Introducing the new Conservation Evidence synopsis: Eel Conservation in Inland Habitats

This blog post was written by Dr Vanessa Cutts, Research Associate with Conservation Evidence and Dr Anders Clarhäll, Senior Analyst at Formas, Sweden

Anguillid eels are some of the most mysterious and fascinating creatures in our rivers and lakes. They are famously elusive and have puzzled humans for centuries. Aristotle, baffled by their apparent lack of reproductive organs, speculated that they emerged spontaneously from mud. Even Sigmund Freud, long before becoming the father of psychoanalysis, dissected 400 eels in search of their male reproductive organs (he found none). It wasn’t until the 20th century that scientists discovered eels reach reproductive maturity late in life. When they do, they vanish from freshwater habitats inland, embarking on an epic journey to spawn in the ocean before dying [1,2]. European eels, for example, travel up to 10,000 km to the Sargasso Sea. Their larvae (leptocephali) then make the long journey back to inland waters, transforming through several stages—glass eels, elvers, yellow eels—before becoming silver eels ready to migrate.

This complex life cycle is one of the reasons eels remained a mystery for so long, and it also makes their conservation quite challenging. Spending time in several different water environments means they face numerous threats associated with each of these environments and the migration between them. The threats may overlap and accumulate through different stages of their lifecycle [3], such as habitat loss, migration barriers like dams and weirs, overfishing, and even illegal trade. The high mortality of adult eels is particularly concerning, as they take years to mature and only reproduce once before dying [4].

Some eel species are now highly threatened, including the three most commercially important ones: the European eel, American eel, and Japanese eel. European eel populations, in particular, have declined dramatically since the 1970s and 80s [2,5].

So, how can we help the eels? The first step, true to Conservation Evidence style, is to look at the evidence: what do we know, and what don’t we know about eel conservation? With funding from Swedish research council Formas, we created a new Conservation Evidence synopsis focusing on the conservation of anguillid eels in inland habitats.

What we found

The synopsis features 114 summarized studies from 88 publications. These summaries cover 36 conservation actions out of a possible 126 actions identified with the help of 14 experts.

Most of the evidence fell into two main themes. The first was mitigating the effects of migration barriers. This includes the use of fish passes, physically capturing and transporting eels past barriers, modifying the operation of barriers, or removing barriers entirely. The good news is that this reflects one of the biggest threats to eels, showing that efforts are being made to find and test solutions. Here, at least, we seem to be on the right track.

The second theme was actions that focused on species management, mainly the restocking of wild caught elvers. While restocking may lead to increased abundance in receiving habitats, the effect might be only local and short term. It remains unclear whether the action leads to a greater spawning migration and thereby contributes to the recovery of eel stocks. Many studies focus on recapture rates of restocked eels but don’t monitor populations over time, and so, although there seems to be a lot of evidence, it is not necessarily conclusive.

Beyond these two themes, research was sparse. Only a handful of studies focused on habitat restoration (seven summaries), and even fewer addressed overfishing (just two summaries). This is surprising given eels’ ecological and economic importance. Furthermore, we found zero evidence on climate change mitigation, habitat protection, education and awareness, or the impacts of residential and commercial development on eels.

Behind the scenes

Creating a synopsis like this involves a deep dive into the scientific literature. Our team screened thousands of studies, selecting those that specifically tested conservation actions. We then summarized the findings in a standardized way, making it easier for conservationists to see what the evidence says at a glance. This structured approach ensures that practitioners have access to reliable information when making conservation decisions.

One of the challenges we faced was dealing with the variation in study quality. Not all studies are created equal—some provide strong, well-controlled evidence, while others offer only tentative insights. Even well-designed studies don’t always tell the full story. For example, just knowing how many eels use a fish pass to bypass a dam doesn’t tell us how many are unsuccessful, or if the action has an effect on eel stock recovery. How many eels need to successfully bypass a barrier for there to be a positive impact on the population? These are the harder questions we need answers to.

Formas and the impact on policy

This work was made possible thanks to funding from Formas, a research council for the Swedish government. Our collaborators at Formas used the Anguillid eel synopsis as supporting evidence for a report to the Swedish government on how to improve eel management.

One of the main recommendations is for habitat connectivity to be improved by providing both upstream and downstream passage at barriers. Furthermore, they also advised discontinuing the restocking of glass eels because, although this can increase eel numbers in the short term, there is little evidence to as to whether it increases spawning and reproductive stock.

Ultimately, they emphasized (as do we) the need for further research to fill key knowledge gaps and test new conservation strategies. Unlike many other fish, eels have an extraordinary life cycle that takes them across vast oceanic distances into freshwater habitats, meaning that any disruption to their journey can have significant consequences for their populations. Conservation efforts therefore need to be both well-informed and well-targeted.

If you’re curious to learn more, you can explore the full synopsis on our website or download the pdf. The report by Formas is freely available on their website.

References

[1] Schmidt J (1923) Breeding places and migrations of the eel. Nature, 111, 51–54. https://doi.org/10.1038/111051a0

[2] Williamson MJ, Pike C, Gollock M, Jacoby DM & Piper AT (2023) Anguillid eels. Current Biology, 33, R888–R893. https://doi.org/10.1016/j.cub.2023.07.044

[3] Jacoby DM, Casselman JM, Crook V, Delucia M, Ahn H et al. (2015) Synergistic patterns of threat and the challenges facing global anguillid eel conservation. Global Ecology and Conservation, 4, 321–333. https://doi.org/10.1016/j.gecco.2015.07.009

[4] MacNamara R & McCarthy TK (2012) Size-related variation in fecundity of European eel (Anguilla anguilla). ICES Journal of Marine Science, 69, 1333–1337. https://doi.org/10.1093/icesjms/fss123

[5] Pike C, Crook V. & Gollock M. (2020) Anguilla anguilla. The IUCN Red List of Threatened Species: e.T60344A152845178. https://dx.doi.org/10.2305/IUCN.UK.2020-2.RLTS.T60344A152845178.en