A fish ladder is a structure that facilitates migrating fish to pass over or around an obstacle on a river. This obstacle can be artificial or natural.
Many fish species’ survival depends on their migrations up and down the rivers. Downstream migration is a feature of early life stages, while upstream migration is a feature of adult life for anadromous fish such as salmon, shad, and sturgeon. The opposite is true for catadromous fish, like the North American eel. Artificial constructions such as culverts, dams, and waterfalls act as river obstructions and have the potential to slow or stop fish migration. These obstructions to fish migration are often responsible for the decline of certain fish stocks.
A fishway or a fish ladder provides a detour route for migrating fish around or over a particular obstruction on the river. Depending on the river flow, obstruction, and the species of fish affected, the designs vary but the general principle for all fish ladders is the same: the ladder consists of a series of ascending pools that can be reached by swimming against a stream of water. Fish leap through the rushing water, rest in a pool, and then repeat the process until they come out of the ladder.
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