Fish Ladders: Stairways That Let Migrating Fish Get Past Dams

For many fish, survival depends on a journey up or down a river, and a dam can end it. Dams, culverts and waterfalls can slow or stop fish on their migrations, and these barriers are often linked to the decline of certain fish stocks. A fish ladder is the way around: a structure that lets migrating fish pass over or around an obstacle in the river.

A fish ladder near the south shore of Long Island, where alewives were recorded migrating. Credit: NOAA.

Who needs to migrate

Different fish travel in different directions at different stages of life:

Either way, a blocked river means a blocked life cycle.

How a ladder works

A fish ladder, also called a fishway, gives migrating fish a detour past a particular obstruction. Designs vary with the obstacle, the flow of the river and the species involved, but every ladder works on the same principle: a series of ascending pools, each reached by swimming against a stream of water. A fish leaps through the rushing cascade, rests in a pool, and repeats the process until it is out of the ladder.

Sources

Based on "What is a fish ladder?" from NOAA's National Ocean Service, a work of the United States government in the public domain.