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

Channel catfish, Ictalurus punctatus (Rafinesque), goldfish, Carassius auratus (L.), striped bass, Morone saxatilis (Walbaum), and Gulf sturgeon, Acipenser oxyrinchus desotoi Vladykov, were acclimatized to fresh water or salinities of 9.0 parts per thousand or less and then exposed to Flavobacterium columnare (formerly known as Flexibacter columnaris), the bacterial pathogen 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 occurred between 1 and 5 days after exposure to F. columnare. Mortality was 97.7% in fresh water and 67.1% in 1.0 parts per thousand salinity for channel catfish (model SE, 1.8) and 66.5% in fresh water and 40.8% in 1.0 parts per 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. Flavobacterium columnare was isolated from the skin and gills of all fish dying during the experiments, but was not isolated from survivors in fresh water and 1.0 parts per thousand salinity 21 days after bacterial exposure. In vitro growth of bacteria was significantly higher in 1.0 or 3.0 parts per thousand salinity than in control medium (0.3 parts per thousand salinity). However, in vitro adhesion of bacteria was reduced with increasing salinity, which could explain the lower mortality of fish at higher salinities.