Effects of stocking density and acute hypoxic stress challenge on growth performance, immune–antioxidant, and physiological responses in Siberian Sturgeon Acipenser baerii ♀ × Beluga Huso huso ♂ hybrids

Abstract Objective This study aimed to evaluate the effects of stocking density on growth performance, body composition, hematological indices, immune–antioxidant responses, and acute stress tolerance in hybrids of Siberian Sturgeon Acipenser baerii ♀ × Beluga Huso huso ♂. Methods Juvenile sturgeon hybrids (mean ± SD = 15.5 ± 0.60 g) were reared at stocking densities of 4, 6, 8, 10, and 12 kg/m3 for 10 weeks. Growth indices (final weight, weight gain, body weight increase, growth rate, specific growth rate, feed efficiency, condition factor, and hepatosomatic index), survival, feed conversion ratio, protein efficiency ratio, CV of body weight, carcass composition, hematological traits, mucus and serum immune parameters, antioxidant enzyme activities, and water quality were measured. Following the 10-week trial, fish were subjected to a 1-min air exposure to assess acute stress responses (cortisol, glucose, and lactate). Results Growth indices declined significantly at densities above 4 kg/m3, with the lowest values observed in the 12-kg/m3 group, while 10 kg/m3 showed intermediate reductions. The condition factor and hepatosomatic index decreased at densities above 8 kg/m3. Survival, feed conversion ratio, protein efficiency ratio, and the CV of body weight were not significantly affected. Carcass fat decreased at higher densities, whereas protein, moisture, and ash remained stable. Hematological traits were largely unchanged except for increases in hemoglobin, mean corpuscular hemoglobin, and mean corpuscular hemoglobin concentration. Mucus and serum lysozyme, total protein, immunoglobulin, and antioxidant enzymes (glutathione peroxidase and superoxide dismutase) decreased at densities above 8 kg/m3. Water quality variables remained generally stable across treatments, except for total ammonia nitrogen (TAN), which increased with stocking density. Increases in TAN and estimated un-ionized ammonia (NH3), calculated from TAN based on temperature- and pH-dependent equilibrium equations, were observed with increasing stocking density. Cortisol and glucose increased at densities above 8 kg/m3 following acute stress, whereas lactate increased only in the 12-kg/m3 group after acute stress. Conclusions The results indicate that 8 kg/m3 is the optimal stocking density for rearing Siberian Sturgeon × Beluga hybrid juveniles, balancing growth performance, immune–antioxidant status, stress tolerance, and water quality management.