Changes of glucose, cortisol, and ions in spotted scat Scatophagus argus under different ambient salinities

Spotted scat, Scatophagus argus is a euryhaline fish and is widely distributed in brackish and marine waters. Salinity influences the standard metabolic rate of fish, fish ingestion rate, feed conversion efficiency, metabolism, and hormone secretion, thus affecting the body growth and the accumulation of nutrients in the fish. This study aims to determine the optimal salinity for growth performance, and physiological characteristics of S. argus. The experiment was conducted in triplicates with 12 composite tanks at 4 different salinity treatments: 10%o; 15%o (control); 20%o, and 25%o, with a completely randomized design method. The weight and total length of fish at different salinities were measured before and after the experiment. The results show that the specific growth rate (SGR), survival of fish, and feed conversion ratio (FCR) varied significantly between treatments of 15% and 25% (p < 0.05). For fish, all treatments had a survival rate as high as 100%. The FCR in the 15% group was lower than that of the 25% group (p < 0.05), however no significant difference in FCR was found among the 10%, 15%, and 20% groups (p > 0.05). There was a significant difference in plasma cortisol levels between the 10% and 15% treatments when compared to 20% and 25% (p < 0.05). There was no significant difference in glucose concentrations among salinity treatments (p > 0.05). Changes in ionic concentrations of Na+ and Cl- were at the highest value at the treatment of 25% and all treatments reached the peak on d-1 and then declined until the end of the experiment. Changes in the concentrations of glucose, Ca++, and K+ during the time course of the experiment were not significantly different under different salinities. As a result, we propose using a salinity of 15% for S. argus farming in order to achieve a higher growth rate and lower the FCR, and thus better production.