How do people know that salmon return to the place where they hatched? - Ask Scarlett

Ranger Rick, Jan, 1997

By "following" the fish. Here's how scientists who study salmon do it:

First they catch some young salmon in a stream before the fish head out to sea. Next the scientists mark them all the same way. (One way to mark a fish is to stick a very tiny wire tag into its body. The tag has information on it that can be read by a special machine.) Then they let the fish go.

Later, scientists catch some adult salmon coming up the same stream and check their tags. The information on the tags proves that the fish returned to their own streams.