Bacteria Associated with Fin Rot Disease in Hatchery-Reared Atlantic Salmon (Salmo salar)

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Abstract
Fin rot is a progressive necrotic disease of many fish species. Several bacterial have been implicated in the etiology of the disease. To determine the bacteria associated with the disease in Atlantic salmon, fish were randomly sampled at weekly intervals at 2 hatcheries from Aug. 1977-April 1978. The degree of fin rot was scored numerically by a standard method and bacteria were enumerated by plate count on several media. Representative colonies were isolated for identification. The extent and severity of fin rot increased as the water temperature decreased. The average total colony counts from healthy fins of fish at both hatcheries were low (2.5 .times. 103/g) and included a variety of bacteria. The average totoal colony counts from fins with severe rot were high (2.0 .times. 105/g) with 1 or more Flexibacter spp. and Aeromonas salmonicida accounting for 50-90% of the total. Statistical analysis showed a highly significant positive correlation of fin rot with A. salmonicida colony counts at hatchery I; the disease was correlated only with Flexibacter colony counts at hatchery II. High colony counts of Flexibacter sp. and A. salmonicida were correlated with low water temperatures at the respective hatcheries. EM examination of fins with advanced lesions revealed evidence of tissue necrosis surrounding bacteria. While no relationship was observed between bacteria and the onset of fin rot disease, evidence was obtained implicating at least 2 bacteria as probable etiological agents of progressive disease.

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