Submitted to: American Fishery Society (Fish Health Section) Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: January 30, 2007
Publication Date: N/A
Technical Abstract: One of the goals of the ARS Catfish Genetics Research Unit is to incorporate disease resistance to ESC, caused by the bacterium, Edwardsiella ictaluri, into our selective breeding program. Through repeated experiments we have determined an optimal challenge dose of E. ictaluri that produces 50-70% mortality in susceptible strains, while causing only 10-30% mortality in resistant strains. During late spring and into early fall these mortality rates remain reproducible, but with the onset of late fall and through early spring, they become increasingly variable in juvenile catfish. In an effort to determine if the variability could be overcome, we performed two studies designed to examine the effects of stocking density, challenge dosage, and fasting on mortality during winter challenges. Juvenile channel catfish weighing an average of 19.4 grams were stocked at 50, 40, 30 and 20 fish per 76 liter tank, challenged with the standard dosage (1X) or double the standard dosage (2X) of E. ictaluri (5 X 106 CFU/ml or 1 X 107 CFU/ml, final concentration), and observed for mortality for 21 days. In a separate study, 12 tanks were stocked with 20 fish weighing an average of 18.0 grams. The fish were challenged with the 2X dosage of E. ictaluri (1 X 107 CFU/ml), then half of the tanks were maintained fasting while the other half were fed to satiation for 21 days during which they were observed for mortality. Stocking density had no significant effect on mortality when fish were challenged with the 2X dosage of E. ictaluri, and at stocking densities of 40 and 50 fish there were no significant differences in mortality rates between the 1X and the 2X dosage. Significant dosage effects on mortality rates were seen in tanks stocked with 20 (p=0.05) and 30 fish (p=0.005), where mortality rates in fish challenged with the 1X dosage were half that seen in the fish challenged with the 2X dosage. Fish that were fasted showed significant differences in mortality rates, with fed fish having an average of 35.8% mortality, while the fasting fish had an average of 7.5% mortality (p=0.0001). These results indicate that variation in mortality rates during winter challenges can be reduced by increasing dosage rates, using higher stocking densities, and maintaining feeding regimens during challenges.