Salinity tolerance as well as osmotic and ionic regulation in juvenile Chinese sturgeon (Acipenser sinensis Gray, 1835) exposed to different salinities

To understand salinity tolerance and osmoregulatory status of juvenile Chinese sturgeon, we transferred the hatchery-reared juveniles (mean 189.9 g, 8-months-old) directly from freshwater (0) to salinity 5, 10, 15, 20, and 25 for 192 h (8 days exposure), and measured the mortality, serum osmolality and ion concentrations of fish exposed to various salinities. No mortality occurred in sturgeon exposed to salinity between 5 and 20 during the course of the trial. Sturgeons exposed to salinity 25 began to die after 12 h, and did not survive 24 h. However, acute environmental salinities <20 should not pose a survival problem for this species. Both serum osmolality and ion concentration (Na+ and Cl−) showed the same pattern in all salinity tests. In salinity 5, serum osmolaity and ion concentrations of juvenile Chinese sturgeon were not significantly different compared to freshwater exposed fish (P > 0.05). Serum osmolality and ion concentrations increased significantly (P < 0.05) in higher salinity treatments, reaching a peak at 12 h for fish in 10 and 15, and at 24 h for fish in 20 salinity. Thereafter, serum osmolality and ion concentrations decreased and reached a new steady state for fish in 10 and 15 salinity at the end of the trial, which was higher than for fish in freshwater. Serum parameters in fish exposed to salinity 20 decreased but remained unstable at 192 h, indicating a longer acclimation time was needed for fish exposed to higher salinities. The results indicate that 8-months-old juvenile Chinese sturgeon must stay in the Yangtze River estuary for a period of time to acclimate and can not move directly towards the sea. This information will help to identify the appropriate salinity where cultured juveniles can possibly be restocked.