Effect of dietary iron (Fe) supplementation on growth performance, hematological parameters and anti-oxidant responses of coho salmon Oncorhynchus kisutch(Walbaum, 1792) post-smolts

Effects of graded dietary iron (Fe) levels on coho salmon Oncorhynchus kisutch(Walbaum, 1792) post-smolts were assessed by determining the growth performance, Fe deposited in tissue, liver and serum antioxidant and immunity responses. Six experimental diets containing graded Fe (FeSO4·7 H2O as a Fe source) levels of 23.70, 42.40, 63.80, 89.30, 102.70 and 125.70 mg/kg were made and fed to three replicate groups of fish. A total 180 coho salmon post-smolts (188.28 ± 0.40 g) were randomly distributed to 18 floating cages (water volume 1000-L) with a flow-through system for 10 weeks, and fed three times daily until satiation. The results showed that the final body weight (FBW) and specific growth rate (SGR) were increased (p< 0.05) with increasing Fe content (from 23.70 to 89.30 mg/kg) in the diet, and significantly decreased (p< 0.05) in these traits were recorded when dietary Fe supplementation was further increased (from 89.30 to 125.70 mg/kg). However, feed conversion ratio (FCR) showed the opposite trend. Survival rate, morphological indices, and body proximate compositions did not differ significantly among the groups (p> 0.05). Nevertheless, the Fe contents in the whole body and muscle exhibited a steady rise (linear, p< 0.05), meanwhile, the liver Fe content significantly increased with increasing dietary Fe levels (linear and quadratic, p< 0.05). Increasing dietary Fe levels increased (p< 0.05) the hemoglobin (Hb), erythrocyte counts and hematocrit. In serum, lysozyme activity was significantly increased in 89.30 mg/kg diets ( p< 0.05). There was a significant increase in liver and serum superoxide dismutase (SOD) and catalase activities in response to the Fe levels in the diet, whereas a significant quadratic decrease in malondialdehyde levels (p< 0.05). Quadratic regression analysis of SGR and FCR against dietary Fe levels indicated that the optimal dietary Fe requirements were estimated to be 100.00 mg/kg and 104.17 mg/kg. Broken-line model analyses of the hepatic Fe content and SOD activity, the Hb content in the blood, and serum SOD activity against dietary Fe levels indicated that the optimal dietary Fe levels are 85.01, 87.53, 87.04, 91.23 mg/kg in Oncorhynchus kisutchpost-smolts, respectively.