Ichthyophthiriasis of Atlantic salmon, Salmo salar L., at the Montta Hatchery in northern Finland in 1978–1979

Abstract. The occurrence of the ciliated protozoan Ichthyophthirius multifiliis on Atlantic salmon yearlings at a hatchery in northern Finland is described. During August 1978 a severe epizootic occurred in two earth ponds resulting in mortality of over 50%. Water temperature at this time was about 15°C. Fish were treated with a 1:4000 formalin bath administered every 3 days. Parasites disappeared from the fish by October after which they were refractory to further infection. The parasite occurred in other stocks in 1979 but was successfully controlled with formalin. Some aspects of the ecology of I, multifiliis are discussed.