Effects of low salinities on Flavobacterium columnare infection of euryhaline and freshwater stenohaline fish

Channel catfish, Ictalurus punctatus (Rafinesque), goldfish, Carassius aura tus (L.), striped bass, Morone saxatilis (Walbaum), and Gulf sturgeon, Acip enser oxyrinchus desotoi Vladykov, were acclimatized to fresh water or sali nities of 9.0 parts per thousand or less and then exposed to Flavobacterium columnare (formerly known as Flexibacter columnaris), the bacterial pathog en that causes columnaris disease. None of the fish acclimatized to 3.0 or 9.0 parts per thousand salinity died, and all deaths in lower salinities oc curred between 1 and 5 days after exposure to F. columnare. Mortality was 9 7.7% in fresh water and 67.1% in 1.0 parts per thousand salinity for channe l catfish (model SE, 1.8) and 66.5% in fresh water and 40.8% in 1.0 parts p er thousand salinity for goldfish (model SE, 1.2); and 96.9% in fresh water and 61.7% in 1.0 parts per thousand salinity for striped bass (model SE, 1 .8). After exposure to F. columnare, none of the Gulf sturgeon died. Flavob acterium columnare was isolated from the skin and gills of all fish dying d uring the experiments, but was not isolated from survivors in fresh water a nd 1.0 parts per thousand salinity 21 days after bacterial exposure. In vit ro growth of bacteria was significantly higher in 1.0 or 3.0 parts per thou sand salinity than in control medium (0.3 parts per thousand salinity). How ever, in vitro adhesion of bacteria was reduced with increasing salinity, w hich could explain the lower mortality of fish at higher salinities.