Agonistic Behavior of Young Perch Perca fluviatilis: The Effects of Fish Size and Macroparasite Load

— The paper reports the study of macroparasite load and aggressiveness of the European perch Perca fluviatilis juveniles of different size captured in the Rybinsk Reservoir (Volga River). A total of 13 macroparasite species have been identified with the Tylodelphys sp . and Ichthyocotylurus sp. trematodes and the Triaenophorus nodulosus cestode being the most abundant ones. A statistically significant correlation between the invasion intensity and fish size and aggressiveness has been found only for T. nodulosus . The possible mechanisms underlying T. nodulosus plerocercoid effects on the size and behavior of their fish hosts, the parasite transmission to the final host, and phenotypic differentiation in perch populations are discussed.