Install fish passes (type not specified)

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

Study locations

Key messages

  • Four studies evaluated the effects of installing fish passes (type not specified) on anguillid eel populations in inland habitats. Two studies were in the UK and one was in each of Germany and Lithuania. 

COMMUNITY RESPONSE (0 STUDIES) 

 

POPULATION RESPONSE (2 STUDIES) 

  • Survival (1 study): One study in Lithuania found that more than half of European eels died after passing downstream through a turbine instead of an unspecified fish pass. 
  • Abundance (1 study): One replicated, before-and-after study in the UK found that, four years after installing unspecified upstream eel passes at barriers in two rivers, European eel abundance increased upstream of the passes.  

BEHAVIOUR (3 STUDIES) 

  • Use (3 studies): Three studies in the UK, Germany and Lithuania found that unspecified upstream fish passes at hydropower stations, along with two other fish pass types, were used by most tagged European yellow eels to travel upstream1 and low numbers of European silver eels to travel downstream. In one of the studies, eels passing through an unspecified fish pass were larger than those that passed through turbines. 

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 in 2013–2015 in a river in Suffolk, UK (Piper & Wright 2017) reported that two unspecified fish passes, along with a baffled Larinier fish pass, were used by European yellow eels Anguilla anguilla to bypass a hydroelectric power turbine when migrating upstream but rarely downstream. Results are not based on tests of statistical significance. Fifty of 56 tagged yellow eels (89%) that approached the hydropower station passed through the fish passes (separate results not reported for fish pass type). Most adult (yellow and silver) eels (82%, number not reported) migrating downstream passed by a lock instead of the fish passes (percentage/number not reported). Of eels that approached the passes, the average delay before passing was 1.8 hours. In March 2013 to March 2014, yellow eels were captured, tagged, and released either upstream (74 eels) or downstream (216 eels) of a hydroelectric power turbine in the River Stour. In addition, 127 actively migrating silver eels were captured, tagged and released upstream in autumn 2013 and 2014. Antennas were installed to monitor eel movements through two unspecified (referred to as ‘trough-type’ elver passes) and one baffled Larinier fish pass.

    Study and other actions tested
  2. A study in 20142015 in a river in Germany (Økland et al. 2017) found that an unspecified upstream fish pass, along with a pool-and-weir fish pass, at a hydropower station were used by less than one-fifth of European silver eels Anguilla anguilla to migrate downstream. Of 111 tagged eels that passed the power station, 16 eels (14%) used a fish pass (design not reported) and 19 eels (17%) used a pool-and-weir fish pass. 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 (60114 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 fish pass adjacent to an ‘Archimedes’ screw turbine and an unspecified upstream fish pass 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
  3. A study (year not stated) in a river in Lithuania (Dainys et al. 2018) found that an unspecified upstream fish pass at a hydropower station was used by one-third of silver European eels Anguilla anguilla during downstream migration, and eels that used the fish pass were larger than those that passed through the turbine. During one night, 22 of 64 tagged silver eels (34%) used the fish pass, whereas 42 eels (66%) passed through the turbine. Twenty-two of the 42 eels (52%) that passed through the turbine sustained lethal injuries. On average, eels passing through the fish pass were larger (length: 72 cm, weight: 681 g) than those passing through the turbine (66 cm, 476 g). The fish pass was constructed for upstream migration of salmonids at a small hydropower station with one turbine. Sixty-four silver eels captured upstream of the power station in fyke nets were tagged and released, 0.8 km upstream of the hydropower station. Tagged eels were recaptured in fyke nets at the turbine outflow and below the fish pass after passing through or around the power station during one night (date not reported). 

    Study and other actions tested
  4. A replicated, before-and-after study in 2012–2018 in two rivers in Greater London, UK (Pecorelli et al. 2019) reported that four years after installing unspecified upstream eel passes, European eel Anguilla anguilla abundance increased upstream. In one river, the average number of eels caught ranged from 0.17–0.27 eels/day in the four years after eel pass installation compared to <0.01–0.08 eels/day in the three years before. In another river, over the same time period, the average number of eels caught ranged from 0.9–6.3 eels/day in the four years after eel pass installation compared to 0 eels/day in the three years before. In 2014, an eel pass (details not provided) was installed at a barrier in each of two rivers. In 2016, a second eel pass of unknown design was installed in one of the rivers. Eel traps were installed upstream of the barriers in each river. A sloping ramp made from plastic roof gutter (1.5–2 m long; 100 mm wide) lined with netting allowed eels to crawl into the trap. Water was supplied to the traps from upstream of the barrier using a 30-mm diameter pipe. Eels were monitored from April to September 2012–2018 by citizen scientists who had received a 2 h training session.  

    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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