Contracaecum osculatum (Nematoda: Anisakidae) in fish and seals in Bothnian Bay (northeastern Baltic Sea)

The occurrence of C. osculatum in 7400 fish specimens of 30 species and in two seal species is reported from Bothnian Bay, Baltic Sea. Seven fish species were infected with C. osculatum, the highest prevalences occurring in the salmon, bull-rout, burbot and cod (20%, 20%, 16% and 15%, respectively). Prevalences tended to increase in the larger fish, but no seasonal variations were found in either the prevalence or the intensity of infection. C. osculatum occur and mature only accidentally in the resident seal species, the ringed seal ( Phoca hispida botnica), but a mean of 640 worms were found in the grey seal ( Halichoerus grypus) with a maximum of 1244 worms. Grey seals regularly visit the present study area for a few weeks in late spring and it is thought that these serve to maintain the C. osculatum infection in the fish of the area. The nematodes were found in the stomachs of the grey seals, where they tended to occur in aggregations of both third- and fourth-stage larvae and adult worms simultaneously. The presence of third-stage larvae demonstrates that the origin of the C. osculatum infection in these seals may also be in Bothnian Bay. in addition to the more southerly areas where grey seals are to be found for most of the year.