I believe the fish, in order to return, have to have their "spawning spot" imprinted. These fish are anadromous. This means they spawn in fresh water and migrate to the ocean to grow. After a certain time internal urges guide them back to their natal place. Without the "imprint" they would have no natal homing. They most definitely benefit from fish ladders. The fish ladder in Manchester is a great place to visit. Very informative.
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