Fróðskaparrit - 01.01.1995, Qupperneq 115

Fróðskaparrit - 01.01.1995, Qupperneq 115
EXPERIMENTAL INFECTION OF MACROPLANKTON FROM FAROESE WATERS 119 several fishes (Pálsson, 1983). This distrib- ution corresponds fairly well with the vast literature reports of A. simplex in fishes (see e.g. Grabda 1974; 1976; Platt 1975; Smith and Wootten 1978). The high levels of the prevalence of infection in krill-eating fish- es species like herring, blue whiting and saithe (see van Banning and Becker, 1978; Højgaard, 1980, 1988, 1995b) is then most likely to be explained by infected M. norvegica. Both Calanus finmarchicus and M. norvegica are common food resources of the herring, Clupea harengus, around the Faroe Islands (Jespersen, 1944). Even if the most important food of blue whiting is Calanus finmarchicus, C. hyperboreus, Thysanoessa inermis and T. longicaudata, M. norvegica constitutes 6.2 % of the food (Timokhina, 1974). The cod, Gadus mor- hua, has also high levels of infection with A. simplex (Rae, 1972; Young, 1972; Platt, 1975; Hauksson, 1992 ). In mature cod one major source of infection will be other prey fishes like the blue whiting (Pálsson, op.cit.), but M. norvegica is also playing a very important role in its diet (Astthorsson and Pálsson, 1987). Experimental infection of fishes with A. simplex from other infect- ed fishes has been demonstrated by Smith (1974) and so most likely also will happen in nature. In conclusion, heavily infected fishes as a rule either seem to be infected through M. norvegica (and possibly Thysa- noessa spp., too) or through infected fish prey. The whales are final hosts in the A. sim- plex-life cycle. An example, is the fin whale, Balaenoptera physalus, which in Davey (1971) is listed as a final host of A. simplex and is predating on M. norvegica (Relini et al., 1992 and Melle et al., op.cit.) Unsolved questions are whether frnal hosts like baleen whales are infected with A. sim- plex directly from their main food, krill, or by accidentally eaten, small fishes. In summary the results presented here are intended to contribute with some clues for further work in the uncovering of the still obscure parts of the A. simplex life-cy- cle in the macroplankton. A lot of experi- mental work still has to be done for to clar- ify the unknown details and mechanisms of infection. Future research in this area should also include systematic examina- tions of M. norvegica for A. simplex larvae in various regions of the North Atlantic. This could provide more detailed explana- tions of the dynamics and relationships be- tween the distribution pattems of A. sim- plex in the euphausiacean intermeditae hosts and in their different fish predators. Acknowledgements I thank the crew onboard “Magnus Heinason” for help in the catching of macroplankton. I am grateful to the staff at Kaldbak Marine Biological Station for different kinds of help during the experiments, especially to Bjørki Geyti, who kindly and skillfully constructed the sampling gear and some of the equipment for the infec- tion experiments. I am indebted to Hjalti í Jákupsstovu, head of Fiskirannsóknarstovan, and to Paul Aspholm, Oslo, for reađing and critisizing the manuscript. The work was supported by the Carlsbeg Foundation (Carlsbergfondet), The Scientific Foundation (Vísinda- grunnur Føroya Sparikassa) and The Scientific Founda- tion for Fishery (Grunnurin fyri Vísindaligum Fiski- vinnukanningum).
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