Nitrogen Metabolism and Trophic Input in Relation to Growth in Freshwater and Saltwatersalmo Gairdneri

If growth is considered over the complete annual cycle, saltwater trout grow at a greater rate than freshwater trout when fed ad libitum. However, during the spring, the growth rate of the freshwater smolt increases to a level equivalent to that of the saltwater fish. This is not primarily a result of differences in temperature of the natural seawater and freshwater environments, nor is it associated with differences in food quality, consumption or assimilation rates, but results from the increased rates of protein synthesis and degradation found in freshwater smolt and saltwater fish. The close association of the increased rates of protein metabolism with the smoltification process in freshwater fish, the continuation of this increase in the saltwater fish and the decrease found in the post-smolts remaining in fresh water, all suggest that the processes which bring about the transformation of a stenohaline freshwater trout to a euryhaline trout increase its rates of protein metabolism. It is known that various hormones play a role in the salinity adaptation of the migratory salmonids. Thus the possibility exists that such hormonal activity, primarily linked with osmotic and ionic regulation, may have a secondary effect on the protein metabolism and specific growth rate of the migratory rainbow trout.