Dietary composition and the potential of food competition between 0- group cod (Gadus morhua L.) and some other fish species in the littoral zone

Abstract

0-group cod (Gadus morhua L.), 0-group whiting (Merlangius merlangus L.) goldsinny wrasse (Ctenolabrus rupestris L.), and black goby (Gobius niger L.) were caught near Arendal in August and October 1991. Dietary composition showed that 0-group cod mainly ate calanoid copepods, gammarids, shrimps or prawns, crabs, and fish. They were generalists with a wide feeding niche. Mostly active benthic, hyperbenthic, and pelagic prey were ingested. 0-group whiting mostly ate catenoid copepods, hyperids, crabs, appendicularians, and fish. They were more specialised feeders than 0-group cod. On some occasions, whiting ate large amounts of megalopas and catenoid copepods. Prey were active benthic, hyperbenthic, or pelagic and consisted of more pelagic prey and fish than that of any of the other fish species. Goldsinny wrasse were generalists and mostly ate bryozoans, gastropods, bivalves, caprellids, gammarids, shrimps or prawns, and crabs. Prey were active, slow moving, or sedentary, and mostly benthic or hyperbenthic. Black goby mostly ate gammarids, crabs, and fish, but many of them also ate gastropods, bivalves, shrimps or prawns, insects, and polychaetes. They were also generalists with a wider feeding niche than 0-group cod and 0-group whiting. Black goby ate active, slow moving, or sedentary prey, mostly benthic or hyperbenthic. Dietary composition differed significantly between locations in each period and between periods in each location for all species (except black goby). The diets of 0-group cod and 0-group whiting were more similar compared to the diets of the other fish, indicating potential food competition only between these two species. Both ate many fish, but also, on some occasions, many megalopas and the copepod species Calanus finmarchicus. Differences in behaviour may, however, reduce competition.