Cystidicola farionis as a swimbladder parasite of the whitefish in the Bothnian Bay

Material comprising 3167 specimens of the parasite Cystidicola farionis was assembled from 1741 individuals of the sea‐spawning whitefish, Coregonus nasus in the shallow northeastern and deeper central areas of the Bothnian Bay. Occasional parasites were found in the immature fish (≦ 175 mm), but infection increased in the two larger classes as the fish aged, reaching 27% in the northeastern area, and as much as 67% in the southern area. This difference is explained partly by the more frequent feeding on Pontoporeia affinis , the intermediate host for the parasite, the deeper waters extending nearer the coast in the majority of the central and southern Bothnian Bay. The increased intensity of infection with age does not vary significantly between the areas studied. No clear seasonal fluctuations in the prevalence or intensity of infection could be found.