A fish ladder, also known as a fishway, fish pass, fish steps or fish cannon, is a waterway structure on or around artificial and natural barriers (such as dams, locks and waterfalls) that provides aquatic animals (particularly fish) a "detour" to facilitate natural movements and migrations of diadromous and potamodromous species.
Fishways enable fish to pass around the water level difference around barriers by swimming and/or leaping up a series of relatively low stepped spillway (hence the term ladder), which is built in the form of a small, serpentine, low-discharge canal to the side of the barrier. Each "step" is created by a low weir and each section between the steps functions as a stream pool, allowing fish to rest temporarily and make graduated ascent/descent until they finally reach the main body of water on the other side of the barrier. The velocity of water falling over the step 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 make steps in steep channels to bypass obstructions.
A 1714 construction of an old channel bypassing a dam, "originally cut for the passage of fish up and down the river", is mentioned in the 1823 U.S. Circuit Court Case Tyler v. Wilkinson. This example predates the 1880 fish ladder at Pawtuxet Falls. The 1714 channel "wholly failed for this purpose" and, in 1730, a mill was built in its place. The channel and its mill usage became an important legal case in U.S. water law.
A pool and weir salmon ladder was built around 1830 by James Smith, a Scottish engineer on the River Teith, near Deanston, Perthshire in Scotland. Both the weir and salmon ladder are there today and many subsequent salmon ladders built in Scotland were inspired by it.
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 1852âÂÂ1854, 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. USA legislated fishways in 1888.
As the Industrial Age advanced, dams and other river obstructions became larger and more common, leading to the need for effective fish by-passes.
Fish ladders have a mixed record of effectiveness. This varies for different types of species, with one study showing that only three percent of American Shad make it through all the fish ladders on the way to their spawning ground. Effectiveness depends on the fish species' swimming ability, and how the fish moves up and downstream. A fish passage that is designed to allow fish to pass upstream may not allow passage downstream, for instance.
Fish passages do not always work. In practice a challenge is matching swimming performance data to hydrodynamic measurements. Swim tests rarely use the same protocol and the output is either a single-point measurement or a bulk velocity. In contrast, physical and numerical modelling of fluid flow (i.e. hydrodynamics) deliver a detailed flow map, with a fine spatial and temporal resolution. Regulatory agencies face a difficult task to match hydrodynamic measurements and swimming performance data.
In North Carolina, the Cape Fear River watershed, effectiveness of the fish passageway improves habitual conditions for cleaner water and increase fish population. Cape Fear River uses "lock and dam" structures that prevent fishes from migrating upstream. In 2012, at Lock and Dam No.1, the watershed allows the fish to migrate upstream using a fish passage structure that resembles stream rapids. The Cape Fear River Partnership by NOAA's Office of Habitat Conservation, are using the fish passageways to restore fish back into the water shed to improve the habitat with cleaner water and benefiting local communities.
The ecological impact of culverts on natural streams and rivers has been recognised. While the culvert discharge capacity derives from hydrological and hydraulic engineering considerations, this results often in large velocities in the barrel, which may prevent fish from passing through.
Baffles may be installed along the barrel invert to provide some fish-friendly alternative. For low discharges, the baffles decrease the flow velocity and increase the water depth to facilitate fish passage. At larger discharges, baffles induce lower local velocities and generate recirculation regions. It may be possible to accommodate fish passage by alternating baffle slots thereby mimicking natural sinuosity of stream flow. However, baffles can reduce drastically the culvert discharge capacity for a given afflux, thus increasing substantially the total cost of the culvert structure to achieve the same design discharge and afflux. It is believed that fish-turbulence interplay may facilitate upstream migration, albeit an optimum design must be based upon a careful characterization of both hydrodynamics and fish kinematics. Finally the practical engineering design implications cannot be ignored, while a solid understanding of turbulence typology is a basic requirement to any successful boundary treatment conducive of upstream fish passage.
Hydropower plants block fish migration which can adversely affect biodiversity. Fish passageways are used to replenish the affected habit. For example, a project in Southwest Washington reopened 117 miles of salmon and steelhead habitat at its hydroelectric dams. Fish migrating downstream are caught, transported by truck around the three dams and released downstream of the dams. Once adults, a fish ladder and sorting facility allows the fish to migrate upstream.
Another approach, technical fishways, are made with natural material to mimic natural waterways with low slopes and flow and large spaces with low heights for species to move. Technical fishways are constructed to allow fish species to move safely in any direction through barriers. Hydropower plants and their dams can use these types of fishway to adapt to fish species movements and provide efficient maintenance.