The Role of Different Fish Species in the Conservation of Hemipopulations of Metacercariae of the Trematode Apophallus donicus (Skrjabin et Lindtrop, 1919) in the Ecosystems of the Rybinsk and Gorky Reservoirs (Upper Volga Basin)

This article describes a circle of the second intermediate hosts of the alien trematode Apophallus (=Rossicotrema) donicus Skrjabin et Lindtrop, 1919, as well as patterns of the development of rossicotremosis (a type of “black-spotted” fish disease) in the Upper Volga foci in 2019–2022. Using the example of the Rybinsk and Gorky reservoirs, the host differences in the distribution of A. donicus metacercariae in pelagic and coastal groups of the most abundant Percidae species are considered. The most common second intermediate host of A. donicus in both reservoirs was the perch Perca fluviatilis Linnaeus, 1758. It is shown that the high variability in the occurrence of A. donicus metacercariae usually reflects the mosaic pattern of distribution of the first intermediate host of the trematode, the Ponto-Azov prosobranch mollusk Lithoglyphus naticoides (C. Pfeiffer, 1828). In the case of the Rybinsk Reservoir, we found no evidence of significant foci of rossicotremosis outside the relatively small area of the Volga Reach. In adult perch individuals capable of making relatively long feeding migrations, the prevalence of metacercariae in this section of the reservoir was 22.73%. In rarely migrating coastal groups of perch fingerlings, the prevalence of metacercariae ranged from 3.57 to 96.47% at different sites of the Volga Reach. A wider spread of rossicotremosis was observed in the Gorky Reservoir. Even in pelagic trawl catches from the lake-like section of this reservoir, the prevalence of metacercariae in perch fingerlings was found to be between 5.00 and 47.83%. A very large focus of rossicotremosis in the Gorky Reservoir formed upstream of the lake-like expansion, in the area affected by heated waste waters from the Kostroma State District Power Plant, where only juvenile specimens of zander, Sander lucioperca (Linnaeus, 1758), were insignificantly infected—0.49%. At the same time, the prevalence of metacercariae in the integrated sample of perch individuals of various ages was 92.31%, while in the integrated sample of different-aged ruff, Gymnocephalus cernuus (Linnaeus, 1758), it reached 100%. The clearly pronounced localization of rossicotremosis foci may be attributed to the relatively high thermophilic nature of the trematode A. donicus, the limited circle of its second intermediate hosts, and their relatively low population densities. In addition to Percidae species, monitoring of the common parasite fauna, even in the rather large Rybinsk Reservoir, revealed A. donicus metacercariae only in one adult specimen of pike, Esox lucius (Linnaeus, 1758), and one fingerling specimen of the freshwater tubenose goby, Proterorhinus semilunaris (Heckel, 1937).