Can the risk of parasite infection in fish cause the ecological differentiation in the population of roach Rutilus rutilus (Cyprinidae)?

Population characteristics of macroparasite infection (prevalence and intensity) in juvenile roach Rutilus rutilus have been studied in one of the tributaries of the Rybinsk Reservoir. The level of infection and parasite diversity is higher in the mouth part than in the upper reaches of the river. Habitats in the upper reaches are characterized by a high degree of irregularity of the risk of infection. The intensity of parasitic infection varies from the absence of parasites to 100% infection. Our data confirm the hypothesis about adaptaive value of the spawning migration to habitats with lower risk for the infection of offspring with parasites. Heterogeneous conditions in the upper reaches of the river make it possible for juvenile fish from a migrating part of the population to grow in relatively safe conditions. Differences in the strategy of the spawning migration can be based on the ecological differentiation as a result of which the groups oriented either at the local adaptation or at migration to the other habitat are formed in the population.