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 locks) 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.
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. A version was patented in 1837 by Richard McFarlan of Bathurst, New Brunswick, Canada, 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 Sligo in Ireland to draw salmon into a river that had not supported a fishery. In 1880, the first fish ladder was built in Rhode Island, United States, on the Pawtuxet Falls Dam. The ladder was removed in 1924, when the City of Providence replaced the wood dam with a concrete one. Concrete ladders are not always an improvement – the electric field-sensitive organs of the paddlefish are overloaded in the proximity of the rebar and other metal used in concrete construction, preventing them from gaining access to their spawning grounds and contributing to a catastrophic decline in their numbers.
There are five main types of fishways:
- Pool and weir
- Baffle fishway (Denil, Larinier, Alaskan Steeppass, or other baffle configuration)
- Fish elevator
- Rock-ramp fishway
- Vertical-slot fish passage
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 Belgian 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 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.
On the Connecticut River, 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. They have a significant advantage in that they can provide fish spawning habitat.
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.
Fish ladder in Meuse River the Netherlands
Fish ladder in North Vancouver, Canada
Detail of fish ladder in Uppsala, Sweden
Detail of fish ladder on the River Dart in England.
At the Charles River Dam near Boston, United States
See also 
- "What is a Fish Ladder?" (in English). Michigan: Michigan Department of Natural Resources. Retrieved 27 April 2012.
- Mario Theriault, Great Maritme Inventions 1833–1950, Goose Lane, 2001, p. 45
- Office Of Technology Assessment Washington DC (1995) Fish passage technologies : protection at hydropower facilities Diana Publishing, ISBN 1-4289-2016-1.
- Luther P. Aadland (2010). Reconnecting Rivers: Natural Channel Design in Dam Removals and Fish Passage. Minnesota Department of Natural Resources.
- To Save the Salmon (1997) US Army Corps of Engineers.
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- A study of the hydraulics of flow over fishways
- Construction of a vertical slot fish passage and eel ladder for the St. Ours Dam (Richelieu River, Québec)
- Fish Passage Center
- Fish passes. Design, dimensions and monitoring, Food and Agriculture Organization of the United Nations/Deutscher Verband für Wasserwirtschaft und Kulturbau (DVWK), Rome, 2002 ISBN 92-5-104894-0 ISBN 3-89554-027-7 (Zip download from FTP area of the FAO's EIFAC – European Inland Fisheries Advisory Commission)
- Washington Post, January 31, 2007: U.S. Orders Modification of Klamath River – Dams Removal May Prove More Cost-Effective for allowing the passage of Salmon