Gill diseases of cultured salmonids in Ontario.

Between 1977 and 1981, the Fish Pathology Laboratory of the Ontario Veterinary College received 239 cases from trout farms of southern Ontario, 51 (21.3%) of which had diseased gills. Branchial lesions in 86.3% of these 51 cases were characterized by marked lamellar epithelial hyperplasia with epithelial hypertrophy and lamellar fusion. Filamentous bacteria were seen on the surface of the branchial filaments and lamellae in 68.6% of the cases. Our observations highlight the importance of gill diseases as a production problem of farmed salmonids in southern Ontario.