In the last five years the fauna of fish parasites of Lake Ladoga, the largest freshwater body of Europe, has been studied. The research now covers 50% of all ichthyofauna. Some preliminary conclusions can already be made. Parasitological data suggest that certain shifts have occurred in the development of hydrofauna of Lake Ladoga, at least its offshore areas, in a multiyear period of about 50 years. A tendency towards the increase in the abundance of parasitic infusorians (Apiosoma, Trichodina, Epestylis), genus Diplostomum trematodes, entomostracans Ergasilus sieboldi and Caligus lacustris is observed. At the same time, some parasites are now recorded less often and in smaller amounts (Diphyllobothrium, Phyllodistomum). Infestation of fish with parasites whose development involves crustaceans (M.salmonis, C.farionis) has decreased. These and other changes in the fish parasite fauna of Lake Ladoga through the multiyear period are incontestably related to the growing eutrophication of the water body.

Still, the process of eutrophication is characteristic of the offshore areas while the lake on the whole has preserved its oligotrophic status. The lake possesses the whole complex of relic hydrofauna. Parasite fauna of some fish species is characterized by the great species diversity not achieved in other lakes. The greatest similarity is observed between this water body and Lake Onega. The list of parasites in the lake after the studies are finished will probably total no less than 300 species. This value of biodiversity of the fauna is probably due to the preservation of the relic elements as well as the immigration of many representatives from the southwest areas. The most peculiar feature of Lake Ladoga is the presence of genus Corynosoma acanthocephals absent from other Karelian lakes.

A large oligotrophic water body of Janisjarvi located in the Lake Ladoga vicinity and with an outlet into the lake is characterized by a poorer species composition of the fish parasite fauna. The total of 120 species were recorded there (Rumyantsev, Maslov, 1985). The water body is also noted for the low quantitative indices of fish infestation with the parasites which represent the boreal foothill and arctic freshwater faunistic complexes. This situation is caused by the low mineralization of the water body and, above all, its considerable humification. Differences in the fish parasite faunas of the two water bodies are quite explicable knowing that they belong to different classes of oligotrophic lakes. Lake Ladoga is a Gammarus-inhabited water body, Janisjarvi - an oligotrophic dystrophy-bound one. Another fact to mention is the presence of a dangerous parasite of salmonids, the monogenean Gyrodactylus salaris, in the Janisjarvi basin. It caused the mass infestation of landlocked salmon parr bred on the North-Ladoga fish farm.

Lakes Ladoga and Janisjarvi, which are unique freshwater bodies adjacent to the so-called "green zone" of Karelia in the border zone between Russia and Finland certainly deserve to be granted the status of protected natural water areas.