Increasing occurrence of anisakid nematodes in the liver of cod (Gadus morhua) from the Baltic Sea: Does infection affect the condition and mortality of fish?

Cod (Gadus morhua) taken from the southern Baltic Sea between 2011-2014 were visually examined for the presence of anisakid nematodes in the liver (Anisakis sp. and Contracaecum sp.). The prevalence of infection increased with year of sampling, and depended on the length of the fish. The most abundant parasites were Contracaecum sp. larvae. Both the prevalence and intensity of infection increased with fish length up to 70-80 cm but then began to decline, exhibiting dome-shaped dependence. The decline may be attributed to the increasing mortality of large and heavily infected cod. The body condition of infected cod was lower than that of those free of parasites and declined with the intensity of infection; the condition of most infected fish was up to 20% lower than that of uninfected individuals. The biological basis for the negative effects of infection on the condition and survival of cod and the role of the grey seal Halichoerus grypus in the dispersion of anisakid nematodes in the Baltic Sea are discussed.