Parasites of Fish Juveniles in a Small River: The Cost of Fish Omnivory

The infestation of juvenile European perch Perca fluviatilis and northern pike Esox lucius with parasites was compared in different reaches of the Il’d’ River (a second-order tributary of the Rybinsk Reservoir, Russia). The main characteristics of the parasite communities changed significantly in both fish species from the upstream to the downstream reaches of the river. The abundance of adults and larvae of parasite species, total parasite prevalence and adult and larvae prevalence, total abundance and abundance of larvae were increasing. This increase was most pronounced in juvenile northern pike. The abundance of parasite larvae, including the most virulent species and manipulator parasites, was higher for European perch in all reaches of the river. Dynamics of fish infestation with adult parasites were less consistent along the river. The reasons for such differences in the infection rates of juvenile European perch and northern pike in the small river are discussed considering the trophic relationships of fish, their feeding, social and migratory behavioral strategies.