Triaenophorus nodulosus and T. crassus in fish from northern Finland

10,288 fish specimens of 34 species from three water bodies in northern Finland were studied mainly in 1978-1982 Triaenophorus nodulosus and T. crassus parasites. Six of the 31 species from the Bothnian Bay were found to harbour T. nodulosus plerocercoids, four of the 11 from the cold, oligotrophic Lake Yli-Kitka in northeastern Finland and only one of the 6 from the small, eutrophic Lake Kuivasj?rvi. The ruff (Gymnocephalus cernuus) was most essential intermediate host in the lakes, as supported by stomach analysis of predator fish. Although the highest prevalences were found in adult burbot (Lota lota) in two of the areas, these fish are more likely a blind end in the life cycle of T. nodulosus in the present case. T. crassus larvae were found only in three coregonid species and once in the intestine of a lamprey (Lampetra fluviatilis) from the sea. Neither the prevalence nor the intensity of infection in most of the intermediate fish hosts of T. nodulosus was seen to increase with the length of the fish. The only exception was the ruff in Lake Yli-Kitka, where a sharp increase was encountered. No prominent seasonal variation in Triaenophorus larvae infections was found in any of the cases. The prevalence of T. nodulosus infection in its final host, the pike (Esox lucius), was highest in the sea (93%) and lowest in the eutrophic lake (47%). Seasonally, the lowest T. nodulosus infections were found in June-July in the two lakes. A clear maturation cycle was also found, with the youngest worms in the summer months in all cases. T. crassus was found in every third pike in the sea and its proportion in relation to T. nodulosus is higher in the smallest and largest fish. In only 12%t of the pikes did the number of T. crassus exceed that of T. nodulosus, however, the mean ratio being 1:13 to favour of T. nodulosus. The results give an indication that the composition of the fish fauna in each water body has a considerable influence on the T. nodulosus burden and its distribution among its potential intermediate fish hosts. With the most stable environmental conditions and the greatest variety of fish species, the parasite burden in the Bothnian Bay is most evenly distributed among the 6 intermediate hosts. The fewer fish species occur in the water body, as in the present eutrophic lake, the more the significance of a few or even one intermediate host will increase.