Install climbing structures for fish

How is the evidence assessed?
  • Effectiveness
    not assessed
  • Certainty
    not assessed
  • Harms
    not assessed

Study locations

Key messages

  • Twelve studies evaluated the effects of installing climbing structures for fish on anguillid eel populations in inland habitats. Five studies were in the USA, three were in the UK and France and one was in Sweden. 

COMMUNITY RESPONSE (0 STUDIES) 

 

POPULATION RESPONSE (2 STUDIES) 

  • Abundance (2 studies): Two studies (including one before-and-after study) in France and the USA found that, 1–6 years after installing fish ladders/ramps at dams (in one case along with removing two dams10), European eel numbers and American eel density increased upstream of the ladders/ramps. 

BEHAVIOUR (12 STUDIES) 

  • Use (12 studies): Five studies in the USA reported that some American eels used eel ladders/ramps to move upstream at dams over periods of 2–10 years, but in one case use declined over seven years. Seven studies (including two replicated, controlled studies and one before and after study) in France, the UK and Sweden found that some European eels used eel ladders, ramps and tiles to move upstream at dams in rivers and experimental weirs, and in three studies in varying proportions (11–92%). Three of the studies tested the effects of ladder design and found that more European eels used ladders/ramps with a studded surface than a bristle or open weave surface, or with small than large studs8, and smaller eels were more likely to use steeper ramps with larger bristle spacing. One replicated, controlled study found that more European eels used a ladder at an experimental weir at high water velocities than medium or low water velocities.

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

  1. A study (year not stated) in a river in Dordogne, France (Legault 1992) reported that steep ramps at a hydropower dam were used by smaller eels, while shallow ramps were used by larger eels, but this depended on bristle spacing on the ramp surface, with fewer eels using ramps with smaller spacing. Differences in eel size but not abundance were statistically tested. On average, fewer, larger eels used a 15° ramp (20 eels/h, average length 232–256 mm) compared to a 30° (56 eels/h, 190–218 mm) and 45° ramp (23 eels/h, 198–226 mm). Fewer, smaller eels used ramps with smaller spacing between bristles on the ramp surface: 7 mm (151–285 eels, 209 mm), 14 mm (553–1,983 eels, 232 mm) and 21 mm (69–1,347 eels, 220 mm). Eels caught in the ramps were smaller (6,276 eels, 120–380 mm) than eels caught in a fish elevator (202 eels, 140–395 mm). Three experimental eel ramps (each 2.4 m long and 0.3 m wide) with speed bumps were placed downstream of a fish pass at Tuilière dam (105 m long, 12 m high). The ramps contained brushes mounted on a PVC support. Each ramp had different spacing between the brush bristles: 21-, 14- and 7-mm. Eels were trapped after passing over the ramps for 12 days in June–July. The slope of the ramps (15°, 30° or 45°) was modified every 3–4 days. Eels were trapped passing through a fish elevator at the same dam as a comparison. 

    Study and other actions tested
  2. A before-and-after study in 1981 and 19962003 in a river in Brittany, France (Briand et al. 2005) found that some glass and yellow European eels Anguilla anguilla used an eel ladder at a tidal dam to travel upstream, and eel densities increased in upstream areas after the ladder was installed. Each year from 1996 to 2003, approximately 0.22.5 million glass and 6,60037,000 yellow eels were captured on the eel ladder.  On average, eel densities were greater upstream of the dam during six years after installation of the ladder (0.30.8 eels/m2) compared to one year before (0.1 eels/m2). In 1995, an eel ladder (comprising two inclined panels with synthetic bristles located on each side of a pooling pass) was installed on a tidal dam within an estuary. The dam was constructed in 1970 to reduce flooding. Thirteen additional eel ladders were constructed on dams further inland (>100 km from the sea) in 1999 and 2000 (use by eels not reported). Electrofishing surveys were carried out in netted sections (1201,200 m2) in 10 tributaries of the river system before (1981 in 17 sections) and after (19982003 in 19 sections) installation of the eel ladder on the tidal dam. In 19962003, eels were captured in nets on the ladder daily during spring and weekly when catches were low. Yellow eels were counted and glass eel numbers estimated from wet weight.

    Study and other actions tested
  3. A study in 20062007 in an upland river in New York, USA (Schmidt et al. 2009) reported that an eel ladder at a small dam was used by some migrating American eels Anguilla rostrata to move upstream, and most eels were small in size. Overall, 183 eels (751 cm long) used the ladder over two years (51132 eels/year). Most of the eels using the ladder (7386%) were <20 cm in length. In AprilOctober 2006 and 2007, an eel ladder was installed at a 2-m high dam at a mill. The ladder consisted of an aluminium ramp (1.9 m long, 42 cm wide, 35.7° to the river surface). One side was covered in plastic pegs, and the other with plastic mesh. A flow of water (0.5 l/s) was created on the ladder by a pair of siphons. Water from a collection bucket was piped to the base of the ladder to transfer the odour of captured eels as an attractant. In AprilOctober 2006 and 2007, the collection bucket was checked at least twice weekly. Captured eels were measured and released.   

