Metacercariae of the Trematode Apophallus muehlingi (Jägerskiöld, 1899) as Biological Indicators of Migration Activity and Physiological Status of Different Age Individuals of the Black and Caspian Sea Sprat Clupeonella cultriventris (Nordmann, 1840) in Two Upper Volga Populations

The authors used metacercariae of the trematode Apophallus muehlingi (Jägerskiöld, 1899) to assess the migration activity of fingerlings and adults of the thermophilic freshwater form of the Black and Caspian Sea sprat Clupeonella cultriventris (Nordmann, 1840) naturalized in the Upper Volga basin. In the limnic conditions of a large lake-like reservoir (the Rybinsk Reservoir), C. cultriventris showed relatively weak feeding migrations, which is probably due to the need to form local subpopulation spawning groups in small river reaches. This resulted in minimal contacts with littoral settlements of the first intermediate host of the trematode, the Ponto-Azov prosobranch mollusk Lithoglyphus naticoides (C. Pfeiffer, 1828), in 2020, the prevalence of metacercariae in the central part of the Rybinsk Reservoir was only 1.75% in fingerlings and only 2.65% in adult individuals. In 2020, under lotic conditions of the upper section of the Gorky Reservoir, classified as a valley type, the prevalence of metacercariae in fingerling samples varied from 38.46 to 48.95%. This is partially explained by the fact that, in this reservoir, juveniles of C. cultriventris (relatively large dominant individuals) have the ability to make long-distance feeding migrations already in the first months of their life. On the basis of the example of the middle section of the Gorky Reservoir, this paper examines the possibility of using A. muehlingi as a biological indicator for identifying the physiologically weakest individual outsiders in fish samples. In the largest foci of apophallesis, formed in the middle section of the reservoir, the prevalence of metacercariae did not exceed 68.97% in fingerlings, whereas it reached ~100.00% in adult individuals of C. cultriventris. Large age differences, in terms of the prevalence of metacercariae, can be associated with the selective consumption of the most infected and physiologically relatively weak fingerling outsiders by pelagic ichthyophages. Maximally infected (≥50 specimens of metacercariae) fingerlings of C. cultriventris were absent in pelagic trawl catches within the heated discharge water zone of the Kostroma State District Power Plant, but were found in samples collected on the water intake protective grates of the power plant, i.e., among the individuals least prone to migration and not subject to serious pressure from pelagic ichthyophagous fish.