Release wild-caught captive-reared eels to re-establish or boost native populations ('head-starting')

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

Study locations

Key messages

Five studies evaluated the effects of releasing wild-caught captive-reared anguillid eels to re-establish or boost native populations (‘head-starting’) in inland habitats. Two studies were in Denmark, and one study was in each of Sweden, Germany and Japan.

 

COMMUNITY RESPONSE (0 STUDIES)

 

POPULATION RESPONSE (5 STUDIES)

  • Abundance (3 studies): Two studies in Sweden and Denmark reported that wild-caught captive-reared European eels were recaptured at rates of 2–13% up to 13 years after release in two lakes or a fjord. One replicated study in Denmark found that 12 months after release, wild-caught captive-reared European eels were recaptured in greater numbers in ponds than translocated wild eels. One controlled study in Japan reported that wild-caught captive-reared eels decreased in density during two years after release.   
  • Survival (2 studies): One replicated study in Germany found that, 3–6 years after release in five lakes, survival estimates were 8–17% for wild-caught captive-reared European eels compared to 5–45% for translocated wild eels. One study in Denmark found no difference in survival between wild-caught captive-reared small and large European eels during 3–8 years after release in a fjord. 
  • Condition (4 studies): One replicated study in Germany found that 5–6 years after release in five lakes, there was no difference in size between wild-caught captive-reared European eels and translocated wild eels. One study in Denmark found that small wild-caught captive-reared European eels had a higher yearly growth rate than large eels after release in a fjord. One replicated study in Denmark found that over two growing seasons after release in ponds, wild-caught captive-reared European eels grew more than translocated wild eels. One controlled study in Japan found that during two years after release, wild-caught captive-reared eels grew less per day than naturally occurring wild eels.

BEHAVIOUR (1 STUDY) 

  • Movement (1 study): One controlled study in Japan reported that two years after release, wild-caught captive-reared eels had travelled further from their release site than naturally occurring wild eels that had been captured, tagged and released at the same time and place.

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 1980–1994 at two lakes in southeast Sweden (Wickström et al. 1996) reported that 2–11% of wild-caught captive-reared European eels Anguilla anguilla were recaptured over 13 years after release. In one of the two lakes (399 ha, 1.5 m deep), 5,959 of 52,945 translocated eels (11%) were recaptured, most of which were migrating silver eels. In the other lake (299 ha, 18 m deep), 619 of 31,134 translocated eels (2%) were recaptured, most of which were yellow eels. In January–February 1980, European glass eels were imported from a French island and captive-reared for seven months before being released in two previously eel-free lakes (124–156 elvers/ha). In 1980–1994, eels were recaptured in mesh traps at the lake outlets and in 3–8 fyke nets (each 10-m long with two traps) at 2–4 sites/lake (frequency of monitoring not reported).

    Study and other actions tested
  2. A replicated study in 20052010 at five lakes in Brandenburg, Germany (Simon & Dörner 2014) found that 8–17% of released wild-caught captive-reared European eels Anguilla anguilla survived after 3–6 years and were a similar size to translocated wild glass eels after 56 years. After 36 years, the percentage of eels surviving in each lake was estimated to be 817% for wild-caught captive-reared eels and 545% for translocated wild eels (difference not statistically tested). Five and six years after release, wild-caught captive-reared eels had similar average lengths (179347 mm) to translocated wild glass eels (186311 mm) in four of five lakes, despite being released at significantly larger sizes (average length: captive-reared = 165 mm; wild = 72 mm). In the other lake, too few eels were recaptured for analysis. Between 2004 and 2007, wild-caught captive-reared eels (55 eels/ha, average 7 g/eel) and translocated wild glass eels (200 eels/ha, average 0.3 g/eel) were released into each of five lakes (<20 ha) on two occasions in AprilJune. Eels were tagged and marked before release. Captive-reared eels were wild-caught in France as glass eels and reared at commercial eel farms. Wild glass eels were obtained from commercial fisheries in England. The lakes were previously stocked with farmed eels until 19972004. In May 20052009, each lake was sampled three times by electrofishing from a boat along the shoreline. Captured eels were identified, measured and weighed before being released. Survival rates were estimated from a mark and recapture experiment in AprilJune 2010.

