Studieswere conducted to determine aspects of water chemistry responsible for large differences in pathogenesis
and mortality rates in experimental challenges of channel catfish Ictalurus punctatus with Flavobacterium
columnare; challenges were conducted in water supplying either the Stuttgart National Aquaculture Research
Center (SNARC) in Stuttgart, Arkansas, or the Warmwater Aquaculture Research Unit (WARU) in Stoneville,
Mississippi. Waters differed in two major characteristics: compared to SNARC water, WARU water contained
appreciable dissolved organic matter (DOM) and very low concentrations of dissolved calcium and magnesium.
In Experiment 1, fishwere exposed to a bacterial challenge of F. columnare in aquariawith either SNARC orWARU
water. Fish mortality at 4 d was 100% for fish in SNARCwater, while no fish died inWARUwater. Bacterial adhesion
to gill surfaces also differed dramatically in the two waters and was approximately 1900-times higher for
fish challenged in SNARC water (more than 800,000 bacterial CFUs/ng extracted DNA) than for fish challenged
in WARU water (fewer than 450 CFUs/ng). Experiment 2 was conducted with a lower bacterial challenge to
determine which factor, DOM or divalent cation concentration, caused the difference in mortality seen in
Experiment 1. Complete removal of DOMfromWARUwater by carbon filtration did not affect bacterial adhesion
to gills nor resulted in greater mortality in challenged fish. Consequently, DOM did not contribute to the differences
in mortality seen in Experiment 1. However, removal of most calcium and magnesium from SNARC
water by ion-exchange filtration resulted in decreased bacterial adhesion to gill and decreased mortality
compared to un-altered SNARC water. It appears that the concentration of divalent cations (hardness) affects
the pathogenesis of columnaris disease.