Organic and inorganic zinc in the diet of a commercial strain of diploid and triploid rainbow trout (Oncorhynchus mykiss): Effects on performance and mineral retention

Abstract

Zinc is an important micro-mineral for finfish required in trace amounts and is involved in a wide variety of biochemical processes. In aquaculture, triploid fish are utilized for their sterility to decrease the risk of genetic pollution to wild populations and for their potential for faster growth. However, to our knowledge there is no information regarding zinc requirements for triploid rainbow trout (Oncorhynchus mykiss), despite reports from previous studies that mineral requirements may differ. Hence, this study was designed to determine the effects of organic and inorganic zinc on a commercial strain of genetically similar diploid and triploid rainbow Trout. Eleven dietary treatments, including one basal diet (33 mg/kg, Zn33) and 10 experimental diets, supplemented with incremental levels (63, 93, 123, 153 and 183 mg/kg) of inorganic (ZnSO4) and organic (Bioplex® Zn, Alltech) zinc supplemented to the basal diet were fed for 9 weeks (64 days). Twenty-two experimental groups (a unique combination of ploidy, zinc type, and dose level) were run in triplicate, with 11 fish per tank (66 L), with an average starting weight of 53.07 ± 4.35 g. Results showed neither ploidy (diploid vs. triploid) nor type of mineral (organic vs. inorganic) significantly influenced growth parameters. There was a significant interaction between zinc type and dose in diploids for lipid retention, energy retention, and vertebral zinc content. In triploids, there was a significant interaction between zinc type and dose for lipid, energy and protein retention, and vertebral zinc content. In conclusion, numerical data suggest that organic Zn requirement is lower than inorganic Zn in triploid trout, recommended requirement of inorganic Zn is 123 mg/kg whereas organic Zn is 63 mg/kg diet. The results showed that interactions between type of zinc and dose had a significant effect on nutrient retention, representing a potential for increased capacity to utilize feed ingredients.