Seasonal occurrence of Gyrodactylus derjavini (Monogenea) on brown trout, Salmo trutta, and Atlantic salmon, S. salar, in the Sandvikselva river, Norway.

Seasonal variation in prevalence and intensity of the ectoparasite Gyrodactylus derjavini on brown trout parr, Salmo trutta, and Atlantic salmon parr, S. salar, and occurrence of G. derjavini on ascending sea trout, S. trutta m. trutta, in the Sandvikselva river, southeastern Norway, was recorded. In general, both prevalence and intensity increased and decreased correspondingly with the rise and fall in water temperature. However, both prevalence and intensity decreased in warm periods when reproduction and transmission should be high, possibly from host-induced parasite mortality. Sea trout became infected with G. derjavini soon after they entered the river; prevalence tended to increase as the sea trout migrated upstream to the spawning grounds. It is hypothesized that transmission of G. derjavini to sea trout occurred via the river substratum.