Epizootiology of protozoans in farmed salmonids at northern latitudes

Protozoan ectoparasites were examined in a northern salmonid fish farm over a 10-year period, June 1984–May 1994, by the same researcher, with similar catching and sampling procedures throughout. Husbandry procedures remained constant during the study, e.g., fingerlings were kept in steel tanks and yearlings in both steel tanks and earth ponds. Ichthyobodo necator, Chilodonella hexasticha and Ichthyophthirius multifilürs infections were treated with formalin, salt and malachite green-formalin baths, respectively, whenever any parasites were found. Altogether 10 790 randomly sampled salmon ( Salmo solar), sea trout ( S. trutta m. trutta) and brown trout ( S. trutta m. lacustris) were studied. Higher prevalences were found in yearlings than in fingerlings, except in I. necator infections, which were higher in fingerlings (e.g., 26% vs 6% in sea trout). C. hexasticha occurred less often and was found most commonly on brown trout fingerlings. Trichodina nigra occurred more often in salmon of both age groups and Riboscyphidia arctic in trout. The results show that the occurrence of protozoan parasites in a fish farm is predictable and is influenced by the fish species, the age group of the fish, the season and the tank type. Parasite burden increased up to 7 species per brown trout, e.g., when fish were studied from hatching until stocking at the age of 2 years.