Increasing Fish Oil Levels in Commercial Diets Influences Hematological and Immunological Responses of Channel Catfish, Ictalurus punctatus

Growth performance, immune responses and disease resistance were studied in juvenile channel catfish, Ictalurus punctatus, fed a commercial diet (35.3% crude protein and 5.6% lipid) supplemented with menhaden fish oil at levels of 0, 3, 6, and 9% for 15 wk. Dietary fish oil levels did not significantly influence growth performance of catfish. Fatty acid compositions of whole-body and liver reflected dietary fatty acid composition. No differences were found in hematological values, except that fish fed the 9% fish oil diet had significantly lower hematocrit. The resistance of erythrocytes to hemolysis in hypotonic solutions increased with increasing fish oil levels and the highest resistance was seen with the 9% fish oil diet. Fish fed 6 and 9% added fish oil diets had significantly higher serum protein levels than that of control fish. Serum lysozyme activity of fish fed 3 and 6% added fish oil diet was significantly higher than that of the control. Complement activity and chemotaxis ratio significantly decreased in fish fed diets with 6 or 9% added fish oil. The 3% added fish oil diet, however, had significantly highest natural hemolytic complement activity. Mortality from Edwardsiella ictaluri 14 d postchallenge and antibody titers to E. ictaluri did not differ among treatments. In general, freshwater fish require dietary sources of polyunsaturated fatty acids of the n-6 (linoleic acid, 18:2 n-6) and n-3 (linolenic acid, 18:2 n-3) families for optimum growth. Based on their essential fatty acid requirements, cultured freshwater fish, including channel catfish, are commonly fed grain–soybean meal feeds high in n-6 fatty acids. Based on their essential fatty acid requirement, channel catfish should be fed diets containing 0.5–1.0% n-3 fatty acids (NRC 1993). Feeding studies have shown that channel catfish fed diets supplemented with fish oil had body fatty acid composition, and the ratio of n-3 to n-6 fatty acids positively correlated to their levels in the diets (Abdel-Aty Mohamed 1989; Li et al. 1994; Fracalossi and Lovell 1995; Manning and Li 2002). Suboptimal and/or excessive levels of certain nutrients, including n-3 and n-6 fatty acids, may adversely affect immune response and disease resistance of fish (Lim and Webster 2001). Optimum functioning of the immune system depends on balanced and adequate nutrition. N-3 highly unsaturated fatty acids (HUFAs) such as eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3) are readily incorporated into polar lipids in cellular membranes. Their presence in the cell membrane affects the functioning of immune cells. Increased intracellular killing of the bacteria Edwardsiella ictaluri, the cause of enteric septicemia of catfish, by head kidney macrophages has been demonstrated in channel catfish fed diets containing high levels of n-3 HUFAs (Sheldon and Blazer 1991). Fracalossi and Lovell (1994) obtained increased antibody titers 2 wk after immunization against E. ictaluri in catfish fed a fish oil diet compared with those fed diets containing corn oil, linseed oil, or a mixture of fish oil, corn oil and beef tallow. High dietary levels of n-3 HUFAs, however, can negatively affect fish growth as these fatty acids are readily oxidized by reactive oxygen species to lipid peroxides (Porter et al. 1995). Red seabream juveniles fed high levels of n-3 HUFAs showed low growth rates (Takeuchi et al. 1992). Fracalossi and Lovell (1994) and Li et al. (1994) reported that dietary 1 Corresponding author. JOURNAL OF THE WORLD AQUACULTURE SOCIETY Vol. 40, No. 1