A certain percentage of fish will stray away from the stream they were born in. That way if fish from one stream gets wiped out (Tootle River when Mt. St. Helens erupted) the fish population will start up again. Some fish will go up the mouth of a river that isn't their's and turn around and head back out again. I've heard of guy's catching dark fish in the saltwater and this is probably what happened. Also when they net the fish in the rivers they might be netting fish that are endangered from another river.

~ Dr Pepper