Sexual differences in the incidence and severity of ectoparasitic infestation of the brown trout, Salmo trutta L.

Hatchery‐reared and wild brown trout, Salmo trutta L., were examined for skin ectoparasites during their spawning period in 1977 and 1978. A total of eight genera of parasites, comprising five ciliates, one flagellate, one monogenean and one parasitic fungus, were identified with as many as five different genera occurring on a single fish. Sexually mature male fish were more frequently or more severely infested by species of Ichthyophthirius, Scyphidia, Gyrodactylus and Saprolegnia than were immature fish of either sex or mature female fish. The differences in ectoparasitic loading could not be correlated with known, seasonal changes in the mucus‐producing potential of the epidermis. These findings are discussed in relation to the defence mechanisms of teleost fish and to some of the endocrinological changes that occur in salmonid fish during the spawning season.