Seasonal changes in osmoregulation, cortisol, and cortisol receptor activity in the gills of parr/smolt of steelhead trout and steelhead-rainbow trout hybrids, Oncorhynchus mykiss.

Steelhead and hybrid trout (rainbow trout male, steelhead female) presmolts and smolts, Oncorhynchus mykiss, exposed to 30/1000 seawater (SW) for 24 hr manifested progressively lower plasma sodium values from December to May. The increase in plasma sodium after 24 hr in SW diminished significantly after March, with the hybrids (which were heavier in weight than the steelheads) reaching their peak osmoregulatory ability in the middle of April, and the steelheads in late May. In fresh water there was little difference in plasma osmotic concentration but steelhead plasma chloride fell significantly from March 30 levels to May 23 levels, whereas hybrid plasma chloride levels did not change during this time. In steelheads but not hybrids, plasma cortisol correlated negatively with plasma sodium of fish exposed 24 hr in SW. However, in both steelheads and hybrids, plasma sodium values 24 hr after SW exposure had an inverse correlation with cytosol cortisol from the gills. Again, in both steelheads and hybrids, plasma values of fish exposed 24 hr in SW correlated positively with cytosol receptor activity. In hybrids, the nuclear extract receptor activity, Nmax, increased significantly until late March and then dropped significantly thereafter. In steelheads, there were no significant differences but Nmax followed the same trends as occurred in hybrids. Thus seasonal changes in cortisol and cortisol receptors in steelhead occur during smoltification and preparation for marine life and suggest an important role for cortisol. In hybrids there appears to be a reduction in seasonality.