Effect of dietary magnesium supplementation on the growth performance of juvenile gibel carp, Carassius auratus gibelio

Gibel carp (Carassius auratus gibelio) of mean initial weight 3.1 g were fed one of seven casein-dextrin-based diets containing graded levels of magnesium (Mg) (39, 120, 220, 380, 700, 1600 and 2900 mg kg-1) for 3 months with the waterborne Mg concentration of 10.612.7 mg L-1. Magnesium sulphate was used as the supplementation Mg source in the diets. The experiment was carried out in a flow-through system. Growth, survival rate, Na+/K+-ATPase, Mg2+-ATPase and tissue mineral contents were measured to investigate the effect of dietary magnesium in gibel carp. At the end of the experiment, the hepatopancreas of fish were collected for enzyme determination. The hepatopancreas, vertebrae and whole body were collected for tissue magnesium content analysis. After 3 months, dietary magnesium supplementation did not improve the growth performance, including feed intake, weight gain and feed conversion efficiency of juvenile gibel carp. On the contrary, negative impacts on survival, reduced growth performance and dramatically decreased Na+/K+-ATPase, Mg2+-ATPase and superoxide dismutase activities were observed in gibel carp fed a high Mg diet of 2900 mg kg-1. Although serum and hepatopancreas Mg and Ca contents were not affected by dietary Mg supplementation, vertebrae and whole-body Mg contents increased significantly with the increasing dietary Mg concentrations. Based on the relationship between whole-body Mg retention and dietary Mg concentration, a suitable dietary Mg level of 745 mg kg-1 could be estimated for gibel carp. It could be concluded that dietary Mg supplementation did not improve the growth performance, but could increase vertebrae Mg contents of gibel carp. Considering the adverse effects, a dietary Mg concentration of above 2900 mg kg-1 is not recommended and it should be careful to supplement magnesium in practical diets for gibel carp as most feed ingredients contain high magnesium concentrations.