Relationships between the parasites of some wild and cultured fishes in two lakes and a fish farm in central Finland.

A total of 526 Atlantic salmon and 500 brown trout from a fish farm were studied for parasites, as were 272 roach, 251 perch and 150 whitefish from the lake which formed the water source, and 196 roach and 136 perch from the effluent recipient lake. The cultured fish harboured 14 parasite species, of which most were protozoan generalist parasites. Nine species were found on salmon and 12 on brown trout. Epizootically the most important species were Ichthyobodo necator, Chilodonella cyprini and Ichthyophthirius multifiliis. The mean number of species per studied fish was 0.71 for salmon and 0.61 for brown trout. The seasonal occurrences of the ectoparasites were found to be irregular due to preventative bathing of the fish. The wild whitefish harboured 19 parasite species, roach 41 and perch 33. The proportions of the parasite species common to both wild and cultured fishes were 17% of the species from roach, 36% of those from perch and 47% of the species from whitefish. Jaccard's similarity index, indicating similarities between wild and farmed fish, was highest between whitefish and brown trout. It was concluded that the source of parasites in the fish farm is the water supplying lake, but the farm itself was unlikely to effect the fish parasite fauna of the water recipient lake, although some ectoparasites, which had high prevalences in this lake, could originate from the farm. The high standard of maintenance and hygiene in this farm kept problems caused by parasites under control.