Optimum weight of stellate sturgeon (Acipenser stellatus) fingerling to release into brackish water: plasma electrolytes, plasma hormones and histological observation

The present study was conducted to determine the appropriate weight to release the hatchery produced fingerling of stellate sturgeon, Acipenser stellatus, into its natural habitat. For this purpose, prolactin and cortisol hormones, plasma electrolytes and osmolality as well as histological changes in the gill and kidney of stellate sturgeon transferred from fresh to brackish water at different weight (0.5, 1, 2, and 3 g) were assessed during a one-week time course. A total of 2400 fingerlings with different weights (0.5, 1, 2, and 3 g) were equally assigned into 24 aquariums, each one containing 200 L of freshwater or brackish water. After 24 hours, prolactin showed the highest concentration in 0.5 and 3 g fish groups kept in both experimental waters. Cortisol exhibited a time-dependent increase in the 0.5 g group in both experimental waters as well as the 2 g larvae kept in the brackish water. Generally, lower levels of Mg2+, Ca+2; K+, Na+, and Cl- were measured in the fish kept in freshwater aquariums. The lowest levels of osmolarity were found in the 0.5 g fingerling, especially those kept in the freshwater aquariums, whereas the highest levels were observed in 3 g groups in both experimental waters. Histological assessments illustrated that the volume of chloride cells in freshwater increased with increasing fish weight, whereas the cell count decreased. The same held true for glomerular capillary network size. Taken together, the evidence from this study suggests that stellate sturgeon fingerling less than 2 g are almost incapable to be transferred to brackish and saline environment, due to lack of kidney and gill development.