    Study and other actions tested
  4. A study in 2011–2013 in a river in West Virginia, USA (Welsh & Aldinger 2014) reported that an eel ladder at a dam was used by some yellow American eels Anguilla rostrata to migrate upstream. Eight years after the eel ladder was installed, 177 eels were recorded using the ladder over a 15-day period. Ten years after installation, 1,184 eels were recorded over a five-day period. Eels were 223–530 mm long. An eel ladder was installed at a dam in 2003 to assist upstream passage of migrating eels. Eels passing through the eel ladder were photographed using an infrared motion sensor camera (see paper for details). Eels were monitored for 15 days in July 2011 and five days in July 2013. Eel length was recorded for 251 eels. 

    Study and other actions tested
  5. A replicated, controlled study in 2011 in an indoor channel in the UK (Kerr et al. 2015) found that installing vertical bristle fish passes resulted in greater numbers of yellow European eels Anguilla anguilla passing upstream over an experimental weir at high, but not medium or low, water velocities. At high water velocities, the percentage of times eels successfully passed the weir was greater with fish passes installed for small (92%) and large eels (57%) than without fish passes (small eels: 0%, large eels: 5%). The percentage of large eels that successfully passed the weir was also higher with fish passes installed (77%) than without (17%). Successful passage of small and large eels did not differ significantly with and without fish passes installed at medium and low water velocities (see paper for data). A Crump weir (34 cm high) was installed in an indoor channel (21 m long, 1.4 m wide, 0.6 m deep). Vertical fish passes (10-mm thick polypropylene boards), each covered with clusters of 24 synthetic bristles (70 mm long, 1.5 mm diameter, spaced 30 mm apart), were installed along both sides of the weir, facing towards the channel wall. In May–July 2011, wild-caught small (82–320 mm long) and large (322–660 mm long) yellow eels were released in the channel during 2–4 trials (8–12 eels/trial) at each of three water velocities (maximum velocity: 'high' = 2.4 m/s, 'medium' = 1.9 m/s, 'low' = 0.8 m/s), with and without fish passes installed. Each trial was carried out at night and lasted 5.5 h. Eel behaviour was monitored with video cameras. Large eels were radio-tagged. 

    Study and other actions tested
  6. A study in 2007–2014 in a river in West Virginia, USA (Welsh & Loughman 2015) found that an eel ladder at a dam was used by some American eels Anguilla rostrata to travel upstream during eight years of monitoring. Eels used the ladder in all eight years of monitoring (852–5,394 eels/year). A stainless-steel fish ladder (11-m long, 13-cm deep and 41-cm wide with a 50° slope) was installed in a 5-m high dam in the Shenandoah River from late spring (May–July) to autumn (October–November) in 2007–2014 (106–188 days/year). The ladder contained three vertical rows of plastic pipes (5.1 cm diameter) to provide a climbing substrate for eels. It was designed for upstream passage of American eels 15–85 cm in length. Eels were collected and counted from a net bag (3.2 mm mesh) at the upstream end of the ladder. Eels passing through the ladder were also photographed using an infra-red triggered camera. 

    Study and other actions tested
  7. A study in 20092010 in a river in France (Drouineau et al. 2015) found that an elver ladder at a gate was used by up to two-thirds of tagged young European eels Anguilla anguilla to travel upstream. During nine tagging events, 1161% of tagged eels were recaptured after climbing an elver ladder to travel upstream. Recapture rates were greatest when the river level was high (data reported as statistical model results). The brush ladder (6-m long, 45° angle) was installed in 2008. On nine days in MayJune 2009 and 2010, young eels (342551 eels/day, 51245 mm long) collected in a trap at the installed elver ladder were tagged, monitored for 24 h, and released at three locations downstream of the gate (in front of the ladder, or 50 m downstream by either the left or right bank). A trap at the top collected all eels climbing the ladder. For 15 days after each release, tagged eels were collected from the trap every 23 days. 

