DIETARY MENHADEN OIL REDUCED RESISTANCE OF CHANNEL CATFISH (ICTALURUS-PUNCTATUS) TO EDWARDSIELLA-ICTALURI

Fingerling channel catfish were fed practical-type diets supplemented with 2% catfish offal oil, beef tallow, or menhaden oil for 8 or 10 weeks at 24 +/- 1 degrees C under laboratory conditions in two separate experiments. At the end of the feeding period, fish were exposed to live Edwardsiella ictaluri by immersion and mortalities were recorded daily for 3 weeks. Growth, feed efficiency, and survival rate of channel catfish at the end of the feeding period were not significantly different among dietary treatments in either experiment. Fish fed a diet supplemented with menhaden oil had significantly higher levels of 20:5n-3, 22:6n-3, and total n-3 HUFA (highly unsaturated fatty acids), and lower levels of 20:4n-6 and total n-6 HUFA in liver polar lipids than fish fed the other two diets. Average mortality of fish exposed to E. ictaluri was 21 and 15% higher for fish fed supplemental menhaden oil than for fish fed supplemental catfish offal oil in Experiments 1 and 2, respectively. Antibody titers were not different among fish regardless of diet, 3 weeks after exposure to E. ictaluri. The results of this study indicate that channel catfish fed practical diets containing 2% menhaden oil are more susceptible to E. ictaluri. These results do not preclude the use of menhaden fish oil in catfish diets, but it may be prudent to use a mixture of menhaden oil and other animal and/or plant lipids in commercial catfish feeds to provide a balance in the dietary n-3/n-6 ratio.