Differential effects of two seawater transfer regimes on the hypoosmoregulatory adaptation, hormonal response, feed efficiency, and growth performance of juvenile steelhead trout

The differential effects of two seawater (SW) transfer regimes on the hypoosmoregulation, hormonal response, feed efficiency, and growth performance of 50 g juvenile steelhead trout (Oncorhynchus mykiss) were determined to better understand the chronic responses to two salinity increase regimes. The fish reared in freshwater (FW) were exposed to either FW (control) or SW (32 ppt) via two regimes. In the first regime, the fish were exposed to a daily increase in salinity of 11 ppt and transferred to 32 ppt on the 3rd day (3DSW), and in the second regime, the fish were exposed to 20 ppt on the 1st day, which was followed by a daily 2 ppt increase, and transferred to 32 ppt on the 7th day (7DSW). Subsequently, an 8-week growth trial was conducted. At the end of the trial, the body weight of the control fish was higher than that of the 3DSW and 7DSW while the specific growth rate of the control was higher than that of the 3DSW but not significantly different from that of the 7DSW. Similarly, the feed efficiencies of the control were not different from that of the 7DSW but were higher than that of the 3DSW. The results on the plasma Na+ and Cl- levels, osmolality, and gill Na+/K+ ATPase activity indicated that steelhead in the 3DSW and 7DSW developed high hypoosmoregulatory ability at weeks 4 and 8. Kidney Na+/K+- ATPase activity increased in the 7DSW and 3DSW groups at week 4. Circulating growth hormone and insulin-like growth factor I levels in the 7DSW group were elevated at week 4, whereas the cortisol level was not changed. Our results suggest that the 7DSW but not the 3DSW alleviated the chronic effect of osmotic stress on the growth rate and feed efficiency of 50 g steelhead trout following SW transfer.