An eel ladder is type of fish ladder designed to help eels swim past barriers, such as dams and weirs or even natural barriers, to reach upriver feeding grounds. (Many eels are catadromous, living in fresh water but spawning at sea.) The basic design of an eel ladder has the eel swim over the barrier using an eel ascending ramp, which provides the eels a climbing substrate to "push against" while slithering upstream. For some higher barriers, elevator-style systems are also used.
An eel ladder typically consists of four parts: an eel ascending ramp, a supporting structure, a water-feeding system, and a side gutter. The side gutter provides an attraction flow to draw eels toward the ladder, the water-feeding system ensures the proper flow of water to the gutter and the ramp, and the supporting structure mounts the ladder to the barrier.
The eel ascending ramp can be a fairly simple constructions, such as a hollowed out tree filled with recycled fishing net, or a more complex structure designed to accommodate specific species or ages of eels.
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—Benjamin Harrison (1833–1901)