Fish ladder near the south shore of Long Island, where recorded Alewive migration was occurring.

The survival of many fish species depends on migrations up and down rivers. Among anadromous fish such as salmon, shad, and sturgeon, downstream migration is a feature of early life stages, while upstream migration is a feature of adult life. Among catadromous , like the North American eel, the opposite is true. River obstructions such as dams, culverts, and waterfalls have the potential to slow or stop fish migration. Indeed, these impediments to fish migration are often implicated in the decline of certain fish stocks.

A fish ladder, also known as a fishway, provides a detour route for migrating fish past a particular obstruction on the river. Designs vary depending on the obstruction, river flow, and species of fish affected, but the general principle is the same for all fish ladders: the ladder contains a series of ascending pools that are reached by swimming against a stream of water. Fish leap through the cascade of rushing water, rest in a pool, and then repeat the process until they are out of the ladder.