    Study and other actions tested
  3. A study in 20012011 at a fjord in Denmark (Pedersen & Rasmussen 2016) found that wild-caught captive-reared European eels Anguilla anguilla released at two different sizes had similar mortality rates, and small eels grew faster than large eels. During 38 years after release, average annual mortality rates did not differ significantly between captive-reared and released large (64%) and small eels (52%). Released small eels had greater average annual growth rates (52 mm) than large eels (44 mm). During 213 years after release, recapture rates of captive-reared and released eels were estimated to be 13% for small eels and 9% for large eels (difference not statistically tested). In JulySeptember 1998 and JuneJuly 1999, European eels of two sizes (large: 89 g, total 50,000 eels; small: 3 g, total 274,000 eels) were tagged and released into a brackish fjord (water depths of 13 m over vegetation or soft bottom). All eels had been imported as glass eels from France and reared in an aquaculture facility for 36 months before release. In 20012006, a proportion of commercial fisheries catches (15%) were checked for tagged eels. Recaptured eels were weighed and measured in length. Recapture rates for 20072011 were estimated using growth and mortality rates.  

    Study and other actions tested
  4. A replicated study in 20112012 at seven ponds in Denmark (Pedersen et al. 2017) found that wild-caught captive-reared European eels Anguilla anguilla were recaptured in greater numbers and grew faster than translocated wild eels, 12 months after release. In each of two experiments, average recapture rates after five months did not differ significantly between translocated wild-caught captive-reared eels (6173%) and translocated wild eels (5361%). However, after 12 months, in two ponds in one experiment, wild-caught captive-reared eels had greater recapture rates (66%) than wild eels (52%). Average increases in length over two growing seasons were greater for wild-caught captive-reared eels (1.112 cm) than wild eels (0.37 cm). In June 2011 and 2012, European eels (50 wild-caught and captive-reared; 50 wild) were tagged and released into each of 67 freshwater ponds (192–204 m2). Captive-reared eels (each 36 g) were captured in France during the winter before release and reared at a commercial eel farm. Wild eels (each 25 g) were captured in a trap at a hydropower station in Denmark. In one experiment, four ponds were drained after five months, and two ponds after 12 months. In the other experiment, all seven ponds were drained after five months. Eels were captured in nets as ponds were drained and weighed and measured in length. 

    Study and other actions tested
  5. A controlled study in 2016–2019 in four rivers in Japan (Wakiya et al. 2022) found that wild-caught captive-reared Japanese eels Anguilla japonica decreased in density, grew less and travelled more than naturally occurring wild eels during two years after release. Overall, average density of wild-caught captive-reared eels declined from 251 eels/ha (average biomass: 7,449 g/ha) at the time of release to 86 eels/ha (average biomass: 2,993 g/ha) three months after release, and 13–14 eels/ha (average biomass: 671–919 g/ha) 6–24 months after release. For wild eels, average density and biomass did not differ significantly over the same two years (overall 0112 eels/ha, biomass data not reported). Captive-reared eels had lower average daily growth rates (0.04 g/day) and travelled further from release points (500 m upstream to 3,400 m downstream) than captured and tagged wild eels (0.13 g/day, 200 m upstream to 200 m downstream). In June 2017, a total of 1,940 captive-reared eels (wild-caught and reared at an eel farm for six months) were tagged and released at 10 sites in each of four rivers. Wild eels were captured and tagged at the same sites. Captive-reared (total 34 eels) and wild eels (total 26 eels) were recaptured by electrofishing three, six and 24 months after release. Fishing efficiency was used to estimate eel density and biomass.

    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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