An intense spread of apophallesis in fish of the Gorky Reservoir has been observed after the successful naturalization of the prosobranch mollusk Lithoglyphus naticoides (C. Pfeiffer, 1828) and a number of associated trematodes in this water body (in 2005-2015). A freshwater form of the Black and Caspian Sea sprat Clupeonella cultriventris (Nordmann, 1840), was one of the second intermediate hosts of the trematode Apophallus muehlingi (Jagerskiold, 1899) causing this disease. The aim of the work is to study the consequences of dispersal of C. cultriventris for the formation of large permanent foci of apophallesis in the Upper Volga. Data on the spread of this type of "black-spot" disease in the populations of C. cultriventris from the Gorky Reservoir and an unregulated area of the Lower Volga (downstream of the cascade of Volga reservoirs, pelagic trawl coordinates from 47 degrees 08MODIFIER LETTER PRIME N, 47 degrees 17MODIFIER LETTER PRIME E to 46 degrees 51MODIFIER LETTER PRIME N, 47 degrees 41 ' E) were used for comparison. The high rates of occurrence of A. muehlingi metacercariae (the infection prevalence P, %) are found along the entire area of the Gorky Reservoir: from the northernmost site near the city of Rybinsk (58 degrees 01 ' N, 39 degrees 06MODIFIER LETTER PRIME E) to the dam area (56 degrees 41MODIFIER LETTER PRIME N, 43 degrees 21MODIFIER LETTER PRIME E). It is found that the distribution pattern of A. muehlingi metacercariae varies in the sections of the reservoir differing in hydrological characteristics. The prevalence of A. muehlingi metacercariae in groups of the smallest Black and Caspian Sea sprat fingerlings (with a body length of up to 45 mm) in the middle (transitional type) section of the reservoir is close to the maximum possible (30.00-94.49%). The prevalence of metacercariae in this group of fish in the upper (river-type) section of the reservoir does not exceed 24.39%. The minimum values of this indicator are found in the lower (lake-type) section of the reservoir (6.76-14.84%). Judging by the values of the abundance and the coefficient of variation in the infection intensity, the aggregation of metacercariae is more pronounced in the groups of faster growing underyearlings and yearlings of the Black and Caspian Sea sprat than in the groups of the smallest fingerlings. There were no statistically significant differences between the infection prevalence values (13.33-18.11%) in the different size groups of the Black and Caspian Sea sprat fingerlings from the Lower Volga in 2020. Thus, the presence of C. cultriventris in the water body ecosystem can significantly increase the likelihood of apophallesis spread in some cases, although various fish species of the Cyprinidae family are the main second intermediate hosts of A. muehlingi. A continuous zone of apophallesis can form without clearly defined boundaries between the individual foci of disease in conditions of the Upper Volga owing to the high migration activity of large individuals of the Black and Caspain Sea sprat.
