Seasonal variation in presence of Vibrio salmonicida and total bacterial counts in Norwegian fish-farm water.

Total bacterial counts and the number of the fish pathogenic bacterium Vibrio salmonicida have been studied in water samples collected twice a month in 12 Norwegian fish farms from October 1988 to June 1989. Total counts were determined by staining with 4',6-diamidino-2-phenylindole followed by epifluorescence microscopy. Cells of V. salmonicida were enumerated with a fluorescent antibody technique using highly specific monoclonal antibodies. Despite the fact that no outbreak of cold-water vibriosis was reported, V. salmonicida was detected in all 12 farms, in numbers ranging from 12 to 43 bacteria/mL. The number of farms where V. salmonicida was detected was generally highest during the winter. Total bacterial counts in the water samples varied between 4 X 10(4) and 9 X 10(5) bacteria/mL and the lowest numbers occurred during the winter period. The total bacterial counts were comparable with counts in water uninfluenced by fish farming. On the basis of our results, and additional information available about cold-water vibriosis caused by the bacterium V. salmonicida, an asymptomatic carrier state of the disease is proposed.