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Anadromous FishAnadromous fishes are born in fresh water, migrate to salt water for their adult lives, then turn around and migrate back to fresh water to spawn. The Switch from Fresh water to Salt WaterFreshwater fishes are different from salt- water fishes. Since anadromous fishes start out as freshwater fishes, their bodies must go through changes to become a saltwater fishes. All fishes need to balance the amount of salt in and out of their bodies (see sidebar). If a freshwater salmon swam straight out into the ocean without making those changes, it wouldn't last long. Finding the way backSome anadromous fishes have amazing homing instincts. Salmon may swim thousands of miles out to sea, but they always find their way back to the exact same stream where they were born. How they do this is not very well known. Trails of bread crumbs would probably just float away or get eaten up by seagulls. The BasicWe all have salts in our bodies; so do fishes. All fish must adjust to the water around them to keep the salts in their bodies at the right levels. Freshwater fishes have more salts inside their bodies than there are in the surrounding water. Water tries to get into the fish to balance the salts. Freshwater fishes never drink; they make a lot of watery urine. They even pump salts into their bodies through their gills. Saltwater fishes have the opposite problem. The salt levels are higher outside the fish than inside the fish. The water in the fish tries to leak out to make the concentrations equal. To fight this, saltwater fishes drink lots of water; make thick, concentrated urine; and pump salts out of their bodies through their gills.
Anadromous fishes have to switch their strategies before they cross between habitats. Salmon spend time in estuaries, where the water is just a little salty, while their bodies adjust to the new strategy.
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