Install pool-and-weir fish passes

Overall effectiveness category Evidence not assessed
Number of studies: 2
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Background information and definitions

A pool-and-weir fish pass (or fishway) is a structure designed to help fish, such as anguillid eels, gradually ascend barriers in rivers, including dams or weirs. They are composed of a series of small, stepped pools arranged like a staircase (Carling et al. 1992, Larinier 2002). Each pool is separated by a weir, a small barrier over which water flows. The small height difference between pools enables fish to move between them. The weirs are usually designed with gaps or notches that fish can swim or leap through. Pool-and-weir fish passes are typically installed for upstream-migrating fish, but may also be used for downstream passage.

Note: A pool-and-weir fish pass may also be referred to as a pool fish pass, a weir fish pass, pool-and-traverse or step-and-pool fish pass.

For other actions that may help eels to move past barriers, see ‘Install bypass systems alongside exclusion devices’, ‘Install siphon fish passes, 'Install baffled fish passes’, ‘Install pool-and-orifice fish passes’, ‘Install weirs with notches’, ‘Install nature-like fishways’, 'Install vertical-slot fish passes’, ‘Install rock-ramp fish passes’, ‘Install fish elevators/lifts’, ‘Install fish locks’, ‘Install preliminary weirs (pre-barrages, check weirs)', ‘Install climbing structures for fish’, ‘Install airlift fish passes’ and ‘Install fish passes (type not specified)’.

^{Carling P.A. & Dobson J.H. (1992) Fish pass design and evaluation: Phase 1. Institute of Freshwater Ecology, R & D project 304 5. National Rivers Authority, Bristol. https://nora.nerc.ac.uk/id/eprint/18091/1/N018091RE.pdf}

^{Larinier M. (2002) Baffle fishways. Bulletin Francais de la Peche et de la Pisciculture, 364, 21–27. https://doi.org/10.1051/kmae/2002109}

Study locations

Key messages

Two studies evaluated the effects of installing pool-and-weir fish passes on anguillid eel populations in inland habitats. One study was in Belgium and one was in Germany.

COMMUNITY RESPONSE (0 STUDIES)

POPULATION RESPONSE (0 STUDIES)

BEHAVIOUR (2 STUDIES)

Use (2 studies): One study in Belgium found that a pool-and-weir fish pass at a hydropower station was used by more European eels to travel upstream than a vertical-slot fish pass. One study in Germany found that a pool-and-weir fish pass at a hydropower station, along with another fishway, was used by almost one-fifth of tagged European silver eels to migrate downstream.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

A study in 2013 in a river near Visé, Belgium (Nzau Matondo et al. 2017) reported that a pool-and-weir pass at a hydropower dam was used by more European eels Anguilla anguilla to travel upstream than a vertical-slot pass. Unless stated, statistical significance was not assessed. Over six months, 271 eels (196–765 mm long) were captured in the pool-and-weir fish pass, whereas 164 eels (261–836 mm long) were captured in the vertical-slot fish pass. After tagging and releasing the captured eels downstream, there was no significant difference in the number of tagged eels passing through the two types of fish pass for a second time (pool-and-weir: 84 eels detected, 11 eels recaptured; vertical-slot: 60 eels detected, 16 eels recaptured). Alongside a dam at a hydropower station, a pool-and-weir fish pass (48 m long) was installed in 1980 and a vertical-slot fish pass (305 m long) in 1998. From April to September 2013, eels were captured in two cone traps in the-pool-and-weir pass and eight net traps in the vertical-slot pass. The vertical-slot pass had higher discharge, larger pools and deeper slots than the pool-and-weir pass (see paper for details). Captured eels were radio-tagged, released 0.3 km downstream of the dam and either recaptured or detected with an antenna upstream of each fish pass.
Study and other actions tested
Referenced paper
Nzau Matondo B., Benitez J.P., Dierckx A., Philippart J.C. & Ovidio M. (2017) Assessment of the entering stock, migration dynamics and fish pass fidelity of European eel in the Belgian Meuse River. River Research and Applications, 33, 292-301.
A study in 2014–2015 in a river in Germany (Økland et al. 2017) found that two fish passes, including a pool-and-weir fish pass, at a hydropower station were used by almost one-fifth of European silver eels Anguilla anguilla to migrate downstream. Of 111 tagged eels that passed the power station, 19 eels (17%) used a pool-and-weir fish pass, and 16 eels (14%) used another fishway (design not reported). The remaining eels passed via an ‘Archimedes’ screw turbine (45 eels, 41%), through a ‘Francis’ turbine or a debris opening at the turbine (19 eels, 17%) or over the dam (12 eels, 11%). In 2014, a total of 136 silver eels (60–114 cm length) were captured in the river, fitted with radio tags, and released 4.6 km upstream of the power station. Stationary receivers were placed at migration routes past the power station, including a pool-and-weir fishway adjacent to an ‘Archimedes’ screw turbine and a fishway adjacent to a Francis turbine (details not provided). Tagged eels were recorded passing the power station from October 2014 to July 2015. 
Study and other actions tested
Referenced paper
Økland F., Teichert M.A.K., Havn T.B., Thorstad E.B., Heermann L., Sæther S.A., Tambets M. & Borcherding J. (2017) Downstream migration of European eel at three German hydropower stations. Norwegian Institute for Nature Research (NINA) report, 1355.

Please cite as:

Cutts V., Berthinussen A., Reynolds S.A., Clarhäll A., Land M., Smith R.K. & Sutherland W.J. (2024) Eel Conservation in Inland Habitats: Global evidence for the effects of actions to conserve anguillid eels. Conservation Evidence Series Synopses. University of Cambridge, Cambridge, UK.

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This Action forms part of the Action Synopsis:

Eel Conservation in Inland Habitats

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