Install exclusion devices at water intake and discharge points

Overall effectiveness category Evidence not assessed
Number of studies: 2
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Effectiveness
not assessed
Certainty
not assessed
Harms
not assessed

Calculating overall effectiveness category

Background information and definitions

Water intake channels at hydroelectric dams can pose a significant and often lethal threat to migrating anguillid eels, primarily due to the presence of underwater turbines. Eels are at risk of being struck by the fast-moving turbine blades, becoming trapped within the turbine components, or suffering fatal injuries caused by intense shear forces (Pracheil et al. 2016).

Exclusion devices at both water intake and discharge points may be installed with the aim of mitigating these dangers. For example, bar racks may prevent eels from entering intake pipes or channels by physically blocking their passage (Motyka et al. 2024). Other techniques include rotary disk screens, wedge wire screens, travelling screens, geotextile barriers and rock bunds, however, different screening methods may vary in their suitability for different sizes of eel (Turnpenny & O’Keefe 2005, Jellyman et al. 2023).

Studies are included here that test the effects of installing an exclusion device to prevent eels from entering water intakes. Exclusion devices are also often installed alongside a bypass system, e.g. a channel or pipe, to provide safe passage around a water intake/discharge. For studies that test the effectiveness of this combined action, see ‘Threat: Natural system modifications – Install bypass systems alongside exclusion devices’.

For actions that aim to provide safer eel passage by modifying underwater turbines, see ‘Modify design of underwater turbines’ and ‘Modify operation of underwater turbines’.

^{Jellyman P.G., Quilty C.M., Thomas C.R., Green C.B. & Hickford M.J.H. (2023) Laboratory-based comparison of screening materials for excluding juvenile freshwater fishes from New Zealand water intakes. New Zealand Journal of Marine and Freshwater Research, 58, 176–194. https://doi.org/10.1080/00288330.2023.2173256}

^{Motyka R., Watz J., Aldvén D., Carlsson, N., Eissenhauer F., Harbicht A., Karathanou E., Knieps T., Lind L. & Calles O. (2024) Downstream passage performance of silver eel at an angled rack: effects of behaviour and morphology. Hydrobiologia, 851, 3701–3710. https://doi.org/10.1007/s10750-024-05530-5}

^{Pracheil B.M., DeRolph C.R., Schramm M.S. & Bevelhimer M.P. (2016) A fish-eye view of riverine hydropower systems: the current understanding of the biological response to turbine passage. Reviews in Fish Biology and Fisheries, 26, 153–167. https://doi.org/10.1007/s11160-015-9416-8}

^{Turnpenny A.W.H. & O’Keeffe N. (2005) Screening for Intake and Outfalls: a best practice guide. Science Report SC030231. Environment Agency, Bristol, UK. https://assets.publishing.service.gov.uk/media/5a7c9293ed915d6969f45d2d/scho0205bioc-e-e.pdf}

Study locations

Key messages

Two studies evaluated the effects of installing exclusion devices at water intake and discharge points on anguillid eel populations in inland habitats. Both studies were in Sweden.

COMMUNITY RESPONSE (0 STUDIES)

POPULATION RESPONSE (2 STUDIES)

Survival (2 studies): Two studies in Sweden reported that 8% and 29% of European eels died after passing through exclusion devices (angled bar racks) covering turbine intakes.

BEHAVIOUR (2 STUDIES)

Use (2 studies): One of two studies in Sweden reported that exclusion devices (angled bar racks) prevented most European eels from entering turbine intakes. The other study found that all tagged European eels passed through an angled bar rack into turbine intakes.

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 2007 in a river in southwestern Sweden (Calles et al. 2012) reported that an angled bar rack at a hydropower station did not prevent migrating European eels Anguilla anguilla from entering a turbine intake, and over one-third of tagged eels died passing through the bar rack and turbine. All 22 tagged eels passed through a bar rack across a turbine intake, eight of which died (36%). A bar rack (22 mm bar spacing in spring and 40 mm during rest of year, angled 77° from vertical) was installed across an intake leading to a turbine at a powerhouse. No downstream fish passages were present at the powerhouse (eels could only travel downstream via an upstream ‘Denil’ fish pass or when spill gates were open). In October 2007, forty-two silver eels were caught in the river, radio-tagged, and released 24 km upstream of the hydropower station. Sixteen radio-tagged eels were tracked as they passed one of two powerhouses at the hydropower station.
Study and other actions tested
Referenced paper
Calles O., Karlsson S., Hebrand M. & Comoglio C. (2012) Evaluating technical improvements for downstream migrating diadromous fish at a hydroelectric plant. Ecological Engineering, 48, 30-37.
A study in 2008 in a river in southwestern Sweden (Calles et al. 2013) reported that angled bar racks prevented most migrating European eels Anguilla anguilla from entering turbine intakes, although almost one-fifth of eels that entered collection traps in the racks were injured or died. Overall, 6 of 40 radio-tagged eels (15%) passed through the bar racks, three of which (8%) were killed by turbines. The other radio-tagged eels entered collection traps in the racks (31 eels, 78%), passed via dam spill gates (2 eels, 5%) or remained in the reservoir upstream (1 eel, 3%). In the collection traps, 38 of 196 tagged and untagged eels (19%) were injured, dead, or had altered behaviour. Bar racks (18 mm bar spacing, 35° slope, 8.4 m long x 5.4 m wide) were installed across three parallel turbine intakes. Two entrances (0.25 m wide, 1 m long) in each rack led to collection traps. In October 2008, groups of 5–17 silver eels that had been caught in the river were fitted with radio-tags (total 40 eels) or streamer tags (total 45 eels) and released 300 m upstream of the racks. Radio-tagged eels were tracked in October–November 2008. Tagged and untagged eels were collected from collection traps during the same period. 
Study and other actions tested
Referenced paper
Calles O., Karlsson S., Vezza P., Comoglio C. & Tielman J. (2013) Success of a low‐sloping rack for improving downstream passage of silver eels at a hydroelectric plant. Freshwater Biology, 58, 2168-2179.

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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This Action forms part of the Action Synopsis:

Eel Conservation in Inland Habitats

Related Actions

Actions	Effectiveness	Studies	Category
Cease or prohibit renewable energy installations in an area Action Link	No evidence found (no assessment)	0	Synopsis Link
Install exclusion devices at water intake and discharge points Action Link	Evidence not assessed	2	Synopsis Link
Modify operation of underwater turbines Action Link	Evidence not assessed	2	Synopsis Link