Physiological changes related to migration tendency in rainbow trout ( Oncorhynchus mykiss)

Rainbow trout allowed to undergo volitional release at Roaring River Hatchery, Oregon, were separated into migrant and non-migrant populations. In two of the three years of the study, the extent of migration from the raceway was similar to the extent of migration from the area of stocking in a nearby reservoir. Each population in the hatchery was examined for 11 different physiological measurements. Differences between migrant and non-migrant trout were found in condition factor, gill Na +K +-ATPase activity, plasma thyroxine concentrations and muscle and liver glycogen. These results suggest that physiological changes were occurring during the migration of the trout from the raceways. No genetic basis for migration tendency or the physiological changes could be demonstrated.