Remove dams/barriers

Overall effectiveness category Evidence not assessed
Number of studies: 1
Share Tweet Email

View assessment score

How is the evidence assessed?

Effectiveness
not assessed
Certainty
not assessed
Harms
not assessed

Calculating overall effectiveness category

Background information and definitions

Man-made barriers, such as dams and weirs, can impede anguillid eel migration (Itakura et al. 2020, Yokouchi et al. 2022). Barriers to migration are thought to be a major contributor to the decline in eel populations (Limburg et al. 2009). Fish passes and bypass channels may be installed at barriers with the aim of improving eel passage, but they are unlikely to eliminate the threat entirely (Brown et al. 2013). The complete removal of such barriers may be necessary to restore connectivity and allow populations to recover (Hitt et al. 2012).

For actions that aim to provide eel passage at dams or barriers without removal, see ‘Install bypass systems alongside exclusion devices’, ‘Install siphon fish passes, 'Install baffled fish passes’, ‘Install pool-and-weir fish passes’, ‘Install pool-and-orifice fish passes’, ‘Install weirs with notches’, ‘Install nature-like fishways’, ‘Install rock-ramp fish passes’, ‘Install fish elevators/lifts’, ‘Install vertical-slot fish passes’, ‘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)’.

^{Brown J.J., Limburg K.E., Waldman J.R., Stephenson K., Glenn E.P., Juanes F. & Jordaan A. (2013) Fish and hydropower on the U.S. Atlantic coast: failed fisheries policies from half-way technologies. Conservation Letters, 6, 280–286. https://doi.org/10.1111/conl.12000}

^{Hitt N.P., Eyler S., & Wofford J.E.B. (2012) Dam removal increases American eel abundance in distant headwater streams. Transactions of the American Fisheries Society, 141, 1171–1179. https://doi.org/10.1080/00028487.2012.675918}

^{Itakura H. & Wakiya R. (2020) Habitat preference, movements and growth of giant mottled eels, Anguilla marmorata, in a small subtropical Amami-Oshima Island river. PeerJ, 8, e10187. https://doi.org/10.7717/peerj.10187}

^{Limburg K.E. & Waldman J.R. (2009) Dramatic declines in North Atlantic diadromous fishes, BioScience, 59, 955–965, https://doi.org/10.1525/bio.2009.59.11.7}

^{Yokouchi K., Itakura H., Wakiya R., Yoshinaga T., Mochioka N., Kimura S. & Kaifu K. (2022) Cumulative effects of low-height barriers on distributions of catadromous Japanese eels in Japan. Animal Conservation, 25, 137–149. https://doi.org/10.1111/acv.12725}

Key messages

One study evaluated the effects of removing dams or barriers on anguillid eel populations in inland habitats. The study was in the USA.

COMMUNITY RESPONSE (0 STUDIES)

POPULATION RESPONSE (1 STUDY)

Abundance (1 study): One study in the USA reported that after removing two dams, along with adding a fish ladder and ramp to another dam, the number of American eels migrating upstream in a river increased over four years.

Condition (1 study): One study in the USA reported that after removing two dams, along with adding a fish ladder and ramp to another dam, the size of American eels migrating upstream in a river did not change over four years.

BEHAVIOUR (0 STUDIES)

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 2013–2016 in a river in Massachusetts, USA (Turner et al. 2018) reported that removing two dams, along with adding a fish ladder and ramp to another dam, led to an increase in the number of American eels Anguilla rostrata caught migrating upstream to a lake over four years, but their size did not change. The study does not distinguish between the effects of dam removal and installing a fish ladder and ramp. One year after dam removal and modification, 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, two dams were removed from a river and a third intact dam had a fish ladder and eel ramp installed. In July–August 2013–2016, eels were caught in a lake (upstream of the three 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
Referenced paper
Turner S.M., Chase B.C. & Bednarski M.S. (2018) Evaluating the effect of dam removals on yellow‐phase American eel abundance in a Northeastern U.S. watershed. North American Journal of Fisheries Management, 38, 424-431.

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

All the journals searched for all synopses

This Action forms part of the Action Synopsis:

Eel Conservation in Inland Habitats

Related Actions

Actions	Effectiveness	Studies	Category
Modify flow regimes along regulated rivers (e.g. release water to encourage upstream movements) Action Link	Evidence not assessed	1	Synopsis Link
Restrict hydropeaking Action Link	No evidence found (no assessment)	0	Synopsis Link
Install fish passes (type not specified) Action Link	Evidence not assessed	4	Synopsis Link