Infection of Atlantic salmon, Salmo salar L., by Gyrodactylus salaris , Malmberg 1957, in the River Lakselva, Misvær in northern Norway

Gyrodacrylus salaris was most probably introduced to the River Lakselva in 1975 through stocking of Atlantic salmon from an infected hatchery. The parasite population grew rapidly, and the parasite spread throughout the entire watercourse during the summer of 1976. This epidemic situation led to mortality among the young Atlantic salmon, and the density of salmon parr was heavily reduced from 1976 to 1977. The density of salmon parr has remained close to zero since then, while there are no apparent trends toward decrease or increase in the density of brown trout. In spite of the reduced density of young salmon, a new epidemic has developed each year among the few young 0+ and 1+ Atlantic salmon present in the river. Results from successive sampling during the summer of 1987, 1988 and 1989 indicate that most of the presmolt salmon are attacked during their first summer or autumn of life. The infection develops into an epidemic during the first autumn, winter or the next summer. The build‐up of the parasite burden on the fish leads in turn to mortality. Norwegian Atlantic salmon probably have no resistance against G. salaris, since the parasite has recently been introduced to Norwegian rivers.