Nematode infections of maricultured and wild fishes in Danish waters: A comparative study

Rainbow trout (Oncorhynchus mykiss) from a Danish mariculture system (a total of 166 fish) and from a freshwater system (a total of 40 fish before transfer to the marine system) were investigated for the presence of nematode larvae through a 1 year cycle. Correspondingly, wild fishes (a total of 160 fish) (cod, whiting, saithe, haddock, hake, flounder, turbot, common sole, lemon sole, witch, plaice, herring, mackerel, garfish, angler fish, and European eel) obtained from the same geographic region were examined. Morphological and molecular techniques were applied for diagnosis. No nematode larvae were recovered from the aquacultured fish. In contrast, anisakid larvae (Anisakis simplex) occurred frequently in wild fishes (body cavity and musculature) although at varying intensity. Other anisakid species (Pseudoterranova decipiens and Contracaecum sp.) were also found in the wild fishes but at lower frequency. Examination of the stomach content from the cultured trout confirmed that aquacultured fish preferentially feed on pelleted dry feed although algae, invertebrates and small fish occurred in a few cases. The background for the absence of nematode larvae in maricultured rainbow trout (low infection risk due to feeding using heat-treated feed without any infective parasite larvae) and the implications for food safety issues (lower likelihood of infected fish from mariculture systems) are discussed.