Zinc-L-Selenomethionine Improves Growth and Hemato-Biochemical Parameters at Low but not at High Stocking Density for Nile Tilapia Males.

The study aimed to examine the interaction between diets supplemented with zinc-L-selenomethionine (ZnSeMet) and two stocking densities (SD) on Nile tilapia (Oreochromis niloticus) males. Four extruded diets were formulated: 0.0, 0.5, 1.0, and 2.0 mg ZnSeMet kg^-1. Fish (58.00 g) were reared at a low SD (LSD) (15 fish tank^-1 or 4,35 kg m^-3) and a high SD (HSD) (45 fish tank^-1 or 13,05 kg m^-3). They were randomly distributed in eight treatments in triplicate in a recirculatory aquaculture system (200 L) and were fed until apparent satiation thrice daily for 65 days. The interaction between LSD and 1.0 or 2.0 mg ZnSeMet kg^-1 was better than other treatments for final weight (116.50-174.83 vs. 97.33-139.17 g), weight gain (108.83.50-116.83 vs. 39.00-80.83 g), relative weight gain (188.03-201.18 vs. 66.25-139.40%), total final length (20.12-20.63 vs. 17.33-19.00 cm), specific growth rate (1.76-1.83 vs. 0.85-1.45% day^-1) (p < 0.05). In general, the interaction between LSD and 1.0 mg ZnSeMet kg^-1 compared to several treatments, showed an increase in plasma total protein (3.23 vs. 2.67-2.78 g dL^-1) and total cholesterol (93.67 vs. 68.50-76.67 mg dL^-1) levels and a reduction in aspartate aminotransferase activity (14.83 vs. 145.67-155.17 U L^-1) (p < 0.05). The highest values for albumin (0.83 vs. 0.57 g dL^-1), glucose (56.00 vs. 45.09 mg dL^-1), and triglycerides (122.33 vs. 68.87 mg dL^-1) in the plasma, erythrocytes (2.22 vs. 2.13 × 10⁶ µL^-1), hemoglobin (9.82 vs. 9.26 g dL^-1), and hematocrit (33.09 vs. 30.21%) were observed in LSD than HSD-reared fish (p < 0.05). In conclusion, adding 1.0 mg ZnSeMet kg^-1 at LSD improved fish growth and hemato-biochemical parameters, but it was not effective at HSD.