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°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 20:5 n−3, 22:6 n−3, and total n−3 HUFA (highly unsaturated fatty acids), and lower levels of 20:4 n−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.