NITROGENOUS COMPOUNDS AND OXYGEN CONCENTRATION AS THE KEY DENSITY DEPENDENT FACTORS TO OPTIMIZE GROWTH OF BELUGA, HUSO HUSO (ACTINOPTERYGII: ACIPENSERIFORMES: ACIPENSERIDAE), IN CIRCULAR FIBERGLASS TANKS

Background. Beluga, Huso huso, known also as the great sturgeon, is a valuable fish in terms of meat and caviar production. It has been implicated that intensive fish culture is a major culprit for deterioration of water quality through metabolic excretion of fish in the aquatic environment. The aims of this study were: to determine the effect of the stocking density on the water quality, to assess the effect of water quality changes on growth of beluga juveniles, and also to determine the optimum density for culture in fiberglass tanks. Materials and methods. Fish were randomly distributed into 15 tanks at 5 different densities including 1 kg . m(-2) (3.90 kg m(-3)), 2 kg . m(-2) (7.09 kg . m(-3)), 4 kg . m(-2) (14.54 kg . m(-3)), 6 kg . m(-2) (21.19 kg . m(-3)), and 8 kg . m(-2) (28.64 kg . m(-3)) with 3 replicates. Specific growth rate (SGR), yield (Y), average daily growth (ADG), absolute weight gain (AWG), and relative weight gain (RWG) were calculated. Throughout the experiment, water quality parameters including ammonia (NH3), nitrite (NO2), nitrate (NO3) dissolved oxygen (DO), and oxygen saturation (OS) were also measured. Results. Stocking density considerably influences the growth of beluga juveniles and water quality during 56 days of rearing. DO, OS, NO2, NO3, and NH3 were significantly different among treatments. pH and temperature showed no significant effect under different stocking densities. NO2 and NO3 increased, while DO decrease as the stocking density of fish increased. At the end of the 56 days of rearing, all the growth parameters (final weight, total length, SGR, Y, ADG, AWG, and RWG) were significantly different among treatments. Survival was 100% in all of the rearing densities. Conclusion. Results of this study showed that the density of fish had a significant effect on water quality and a high density culture accompanied by intensive feeding may result in high concentrations of nitrogen compounds, and low concentrations of dissolved oxygen in the water of culture tanks. Each of the factors (increase nitrogenous compounds and decrease in oxygen) could suppress the growth of Huso huso juveniles.