Provide refuges

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

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Background information and definitions

Refuges are areas where individuals or populations are protected from stressors and threats, e.g. predators (Reside et al. 2019). Refuges have declined in aquatic systems due to a reduction in habitat diversity, caused by numerous factors, such as the removal of rock and woody material and the straightening of rivers (Emerson 1971). Refuges can be created through the addition of substrates, such as woody debris (logs, branches, entire trees or ‘coarse woody debris’) and boulders or cobbles (>256 mm in diameter), which add structural diversity and increase the number of microhabitats (Harmon et al. 1986, Golpira et al. 2022). Such features can create areas of slow-flowing water for anguillid eels to rest in before continuing their migration and may provide protection from predators (Sakanoue 2021).

Rock gabions, mesh cages filled with rocks or other hard materials, can also be used to create refuges, as well as to increase habitat structure and complexity (Anderson & Cameron 1980). Gabions can be filled with a mixture of stone sizes and types, which may attract different species, or life stage, of eels (Oto et al. 2022).

^{Anderson J.W. & Cameron J.J. (1980) The use of gabions to improve aquatic habitat: technical note no. 342. US Department of the Interior, Bureau of Land Management, Oregon, USA. https://ia600400.us.archive.org/35/items/useofgabion5182v00ande/useofgabion5182v00ande.pdf}

^{Emerson J.W. (1971) Channelization: a case study. Science, 173, 325–326. https://doi.org/10.1126/science.173.3994.325}

^{Golpira A., Baki A.B.M., Ghamry H., Katopodis C., Withers J. & Minkoff D. (2022) An experimental study: effects of boulder placement on hydraulic metrics of instream habitat complexity. Scientific Reports, 12, 13156. https://doi.org/10.1038/s41598-022-17281-1}

^{Oto Y., Sakanoue R., Hibino Y., Matsushige K., Utida K. & Mochioka N. (2022) Preferred gap structure within stone piles of fishing gear by Japanese eel Anguilla japonica at each life history stage: the search for an effective method to restore estuarine habitats. Nippon Suisan Gakkaishi (Japanese edition), 88, 152–161. https://doi.org/10.2331/suisan.21-00043}

^{Reside A.E., Briscoe N.J., Dickman C.R., Greenville A.C, Hradsky B.A., Kark S., Kearney M.R, Kutt A.S, Nimmo D.G., Pavey C.R., Read J.L., Ritchie E.G., Roshier D., Skroblin A., Stone Z., West M. & Fisher D.O. (2019) Persistence through tough times: fixed and shifting refuges in threatened species conservation. Biodiversity Conservation, 28, 1303–1330. https://doi.org/10.1007/s10531-019-01734-7}

^{Sakanoue R., Satoh S., Matsushige K., Yasutake Y., Hibino Y., Manabe M., Utida K. & Mochioka N. (2021) Shelter effect of the gaps within rock piles to reduce the predation on Japanese eel Anguilla japonica. Nippon Suisan Gakkaishi (Japanese edition), 87, 255–264. https://doi.org/10.2331/suisan.20-00054}

Study locations

Key messages

Two studies evaluated the effects of providing refuges on anguillid eel populations in inland habitats. Both studies were in Japan.

COMMUNITY RESPONSE (0 STUDIES)

POPULATION RESPONSE (1 STUDY)

Survival (1 study): One replicated, controlled, before-and-after study in Japan found that more Japanese eels survived in ponds with a basket of rocks in the centre (acting as a refuge), and suffered fewer attacks from herons, than in ponds without a rock pile.

BEHAVIOUR (1 STUDY)

Use (1 study): One replicated, controlled study in Japan found that baskets filled with stones were mostly used by yellow Japanese eels (as well as low numbers of glass and silver eels), and more yellow eels used baskets with small stones compared to medium or large stones.

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 replicated, controlled, before-and-after study in 2019 in eight experimental ponds in Ibusuki City, Japan (Sakanoue et al. 2021) found that placing baskets filled with rocks in the centre of ponds increased the survival rate of yellow Japanese eels Anguilla japonica and reduced the number of attacks by herons Ardea spp. Overall, 25–27 of 28 eels (89–96%) survived in ponds with baskets of rocks compared to 21–22 of 28 eels (75–79%) in ponds without. Eels were subjected to fewer attacks from herons in ponds with rocks (0.8 attacks/pond) compared to ponds without (3.9 attacks/pond), but there was no significant difference in the success rate of heron attacks (with rocks: 54%: without: 56%). Four experiments, each lasting seven days, ran from July–August 2019. In each experiment, seven wild yellow eels caught in an estuary were placed in each of eight concrete ponds (300 x 150 cm, depth: 78 cm). Four ponds contained a refuge comprising a mesh basket (100 x 100 × 50 cm) filled with 135 stones in the centre, while four other ponds contained no refuge. Before the experiment, 110 farmed loaches (Cobitoidea) were released into each pond to attract birds. Ponds were drained at the end of each experiment, and surviving eels counted. During the experiments, bird predation behaviour was monitored using a video camera.
Study and other actions tested
Referenced paper
Sakanoue R., Satoh S., Matsushige K., Yasutake Y., Hibino Y., Manabe M., Utida K. & Mochioka N. (2021) Shelter effect of the gaps within rock piles to reduce the predation on Japanese eel Anguilla japonica. Nippon Suisan Gakkaishi (Japanese edition), 87, 255-264.
A replicated study in 2020–2021 in a river in Fukuoka Prefecture, Japan (Oto et al. 2022) found that baskets filled with stones were used by Japanese eels Anguilla japonica, and that more yellow eels were found in baskets with small stones compared to medium or large stones but there was no difference for silver eels. Over nine months, five glass eels (average length: 6 cm), 164 yellow eels (average length: 24 cm) and 10 silver eels (average length: 53 cm) were observed using baskets filled with stones. On average, more yellow eels were found in baskets with small stones (3.4 eels) compared to medium (1.2 eels) or large stones (0.8 eels). There was no difference in the average number of silver eels found in baskets with different stone sizes (small: 0 eels, medium: 0.7 eels, large: 0.3 eels). Overall, the length of yellow eels did not differ significantly between stone sizes (small: 10–51 cm, medium: 11–56 cm, large: 11–41 cm), nor did yellow eel weight (small: 2–175 g, medium: 3–295 g, large: 2–91 g). Six baskets, two of each filled with small (10 cm), medium (20 cm) and large stones (30 cm), were installed 550 m upstream of the river mouth. Baskets were placed on rubber mats on the riverbed at 1-m intervals and 1-m from the bank. Baskets were surveyed 15 times from August–December 2020 and April–July 2021. Eels were counted, anaesthetised and measured.
Study and other actions tested
Referenced paper
Oto Y., Sakanoue R., Hibino Y., Matsushige K., Utida K. & Mochioka N. (2022) Preferred gap structure within stone piles of fishing gear by Japanese eel Anguilla japonica at each life history stage: the search for an effective method to restore estuarine habitats. Nippon Suisan Gakkaishi (Japanese edition), 88, 152-161.

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?

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This Action forms part of the Action Synopsis:

Eel Conservation in Inland Habitats

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