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Fishways, most commonly called '''fish ladders''' but also known as '''fish passes''' and in Australia they are referred to as '''fish steps''', are structures on or around artificial barriers (such as Dam s and Weir s) 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 France , where bundles of branches were used to create steps in steep channels to bypass obstructions. In 1852–1854, the Ballisodare Fish Pass was built in County Sligo , Ireland , to draw Salmon into a river that had not supported a fishery. In 1880, the first fish ladder was built in Rhode Island on the Pawtuxet Falls Dam. The ladder was removed in 1924, when the City Of Providence 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. fish ladder, viewed from the Washington (north) side of the Columbia River .]] TYPES There are five main types of fishways:
See also Eel Ladder and Fish Migration . .]] 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 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 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. A Denil 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. Denil 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. The original design for a Denil fishway was developed in 1909 by a Belgian scientist, G. Denil; it has since been adjusted and adapted in many ways. The ''Alaska Steeppass'', for example, is a modular prefabricated Denil-fishway variant originally designed for remote areas of Alaska . 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. Fish Ladder Safety Sign]] On the Connecticut River in Holyoke , Massachusetts , 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. |
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