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

Abstract

Fin rot is a progressive necrotic disease of many fish species. Several bacteria 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 two hatcheries from August 1977 to 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 × 103/g) and included a variety of bacteria. The average total colony counts from fins with severe rot were high (2.0 × 105/g) with one or more Flexibacter species and Aeromonas salmonicida accounting for 50–90% of the total. Statistical analysis showed a highly significant positive correlation of fin rot with Aeromonas salmonicida colony counts at hatchery I, whereas the disease was correlated only with Flexibacter colony counts at hatchery II. High colony counts of both Flexibacter sp. and A. salmonicida were correlated with low water temperatures at the respective hatcheries. Electron microscopical examination of fins with advanced lesions revealed evidence of tissue necrosis surrounding bacteria. Thus, while no relationship was observed between bacteria and the onset of fin rot disease, evidence was obtained implicating at least two species of bacteria as probable etiological agents of the progressive disease.Key words: bacteria, fin rot disease, Atlantic salmon