Change capture, transport or release methods to increase survivorship of translocated (‘stocked’ or ‘restocked') eels

Overall effectiveness category Evidence not assessed
Number of studies: 3
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Background information and definitions

Changing the methods used to capture, transport or release translocated (‘stocked’ or ‘restocked’) anguillid eels may increase survival after translocation. For example, this may include releasing eels of different sizes or life stages, introducing a quarantine period or allowing eels to acclimatize prior to release. The studies included here compare the effects of these different methods. For studies that test the overall effects of translocations, see ‘Translocate wild eels to re-establish or boost native populations (‘stocking’ or ‘restocking’)’.

Note that translocated eels are typically monitored through mark and recapture, but different methods of marking or tagging can influence recapture rates, or even survival, of translocated eels (Josset et al. 2016, Kullman et al. 2018, Jepsen et al. 2022).

^{Jepsen N., Richter L., Pedersen M.I. & Deng Z. (2022) Survival, growth and tag retention of juvenile European eel (Anguilla anguilla L.) with implanted 12 mm passive integrated transponder tags and acoustic tags. Journal of Fish Biology, 101, 1375–1380. https://doi.org/10.1111/jfb.15183}

^{Josset Q., Trancart T., Mazel V., Charrier F., Frotte L., Acou A. & Feunteun E. (2016) Pre-release processes influencing short-term mortality of glass eels in the French eel (Anguilla anguilla, Linnaeus 1758) stocking programme. ICES Journal of Marine Science, 73, 150–157. https://doi.org/10.1093/icesjms/fsv074}

^{Kullmann B., Hempel M. & Thiel R. (2018) Chemical marking of European glass eels Anguilla anguilla with alizarin red S and in combination with strontium: In situ evaluation of short-term salinity effects on survival and efficient mass-marking. Journal of Fish Biology, 92, 203–213. https://doi.org/10.1111/jfb.13508}

Study locations

Key messages

Three studies evaluated the effects of changing the capture, transport or release methods to increase the survivorship of translocated (‘stocked’ or ‘restocked’) anguillid eels in inland habitats. One study was in France, one was in Belgium and one in Denmark.

COMMUNITY RESPONSE (0 STUDIES)

POPULATION RESPONSE (3 STUDIES)

Survival (3 studies): One of two replicated studies (one controlled) in France and Belgium found that holding wild-caught European eels in captivity for longer periods led to higher survival during 15 days after translocation. The other study found lower survival of European eels following quarantining for 30 compared to 15 days, and no effect of water temperature (20°C vs 24°C) or providing food (cod roe and pellets) during quarantine. One replicated, randomized study in Denmark found that 18 months after translocation, survival was higher when European eels were released at lower stocking densities.

Condition (1 study): One replicated, randomized study in Denmark found that 18 months after translocation, body mass, condition and length were greater when European eels were released at lower stocking densities.

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 replicated, controlled study in 2011–2013 in 17 river sites in France (Josset et al. 2016) found that holding wild-caught European glass eels Anguilla anguilla in captivity for longer periods before translocation resulted in lower mortality rates after release. During 15 days after release, translocated wild glass eels that had been held in captivity for longer periods (maximum 45 days) had lower mortality rates than those held in captivity for shorter periods (minimum 7 days; data reported as statistical model results). Wild-caught glass eels were held in captivity for 7–45 days (average 21 days) before being translocated to six closed hoop nets (length: 1.5 m, diameter: 30 cm; 50 eels/net) at each of 17 sites across multiple rivers. Half of the eels at each site were marked with Alizarin Red dye for identification. After 15 days, remaining live eels in each net were counted to calculate mortality rates.
Study and other actions tested
Referenced paper
Josset Q., Trancart T., Mazel V., Charrier F., Frotté L., Acou A. & Feunteun E. (2016) Pre-release processes influencing short-term mortality of glass eels in the French eel (Anguilla anguilla, Linnaeus 1758) stocking programme. ICES Journal of Marine Science, 73, 150-157.
A replicated study in 2017–2018 at a laboratory in Belgium (Delrez et al. 2021) found that wild-caught European glass eels Anguilla anguilla had higher survival rates following release into outdoor basins after quarantine periods of 15 rather than 30 days, and providing food or modifying water temperatures during quarantine had no effect on survival rates. In each of two years, average eel survival rates were higher following release after a 15-day quarantine period (2017: 99–100%, 2018: 98–99%) than a 30-day quarantine period (2017: 95–97%, 2018: 91–94%). Survival rates during quarantine did not differ significantly between eels that were fed (2017: 99–100%, 2018: 98–99%) or not fed (2017: 99–100%, 2018: 100%), or kept in tanks with water temperatures of 20°C (2017: 95–99%, 2018: 94–100%) or 24°C (2017: 97–100%, 2018: 91–100%). Glass eels were wild caught in estuaries in France (2017) and the UK (2018) and transported to the laboratory. After acclimatization, 400 eels were transferred to each of six 40-l quarantine tanks containing PVC pipes. Eels were quarantined for 15 days (four tanks) or 30 days (two tanks). Water in half of the six tanks was kept at 20°C, and half at 24°C. Eels in half of the tanks quarantined for 15 days were provided with food (cod roe and pellets), while the other half were unfed. Following quarantine, three groups of 50 eels were transferred from each tank to separate plastic cages randomly placed in three outdoor basins (110 cm long x 110 cm wide, 33 cm water depth) containing groundwater. Surviving eels were counted after 15 days.
Study and other actions tested
Referenced paper
Delrez N., Zhang H., Lieffrig F., Mélard C., Farnir F., Boutier M., Donohoe O. & Vanderplasschen A. (2021) European eel restocking programs based on wild-caught glass eels: Feasibility of quarantine stage compatible with implementation of prophylactic measures prior to scheduled reintroduction to the wild. Journal for Nature Conservation, 59, 125933.
A replicated, randomised study in 2013–2016 in eight experimental ponds in central Jutland, Denmark (Pedersen et al. 2023) found that releasing translocated wild European glass eels Anguilla anguilla at low densities resulted in higher survival rates, body mass, length and condition compared to eels released at higher densities. After 18 months, eels released at the lowest density (0.5 eels/m²) had the highest survival rates (44–84%), average body mass (9–12 g) and body condition (data reported as condition factor) compared to eels released at 1 eel/m² (survival: 22–52%, mass: 7–11 g), 1.5 eels/m² (survival: 32–48 %, mass: 6–8 g) and 2 eels/m² (survival: 13–36 %, mass: 6–7 g). In addition, eels released at 0.5 eels/m² grew to greater average lengths (16–20 cm) than those released at 2 eels/m² (15–16 cm). Glass eels were captured from streams using electrofishing and kept in a mesh bag within the stream for 1–2 nights before being transferred to one of eight 200-m² shallow open ponds. Eels were released in the ponds at 0.5, 1, 1.5 and 2 eels/m² (3–5 ponds/density). Surviving eels were recaptured and measured after 18 months via pond draining and electrofishing. The study was run over two 18-month periods: June 2013 to November 2014 and June 2015 to November 2016.
Study and other actions tested
Referenced paper
Pedersen M.I., Rasmussen G. & Jepsen N. (2023) Density‐dependent growth, survival, and biomass production of stocked glass eels (Anguilla anguilla) in seminatural ponds. Fisheries Management and Ecology, 00, 1-7.

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.