Salmon eggs hatch in freshwater streams and, during the first year of their life, the young salmon migrate up to 1,000 km to the ocean. They spend varying amounts of time in the ocean (ranging up to five years), where they feed and grow, rapidly acquiring more than 95% of their biomass during this period. During the summer of their maturing year, they begin the long journey back to their home streams where they spawn. Although it is still uncertain how salmon navigate back to their spawning grounds, current hypotheses suggest that they have a highly developed sense of smell. At the spawning grounds, females use their tails to form a hollow cavity where they lay up to 8,000 eggs. The male fertilizes the eggs, and both adults typically die soon thereafter.
The physiological response that allows salmon to survive in fresh water, then in salt water, and then fresh water again is an example of ______.