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

Abstract

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: (1) 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.

Experimental infection of coho salmon Oncorhynchus kisutch by exposure of skin, gills and intestine with Piscirickettsia salmonis

“…Despite the absence of wounds in the skin of the experimental fish, Piscirickettsia salmonis may have initiated their infection process by attaching to the surfaces of microscopic injuries which are frequently found in cultured salmon (Turnbull et al 1996). Kiryu & Wakabayashi (1999) reported that in intact rainbow trout, latex microspheres only adhered to the surface of natural microscopic injuries of these fish and not to healthy sites of the skin properly covered by the cuticular secretion.…”

Factors Affecting Fin Damage of Farmed Rainbow Trout

“…Water temperature plays a role in the process of fin damage and affects the rate of healing and regeneration, but there is no common trend and the effects differ between species (32,35). If the effect on all fins were combined, increased WT resulted in higher degrees of fin damage (Fig 1b), nota bene: within the temperature ranges included in this study.…”

Fin development in stream- and hatchery-reared Atlantic salmon

“…The pectoral and dorsal fins were most affected by the hatchery environment and were as much as 60% shorter than in stream-reared fish by the third sampling. In some instances, the pectoral and dorsal fins exhibited bleeding, exposed fin rays and whitened margins, attributes of hatchery-reared Atlantic salmon (Schneider and Nicholson, 1980;Turnbull et al, 1996). The pelvic and anal fins were 10-20% shorter by the third sampling, but displayed less extensive trauma than either the pectoral or dorsal fins.…”