Effects of acclimation to brackish water in tolerance of salinity challenge by young-of-the-year Adriatic sturgeon (Acipenser naccarii)

The effects of a 24-h salinity challenge in water at 28 g.L-1 on plasma osmotic and ion homeostasis, swimming performance, and exercise respirometry were compared in young-of-the-year Adriatic sturgeon (Acipenser naccarii) maintained in freshwater (FW) or acclimated to brackish water (BW) that was slightly hypertonic to sturgeon plasma at a salinity of 11 g.L-1. Salinity challenge caused a significant increase in plasma osmolality and Na+ and Cl- concentrations in both groups, but this was significantly less severe in the group acclimated to the BW as compared with FW. Salinity challenge elicited a significant and profound decline in maximum sustainable swimming speed (U-crit) in sturgeon from FW, whereas there was no significant effect on U-crit in the sturgeon acclimated to BW. A negative linear relationship was revealed between plasma osmolality, Na+ or Cl- concentrations, and U-crit. These variables appeared to influence U-crit by increasing costs for locomotion while reducing the sturgeon's capacity for oxygen uptake and muscular work. The results indicate that prior acclimation to mildly hypertonic BW promoted osmoregulatory adaptations that improved the ability of the sturgeon to perform exercise following further increases in water salinity.