Smolting in Coastal Cutthroat Trout

Coastal cutthroat trout (Oncorhynchus clarkii clarkia) are found on the West Coast of North America from Alaska to northern California. These salmonines exhibit tremendous diversity in life history strategies both within a watershed and throughout their range.  Sympatric resident and sea-run migrants can be found in the same system.  Though migrating juveniles are often characterized as “smolts,” it is unclear whether juveniles undergo a parr-smolt transformation process similar to those observed in other anadromous salmonines.  Shifts in physiology (e.g., elevated gill Na⁺,K⁺-ATPase activity, increased seawater tolerance) concurrent with migratory behavior associated with smolting are not well documented. We present data from both laboratory and the field studies (several tributaries of the Columbia River).  In 2002, Juveniles from the Cowlitz Trout Hatchery were subjected to a series of 24 h isothermal 35 ppt seawater transfers through the period of spring migration (11-Mar through 25-Jul). Survival, plasma ion perturbations and gill Na⁺,K⁺-ATPase activity were measured. Seawater tolerance was lowest in March (plasma osmolalities >420 mOsmol/kg), increased from April-June, and diminished in July.  Even at the peak of tolerance, osmotic perturbations