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A fish ladder, also known as a fishway, fish pass or fish steps, is a structure on or around artificial barriers (such as dams and lock) to facilitate diadromous fishes' natural migration. Most fishways enable fish to pass around the barriers by swimming and leaping up a series of relatively low steps (hence the term ladder) into the waters on the other side. The velocity of water falling over the steps has to be great enough to attract the fish to the ladder, but it cannot be so great that it washes fish back downstream or exhausts them to the point of inability to continue their journey upriver.

History

Fish ladders are known to be very effective. Written reports of rough fishways date to 17th-century Francemarker, where bundles of branches were used to create steps in steep channels to bypass obstructions. A version was patented in 1837 by Richard McFarlan of Bathurst, New Brunswickmarker who designed a fishway to bypass a dam at his water-powered lumber mill. In 1852–1854, the Ballisodare Fish Pass was built in County Sligomarker, Irelandmarker, to draw salmon into a river that had not supported a fishery. In 1880, the first fish ladder was built in Rhode Islandmarker on the Pawtuxet Fallsmarker Dam. The ladder was removed in 1924, when the City of Providencemarker replaced the wood dam with a concrete one.

As the Industrial Age advanced, dams and other river obstructions became larger and more common, leading to the need for more-efficient fishways.

Types

There are five main types of fishways:
  • Pool and weir
  • Baffle fishway (Denil, Larinier, Alaskan Steepass, or other baffle configuration)
  • Fish elevator
  • Rock-ramp fishway
  • Vertical-slot fish passage
See also Eel Ladder and Fish migration.


A pool and weir is one of the oldest styles of fish ladders. It uses a series of small dams and pools of regular length to create a long, sloping channel for fish to travel around the obstruction. The channel acts as a fixed lock to gradually step down the water level; to head upstream, fish must jump over from box to box in the ladder.

A baffle fishway uses a series of symmetrical close-spaced baffles in a channel to redirect the flow of water, allowing fish to swim around the barrier. Baffle fishways need not have resting areas, although pools can be included to provide a resting area or to reduce the velocity of the flow. Such fishways can be built with switchbacks to minimize the space needed for their construction.Baffles come in variety of designs. The original design for a Denil fishway was developed in 1909 by a Belgianmarker scientist, G. Denil; it has since been adjusted and adapted in many ways. The Alaskan Steeppass, for example, is a modular prefabricated Denil-fishway variant originally designed for remote areas of Alaskamarker.

A fish elevator or fish lift, as its name implies, breaks with the ladder design by providing a sort of elevator to carry fish over a barrier. It is well suited to tall barriers. With a fish elevator, fish swim into a collection area at the base of the obstruction. When enough fish accumulate in the collection area, they are nudged into a hopper that carries them into a flume that empties into the river above the barrier.

On the Connecticut River in Holyokemarker, Massachusettsmarker, for example, a fish elevator lifts up to 500 fish at a time, 52 feet (15.85 m), to clear the Holyoke Dam. In its first year of operation, 1955, the Holyoke fish elevator carried 4,899 shad over the dam; by 2004, the typical annual number of fish lifted had risen to more than 500,000.

A rock-ramp fishway uses large rocks and timbers to create pools and small falls that mimic natural structures. Because of the length of the channel needed for the ladder, such structures are most appropriate for relatively short barriers.

A vertical-slot fish passage is similar to a pool-and-weir system, except that each "dam" has a narrow slot in it near the channel wall. This allows fish to swim upstream without leaping over an obstacle. Vertical-slot fish passages also tend to handle reasonably well the seasonal fluctuation in water levels on each side of the barrier.

Image:John Day Dam fish ladder.jpg|John Day Dammarker fish ladder, viewed from the Washington (north) side of the Columbia River, United Statesmarker.Image:Fischlift1.jpg|Fish elevatorImage:Grave vistrap inlaat scheepswerf.jpg|Fish ladder in Meuse Rivermarker, Gravemarker, the Netherlandsmarker.Image:Grave vistrap laag.jpg|Fish ladder in Meuse River, Grave, the Netherlands.Image:CapilanoRiverRegPark-salmonladder.jpg|Fish ladder in North Vancouvermarker, British Columbiamarker, Canadamarker.Image:Cmglee_Uppsala_fish_ladder.jpg|Detail of fish ladder in Uppsalamarker, Swedenmarker.File:Dart at Salmon leap 2.JPG|Lower part of the fish ladder on the River Dart in Devonmarker, United Kingdommarker.File:FishLadder7.JPG|At the Charles River Dammarker in Bostonmarker, United Statesmarker.

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