    Study and other actions tested
  8. A replicated, controlled study in 2013 in an indoor channel in the UK (Vowles et al. 2015) found that installing eel tiles resulted in greater numbers of juvenile European eels Anguilla anguilla passing upstream over an experimental weir, and more eels passed over tiles with small than large studs. On average, 20 eels/trial successfully passed over the weir with eel tiles installed, whereas no eels passed over the weir without eel tiles. Similar numbers of eels attempted to pass the weir with and without tiles installed (both 30 eels/trial). More eels passed the weir over tiles with small (12 eels/trial) than large studs (8 eels/trial). In May 2013, a Crump weir (25 cm high) was installed in an indoor channel (12 m long, 0.3 m wide, 0.4 m deep). Eel tiles with small studs (1.5 cm diameter studs spaced 4.5 cm apart) were installed on half of the downstream face of the weir, while tiles with large studs (3 cm diameter studs spaced 8.5 cm apart) were installed on the other half. All studs were 5-cm high. Behaviour of 30 wild-caught eels released in the channel was recorded by an overhead video camera during each of ten 10-min trials with tiles installed on the weir and without. 

    Study and other actions tested
  9. A study in 2013–2015 in a river in southeast England, UK (Piper et al. 2018) found that two upstream eel ladders were used by around one-fifth of tagged yellow European eels Anguilla anguilla to travel upstream past a hydropower station. Of 34 tagged yellow eels that approached from downstream, 6–7 eels (18–21%) travelled through two upstream eel passes. Two yellow eels were also recorded travelling downstream through one of the eel passes. Two upstream eel passes consisting of troughs (6 m long, 0.2 m wide, 26° slope) with nylon bristles (100 mm long, 18 mm spacing) were installed along the channel walls on both sides of an 'Archimedes' screw turbine (one in 2012, one in April 2014). In April–September 2013 and March–July 2014, yellow eels captured in the river were tagged and released 100 m upstream (total 74 eels) and downstream (total 216 eels) of the hydropower station. Eels were tracked using radio antennas from April 2013 to March 2015, and with imaging sonar from November to March 2015. 

    Study and other actions tested
  10. A study in 2013–2016 in a river in Massachusetts, USA (Turner et al. 2018) reported that adding a fish ladder and ramp to a dam, along with removing two other dams, led to an increase in the number of American eels Anguilla rostrata migrating upstream to a lake over four years, but their size did not change. The study does not distinguish between the effects of installing a fish ladder and ramp and dam removal. One year after installing the fish ladder and ramp, and removing dams, 16 eels were caught in an upstream lake compared to 99 eels after four years, although the difference was not tested for statistical significance. Eel length did not differ significantly between the four sampling years (after one year: 26–61 cm, after four years: 28–70 cm). In 2012, a fish ladder and eel ramp were installed at an intact dam and two other dams were removed from a river. In July–August 2013–2016, eels were caught in a lake (upstream of the ladder, ramp and dams) in 16–30 eel pots placed in shallow habitats along the lake perimeter, including islands. Pots were checked three times/week for 4–6 weeks. 

    Study and other actions tested
  11. A study in 2017 in a river in Laholm, Sweden (Watz et al. 2019) found that more juvenile European eels Anguilla anguilla used upstream ramps with a studded surface than an open weave or bristle surface, and in slow than fast flowing water. More eels were caught using a ramp with a studded surface (2 eels/night) compared to ramps with an open weave (0.9 eels/night) or a bristle surface (0.4 eels/night). There was no significant difference between the number of eels using open weave or bristle surfaced ramps. More eels were caught on all ramps when they were placed in slow flowing water (1.7 eels/night) compared to fast flowing water (0.5 eels/night). Six ramps attached to floating devices were placed in a channel carrying water away from a hydropower plant (‘tailrace’) in the River Lagan. Three ramps were placed at each side of the channel, 1–2 m from the bank (0, 30 and 60 m downstream of the hydropower plant). Each ramp consisted of three aluminium lanes (2 m long, 32 cm wide), each with a studded, open weave or bristle wetted surface (see paper for details). Water flow was fast on one side and slow on the other side of the channel. Eels were caught in buckets at the end of the ladders for five nights in July 2017. 

    Study and other actions tested
  12. A study in 2010–2019 in a river on the border of the USA and Canada (Mack & Cheatwood 2019) reported that two eel ladders at a hydropower station were used by some American eels Anguilla rostrata to migrate upstream but use declined over seven years. The average number of eels using the eel ladders was 376 eels/day in 2011 and 7 eels/day in 2018. Eels used the ladders from June to October each year. Overall, 215,300 eels used the eel ladders from 2010–2014, compared to 73,000 eels in 2015–2019. Two eel ladders were installed either side of a dam, one in 1974 and one in 2006. The ladders deposited eels into the river 300 m upstream of the dam. Eels were counted moving through the ladders using a photoelectric counter. Eel capture data from 2010 to 2019 were obtained from public records or project operators (see paper for details).

    Study and other actions tested
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.

Where has this evidence come from?

List of journals searched by synopsis

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Eel Conservation in Inland Habitats

This Action forms part of the Action Synopsis:

Eel Conservation in Inland Habitats
Eel Conservation in Inland Habitats

Eel Conservation in Inland Habitats - Published 2024

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