A combined salt and confinement stress enhances mortality in rainbow trout ( Oncorhynchus mykiss) selected for high stress responsiveness

Significant differences in mortality was observed among lines of rainbow trout ( Oncorhynchus mykiss Walbaum) subjected to confinement stress shortly after their transfer to seawater. The fish belonged to one of four lines which were selected with respect to their capability to express high or low post-stress serum levels of either cortisol or lysozyme. Fish of the high-cortisol responding line showed a significantly higher mortality than those of the other selection lines, whereas a significantly lower mortality than in the remaining fish was found in the line selected for high lysozyme levels. In parallel with this, osmolality and Cl − concentrations were highest in the line exhibiting the highest mortality, and lowest in the line exhibiting the lowest mortality. Both osmolality and chloridity were significantly higher in fish that died than in fish which survived following the stress treatment, no matter which line. As to the two lines selected for divergent cortisol stress responsiveness, mean osmolality in the high-cortisol line was significantly higher than in the low-cortisol line. In subsequent stress exposures, 3 and 9 months following transfer to seawater, differences in osmolality between high- and low-cortisol responders were no longer detectable. The results suggests that fish with high post-stress serum cortisol levels are less well suited to tolerate multiple stressors and would also require a longer time period to successfully adapt to seawater.