Dietary selenium requirement and toxicity levels in juvenile Nile tilapia, Oreochromis niloticus

A 10-weeks of feeding trial was conducted to evaluate the dietary selenium(Se) requirement and toxicity levels in juvenile Nile tilapia, Oreochromis niloticus using seleno-methionine as the source of Se. Graded level of Se-Met were added to the basal diet at 0, 0.25, 0.5, 0.75, 1.0, 2.0, 4.0, 8.0 and 16.0 mg Se/kg diet providing the analyzed Se of 0.30 (Se-0.30), 0.55 (Se-0.55), 0.73 (Se-0.73), 0.82 (Se-0.82), 1.04 (Se-1.04), 2.06 (Se-2.06), 2.95 (Se-2.95), 6.31 (Se-6.31) and 14.7 (Se-14.7) mg Se/kg diet respectively. Twenty fish averaging 1.85 +/- 0.1 g (mean +/- SD) were fed one of the nine experimental diets in triplicate groups. At the end of the feeding trial, weight gain (WG), specific growth rate (SGR) and feed efficiency (FE) of fish fed Se-2.06 were significantly higher than those of fish fed other diets (p < 0.05). The survival rate of the groups of fish fed Se-6.31 and Se-14.7 diets were significantly lower than those of fish fed other diets (p < 0.05). Selenium concentrations of liver, muscle and gill tissues were recorded to increase in the dose dependent manner. Whereas, lysozyme activity of fish fed Se-0.30 diet was significantly lower than those of fish other diets. Observations for myeloperoxidase (MPO) and superoxide dismutase (SOD) activities showed an increasing pattern up to 6.31 mg Se/kg (Se-6.31) inclusion and afterwards decline. Broken-line regression analysis of WG indicated that the minimum Se requirement level could be 1.06 but mgSe/kg diet. Therefore, these results may suggest that, optimum dietary Se level could be >1.06 but <2.06 mg Se/kg for in juvenile Nile tilapia. Whereas, the toxicity level for dietary Se level could be between 6.31-14.7 mg Se/kg diet for juvenile Nile tilapia, Oreochromis niloticus characterized by reduced growth performance and nonspecific enzymatic activity using Se-Methionine as the source of Se. Statement of relevance: The importance of trace mineral, selenium(Se) supplementation in fish feed formulation has been well accepted since Se from ambient water and feed itself cannot supply the optimal level required by the cultured aquatic species. Even though, Se is typically required in much lower quantity in fish diet but its supplementation at optimum level is a critical issue in fish feed formulation. Scientific reports gathered over last two decades indicate, Se is quite sensitive and proper care should be taken to ensure their inclusion at optimum level in fish feed formulation. The present experiment clearly demonstrated that, freshwater fish juvenile Nile tilapia has a specific requirement level for dietary Se which cannot be met by regular feed and ambient water. Inclusion of dietary Se at optimum level is imperative to promote the maximum growth, tissue saturation and to ensure an optimum enzyme activity in Nile tilapia. Supplementing Se in fish feed beyond the requirement level may have toxic effects in freshwater aquaculture. (C) 2016 Elsevier B.V. All rights reserved.