Survival, growth, food conversion efficiency and plasma osmolality of juvenile Siganus guttatus (Bloch, 1787): experimental analyses of salinity effects

We studied the effects of salinity on survival, growth, food conversion efficiency and plasma osmolality of juvenile Siganus guttatus in two independent experiments. In the first experiment, fish were transferred from 30 aEuro degrees salinity to freshwater, 5, 10, 20 and 30 aEuro degrees salinities for 192 h. No fish died when transferred directly from 30 aEuro degrees to salinities > 5 aEuro degrees. However, all fish died in the freshwater treatment. In the second experiment, survival, growth, feeding rate, food conversion efficiency and plasma osmolality of fish were analyzed during 6 weeks in salinities of 5, 10, 20 and 30 aEuro degrees (control). At the end of this experiment, the final weight and the specific growth rate of fish were significantly greater at 10 aEuro degrees than fish in all other treatments. Feeding rate increased significantly with decreasing salinity: 10 aEuro degrees > 20 aEuro degrees > 30 aEuro degrees. However, the food conversion efficiency was not significantly different between fish in any treatment. Plasma osmolality of fish in 20 and 30 aEuro degrees salinity was significantly greater than fish reared at 10 or 5 aEuro degrees. A salinity of 13.95 aEuro degrees (411.88 mOsmol/kg) was the point of isosmolality for juvenile S. guttatus.