DIETARY REQUIREMENTS OF ORGANIC AND INORGANIC ZINC IN A COMMERCIAL STRAIN OF DIPLOID AND TRIPLOID RAINBOW TROUT Oncorhynchus mykiss
As the world population increases, finding adequate and sustainable food sources is of paramount importance, and aquaculture offers an appealing solution. Among essential nutrients, zinc is an important micro-mineral for fin fish and is required in small quantities, participating in a wide variety of biochemical processes. In aquaculture, triploids are utilized for their sterility to decrease risk of genetic pollution and for their potential for faster growth. However, to our knowledge there is no information regarding zinc requirements for triploid trout, despite support from previous studies that mineral requirements may differ. This experiment aims to determine zinc requirements in a commercial strain of genetically similar diploid and triploid rainbow trout (Oncorhynchus mykiss) using various levels of both organic and inorganic zinc. Eleven diets, including one basal diet (33 mg/kg, Zn33) and 10 experimental diets, with incremental levels of inorganic (ZnSO4) and organic (Alltech) zinc supplemented to the basal diet (Zn63, Zn93, Zn123, Zn153, Zn183) were fed for 9 weeks. Treatments (22 total) were run in triplicate for a total of 66 tanks, with 11 fish per tank (66 l).
Results showed that neither ploidy (triploid vs. diploid) nor type of mineral (organic vs. inorganic) significantly influenced the growth performance of fish. However, in diploid trout, final whole-body protein (%) was significantly higher with organic Zn123 compared to other groups.
Fish fed organic Zn183 had significantly higher lipid retention than those fed Zn33 (control), inorganic Zn153, and organic Zn 123 and a trend of higher lipid retention than inorganic Zn183 (p=0.00387) (Fig. 1). Dietary zinc had no significant effects on histology of the distal intestine or cataract formation and lens histology. Only one fish developed bilateral incipient cataracts. Other parameters being analyzed are density of vertebrae and opercula , mineral analysis of whole-body and vertebrae, oxidative stress enzymes in plasma (superoxide dismutase, glutathione peroxidase) and gene expression in liver (lipoxygenase, NADPH oxidase, NF-kB, xanthine oxidase) and opercula/vertebrae (osteopontin , osteonectin, osteocalcin, calcitonin).
In conclusion , significantly higher nutrient retention was observed in fish fed organic zinc than inorganic zinc. Zinc requirements tended to be numerically higher in the inorganic diets when compared with the organic diets. Results of this study will increase the portfolio of high-quality organic zinc available to the fish feed industry.
