Do the dietary ingredients of low-protein formulated diet provide a sufficient selenium source in Australian snapper Pagrus auratus diet (Bloch & Schneider 1801)?

A 10-week feeding trial was conducted to evaluate the necessity of supplementing selenium (Se) in a low-protein basal diet for Australian snapper Pagrus auratus, a slower-growing fish species. Dietary Se supplementation was tested against commercial barramundi diet, most commonly fed to Australian snapper. Se-yeast was added to a reconstituted commercial diet and to a formulated basal diet at 0, 0.8 and 1.0 mg/kg Se to form six isoenergetic test diets to evaluate the effects of diet types, dietary Se supplementation and their interaction on the growth, feed utilisation, body composition and liver morphology of juvenile Australian snapper. These Se supplementation levels resulted in total dietary Se levels of 0.92, 1.68, 1.76, 1.67, 2.59 and 2.77 mg/kg, respectively. Each test diet was fed to triplicate groups of Australian snapper in a flow-through seawater system. There were no significant interaction between diet types and Se supplementation on all tested parameters of the snapper. Regardless of the diet types, snapper fed 1.0 mg/kg Se-supplemented diet attained significantly lower growth rate than the fish fed diets supplemented with 0 and 0.8 mg/kg of Se. Feed conversion ratio was significantly increased in snapper fed diets supplemented with 1.0 mg/kg Se. There were no effects of both diet types and Se supplementation on the proximate composition of the muscle and whole-body except the reduced moisture and gross energy levels in the whole-body of snapper fed 1.0 mg/kg Se-supplemented diet. The Se contents in the muscle and liver tissues showed positive linear relationships with dietary Se levels. Fish fed diet supplemented with 1.0 mg/kg Se showed increased hepatic lipid compared with the fish fed 0 and 0.8 mg/kg Se-supplemented diets. Histopathological alterations were observed in liver tissues of snapper fed the basal diet containing 2.77 mg/kg Se. The results concluded that the protein and Se requirements of Australian snapper are low and can be met by lower protein ingredients with the endogenous Se. Further, the dietary Se threshold level in Australian snapper fed a basal diet was 2.77 mg/kg, whereas this level was only 1.76 mg/kg in the reconstituted commercial diet.