FEEDING-BEHAVIOR OF COD, GADUS-MORHUA - ACTIVITY RHYTHM AND CHEMICALLY MEDIATED FOOD SEARCH

I studied the feeding behaviour of cad in late spring in a fjord in no rthern Norway by means of a stationary positioning system. The activit y rhythm of eight cod tagged with acoustic transmitters was observed c ontinuously for 3-8 days in terms of swimming speed and size of area o ccupied. I studied responses to food odour by setting a line with mack erel. baits in the experimental area. The cod had a diurnal rhythm of swimming activity with a slightly lower swimming speed during a 5-h pe riod at night, indicating that they searched more actively for food du ring 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 behavio ur. The diurnal rhythm in the range of area occupied varied more than the swimming activity rhythm and there was no significant difference i n range between day and night indicating that factors other than the d iel light cycle also influenced periodicity in activity. A higher prop ortion 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 m etres, and responded immediately to the odour plume by carrying out a rheotactic search at a moderately higher swimming speed. (C) 1998 The Association for the Study of Animal Behaviour.