[Trematode fauna patterns of the freshwater fish on the border of the geographical range]

The trematode fauna of the fishes inhabiting Lake Ladoga are represented by forty-six species; nine species of which are found here on the northern boudary of their geographic range. The depauperization of the trematode fauna compared to the southern pools is a result both of the decreasing impact of the Pont-Caspian group and of the introduction of the boreal piedmont species complex. The increase in the biological productivity of Lake Ladoga for the last 50-year period leads to enrichment of the species biodiversity. The same process increases the invasion extensiveness by trematodes, which use fish as an intermediate host. The Diplostomida, the presence of which is crucial for the young fish survival rate, dominate among the trematodes.