Gross, histological and scanning electron microscopic appearance of dorsal fin rot in farmed Atlantic salmon, Salmo salar L, parr

The gross, histological and scanning electron microscopic appearance of dorsal fin rot in farmed Atlantic salmon parr, Salmo salar L. is described. The lesions were grouped into seven categories: (I) peripheral erosion and ray splitting; (2) peripheral erosion with some nodularity; (3) severe nodularity with differing degrees of tissue loss; (4) extensive to total loss of the dorsal fin; (5) smooth thickening of the dorsal fin; (6) haemorrhagic dorsal fin lesions; and (7) healed dorsal fin rot lesions. The main sign of injury was clefts extending through the epithelium. These injuries were consistent with bites from other parr. During healing from such wounds, damaged cells sloughed from the surface, and there was swelling and hyperplasia in the remaining cells. The majority of the thickening in the fins was the result of epithelial hyperplasia with a variable cellular inflammatory response. The distal epithelium of fins with severe fin rot (i.e. nodular and eroded) was rough, irregular and swollen with superficial nodular extensions. Wounds in all stages of repair were more numerous in such areas. Fin rays were frequently observed protruding from the abnormal epithelium at the distal edge of the fin. With the exception of the isolation of Aeromonas salmonicida from a small number of cases, no significant bacterial involvement was detected. Under scanning electron microscopy, bacteria were only detected on the exposed fin rays and not in association with the abnormal epithelium. In the majority of cases, the dorsal fin was either the only fin damaged or the most severely damaged. It is suggested that the hyperplastic response to numerous bite wounds is responsible for the accumulation of abnormal epithelium typical of dorsal fin rot in farmed Atlantic salmon parr.