Finnish salmon resistant to Gyrodactylus salaris: a long-term study at fish farms

The occurrence of Gyrodactylus salaris on Baltic salmon ( Salmo salar), sea trout ( S. trutta m. trutta) and brown trout ( S. trutta m. lacustris) was examined from 1984 to 1993 at 4 fish farms (A, B, C and D) that produce smolts for stocking in northern Finland. No G. salaris was found on the sea or brown trout, but it did occur on salmon for 6–7 years at farms B, C and D, the prevalences of infection being 9.5%, 17.7% and 8.8% for salmon yearlings and smolts during that time, respectively, but less than 1.2% for fingerlings at farms B and C. Only brood stock salmon were infected at farm A in 4 years. The abundances of G. salaris increased during the second winter of each year class and were highest in the spring before stocking. The combination of variables farm, water temperature, age of fish and other ectoparasites did not suffice to explain the occurrence of G. salaris when studied by step-wise logistic regression analysis. It is suggested that other irregular factors at the farm, such as a change in the water intake system, transfers of fish to the farm and the keeping of wild fish at the farm are also important. The present extensive data strongly support previons suggestions that the Baltic salmon is more resistant to G. salaris than salmon migrating into the Atlantic Ocean.