for food during the day. A shift towards a larger proportion of active prey taken in the day, accompanied by a change in hunting strategy where vision became more important, may explain this rhythmicity in feeding behaviour. The diurnal rhythm in the range of area occupied varied more than the swimming activity rhythm and there was no significant difference in range between day and night indicating that factors other than the diel light cycle also influenced periodicity in activity. A higher proportion of fish encountered by the bait odour plume located baits than that of fish that were out of range of the odour plume, indicating the importance of chemically mediated food searching in this species. The cod detected a food odour source from a distance of several hundred metres, and responded immediately to the odour plume by carrying out a rheotactic search at a moderately higher swimming speed.