When the yolk sacs are used up and disappear, they wiggle out of the gravel and move into deeper water. The smolts migrate downstream to the sea, and must undergo a lot of physiological changes so that they can live in salt water. Diagram courtesy of College of Ocean and Fishery Sciences, University of Washington . Fish which are born in fresh water, but migrate to the ocean where they spend most of their adult life are called anadromous fish . For several weeks or months, the smolts live in the estuaries.
Estuaries are where the river meets the ocean.
Eating tiny shrimp, other crustaceans and the young of other saltwater fishes, they grow very fast. Finally, they disappear into the vast ocean. Salmon make huge migrations in the ocean. They can migrate thousands of miles during their residence in the ocean. Jacks , or salmon returning in their first year of ocean residence, may not even leave Puget Sound.