Physiological Characterization of Hatchery-Origin Juvenile Steelhead Oncorhynchus mykiss Adopting Divergent Life-History Strategies
Abstract Smoltification by juvenile Pacific salmonids has been described as a developmental conflict whereby individuals face several life-history decisions. Smoltification occurs as a result of interactions between organismal condition and environmental cues, although some fish may forgo ocean migration and remain in freshwater streams for some time (residualize). We compared the physiological profiles of steelhead that were actively migrating to the ocean (migratory fish) and those that remained in fresh water (residuals) for at least a period of between 2 wk and 3 mo after release from a hatchery facility. In addition, we investigated the physiological characterization of residuals that further differentiated into precocial freshwater residents or parr that will either precocially mature in fresh water or migrate to the ocean in the future. Residuals had higher condition factors and gonadosomatic index than migratory fish and were characterized as less prepared for saltwater due to low levels of gill Na+,K+-ATPase activity and Na+,K+-ATPase α1b-subunit expression. Residuals tended to be males with the highest condition factors. Sex-specific differences are probably reflective of male fish adopting an alternative life-history strategy foregoing outmigration as a result of condition at the time of release. Collection of residuals throughout the fall suggested that residual hatchery fish further diversify into precocially mature fish that will presumably attempt to spawn without ever migrating to the ocean or into parr that will precocially mature or migrate in a future year.