Submitted to: Journal of the World Aquaculture Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2007
Publication Date: 2/1/2009
Citation: Aksoy, M., Lim, C.E., Shelby, R.A., Klesius, P.H. 2009. Increasing fish oil levels in commercial diets influences hematological and immunological responses of channel catfish, Ictalurus punctatus. Journal of the World Aquaculture Society. 40(1):76-86.
Interpretive Summary: Channel catfish is commonly fed grain-soybean meal feeds high in n-6 fatty acids due to its essential fatty acid requirements. 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 with dietary levels. However, 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. Published information on this subject is inconsistent and often contradictory. There is some evidence for immunostimulatory effects of dietary n-3 fatty acids in fish by influencing cell membrane flexibility and permeability, while immunosuppressive effects associated with excessive levels of n-3 HUFA have also been reported in several fish species. To sort out these conditions we evaluated the effects of various levels of menhaden fish oil (0, 3, 6 and 9%) supplemented to a commercial diet (35.3% crude protein and 5.6% lipid) on growth performance, hematology, immune response and resistance of channel catfish to Edwardsiella ictaluri challenge. Each diet was fed to juvenile channel catfish in four aquaria to apparent satiation (i.e., until fish no longer accept feed) twice daily for 15 weeks. Result showed that 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. Red and white blood cells, and hemoglobin were not affected by dietary levels of added fish oil. Fish fed the 9% fish oil diet, however, had lower hemotocrit. Increasing dietary levels of fish oil resulted in increased resistance of red blood cells to hypotonic salt solutions (less electrolyte concentration than that in cells), and the highest resistance was seen with the 9% fish oil diet. Measured immune parameters were enhanced by supplementation of fish oil at levels of 3-6%, however, immunosuppression effects associated with excessive levels of fish oil were also observed at levels from 6% to 9% depending on the immune parameters evaluated. Resistance of fish to E. ictaluri challenge was not influenced by additional levels of fish oil. There is a need for more basic understanding of the lipid metabolism of catfish and the mechanism in which dietary levels of n-3 HUFA influence immune cells.
Technical Abstract: 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 weeks. 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 hemotocrit. 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, on the other hand, had significantly highest natural hemolytic complement activity. Mortality from Edwardsiella ictaluri 14 days post-challenge and antibody titers to E. ictaluri did not differ among treatments.