Effect of water temperature, salinity, and their interaction on growth, plasma osmolality, and gill Na+, K+-ATPase activity in juvenile GIFT tilapia Oreochromis niloticus (L.)

•The combined effect between temperature (T) and salinity (S) on plasma osmolality was significant.•The quadratic effect of T and S on SGR, osmolality and Na+, K+-ATPase activity was significant.•We established the models of these responses towards the T and S.•The T/S level of the highest growth was close to the lowest osmolality and Na+, K+-ATPase activity.

We used a central composite rotatable experimental design and response surface methodology to evaluate the effects of temperature (18–37 °C), salinity (0–20‰), and their interaction on specific growth rate (SGR), feed efficiency (FE), plasma osmolality, and gill Na+, K+-ATPase activity in GIFT tilapia juveniles. The linear and quadratic effects of temperature and salinity on SGR, plasma osmolality, and gill Na+, K+-ATPase activity were statistically significant (P<0.05). The interactive effects of temperature and salinity on plasma osmolality were significant (P<0.05). In contrast, the interaction term was not significant for SGR, FE, and gill Na+, K+-ATPase activity (P>0.05). The regression equations for SGR, FE, plasma osmolality, and gill Na+, K+-ATPase activity against the two factors of interest had coefficients of determination of 0.944, 0.984, 0.966, and 0.960, respectively (P<0.01). The optimal temperature/salinity combination was 28.9 °C/7.8‰ at which SGR (2.26% d1) and FE (0.82) were highest. These values correspond to the optimal temperature/salinity combination (29.1 °C/7.5‰) and the lowest plasma osmolality (348.38 mOsmol kg−1) and gill Na+, K+-ATPase activity (1.31 µmol Pi. h−1 g−1 protein), and resulted in an energy-saving effect on osmoregulation, which promoted growth.