Fish ladder
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Fishways, most commonly referred to as fish ladders but also known as fish passes, are structures placed on or around human-made barriers (such as dams and weirs) to assist the natural migration of diadromous fishes. Most fishways enable fish to pass around the barrier 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 as to wash fish back downstream or to exhaust them to the point where they cannot continue their journey upriver.
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[edit] History
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 1852–54, the Ballisodare Fish Pass was constructed in County Sligo, Ireland, to draw salmon into a river that previously did not support a fishery.
As the Industrial Age advanced, dams and other river obstructions became larger and more common, leading to the need for more efficient fishways.
[edit] Types
There are five primary types of fishways:
- Rock Ramp Fishway
- Pool and Weir
- Vertical Slot Fish Passage
- Denil Fishway
- Fish Elevator
- See also: Eel Ladder, 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. Effectively, 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 having to leap over an obstacle. Vertical slot fish passages also tend to handle reasonably well seasonal fluctuation in water levels on either 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 in one to provide a resting area or to help 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, but it has been adjusted and adapted in many ways since then. The Alaska Steeppass, for example, is a modular prefabricated Denil fishway variant originally designed for remote areas of Alaska.
As its name implies, a fish elevator or fish lift breaks with the ladder design by providing a sort of elevator to carry fish over a barrier. They are 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 Holyoke, Massachusetts, for example, a fish elevator lifts up to 500 fish at a time 52 ft (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.
 Fish ladder in North Vancouver, British Columbia. |
[edit] See also
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