The European eel (Anguilla anguilla) is one of the most extraordinary and poorly understood fish on the planet. Born in the Sargasso Sea, a region of the western Atlantic Ocean bounded by rotating currents rather than coastlines, its larvae drift thousands of kilometers to the rivers, estuaries, and coastal waters of Europe and North Africa, where they spend decades before making a one-way return trip to spawn and die. No one has ever witnessed a European eel breeding in the wild. The species is now critically endangered, and the combination of an almost absurdly complex life cycle, a mysterious reproductive biology, and escalating human pressures has made it a flashpoint in conservation science.
A Life Cycle Unlike Any Other Fish
European eels pass through a series of dramatically different body forms as they age, and each stage has its own name. After hatching somewhere in the Sargasso Sea, the larvae are flat, transparent, leaf-shaped creatures called leptocephali. They ride the Gulf Stream and North Atlantic currents eastward, a journey that ocean circulation models estimate takes roughly two years at minimum, with the route carrying most larvae past the east coast of North America before they reach European shores.1Canadian Journal of Fisheries and Aquatic Sciences. How does the European eel (Anguilla anguilla) retain its population structure during its larval migration across the North Atlantic Ocean? As they approach the continental shelf, they metamorphose into tiny, translucent “glass eels,” which push into estuaries and rivers. Over months, they gain pigment and become “elvers,” then grow into “yellow eels” with a brownish-gold belly. Yellow eels are the sedentary, feeding stage, and they can stay in this phase for anywhere from five to over twenty years, depending on the individual and its habitat.
When conditions are right, the yellow eel begins a final transformation into a “silver eel.” Its eyes enlarge, its body darkens on top and turns silvery underneath, its gut degenerates, and its pectoral fins lengthen. These changes are preparatory for deep-ocean swimming and are accompanied by real shifts in how genes are expressed. Research has shown that the transitions between glass, yellow, and silver stages involve changes in genome-wide DNA methylation patterns, meaning the eel’s body is not just growing but undergoing epigenetic reprogramming at each stage.2Comparative Biochemistry and Physiology Part D: Genomics and Proteomics. Metamorphosis and transition between developmental stages in European eel (Anguilla anguilla, L.) involve epigenetic changes in DNA methylation patterns Once silvered, the eel stops feeding permanently and begins its migration back to the Sargasso Sea.
The Long Road to the Sargasso Sea
For centuries, the spawning grounds of the European eel were a matter of speculation. It was the Danish biologist Johannes Schmidt who, in the early twentieth century, traced progressively smaller larvae back to the Sargasso Sea. But direct evidence of adult eels actually reaching those waters remained elusive until very recently. A large-scale satellite tagging study tracked over 700 silver eels released from four regions in Europe. Many tags were lost early due to predation, but researchers reconstructed detailed routes for more than 80 eels. Migrations extended from western mainland Europe to the Azores region, covering more than 5,000 kilometers toward the Sargasso Sea.3PubMed Central. Empirical observations of the spawning migration of European eels: The long and dangerous road to the Sargasso Sea
Even more striking, a subsequent tagging study provided the first direct evidence of adult European eels reaching the breeding area itself. Of 21 tags that recorded substantive data, five pop-up locations fell within the Sargasso Sea boundaries, and one eel was located inside the presumed breeding zone. Average migration speeds ranged from about 3 to 12 kilometers per day, with tags staying attached for up to 366 days.4Scientific Reports. First direct evidence of adult European eels migrating to their breeding place in the Sargasso Sea The journey is not a straight line. Eels travel on a broadly south-westerly trajectory, and the route takes them across open ocean at considerable depth. They exhibit a daily vertical migration pattern, swimming deeper during the day and rising at night, presumably to avoid predators and possibly to navigate using temperature gradients.
How Eels Navigate Across an Ocean
One of the persistent puzzles has been how an eel finds its way across thousands of kilometers of featureless open water. Evidence now points to Earth’s magnetic field as a key navigational tool. Experiments with juvenile European eels showed that their orientation shifted in response to subtle changes in magnetic field intensity and inclination angle corresponding to different locations along their migration route. Ocean circulation simulations confirmed that even weak swimming in the observed directions at those magnetic coordinates would increase the chances of juveniles entering the Gulf Stream system.5PubMed. A Magnetic Map Leads Juvenile European Eels to the Gulf Stream
The magnetic sense appears early in the life cycle and is intertwined with tidal rhythms. In experiments on glass eels, nearly all of the individuals tested in the sea showed a preferred orientation tied to the tidal cycle. When the same eels were tested in a laboratory under a manipulated magnetic field with no other environmental cues available, about 70% still oriented the same way during ebb tide. This demonstrates that glass eels possess a magnetic compass linked to an internal circatidal clock.6PubMed Central. Glass eels (Anguilla anguilla) have a magnetic compass linked to the tidal cycle Whether adult silver eels migrating in the open Atlantic rely on the same magnetic map, or supplement it with olfactory cues or celestial orientation, remains an open question.
Fueling a One-Way Trip Without Eating
Silver eels stop feeding before they leave freshwater, and their digestive tract physically degenerates. Every calorie needed to swim 5,000-plus kilometers, mature gonads, and spawn must come from energy stored in the body. The fuel is primarily fat, accumulated during the yellow eel stage and stored as triglycerides, mainly in muscle tissue. These lipids are later broken down to power both the migration and the development of eggs or sperm.7PubMed. Impairment of lipid storage by cadmium in the European eel (Anguilla anguilla) Hormonal signals, including androgens, appear to regulate the mobilization of these fat stores and direct lipids into the developing gonads during the journey.8PubMed Central. Releasing stored lipids to fuel migration and reproduction in the eel, Anguilla australis-a role for 11-ketotestosterone?
Swimming itself appears to trigger the early steps of sexual maturation. In laboratory swim trials simulating migration, female silver eels showed lipid deposition in their developing egg cells, the first stage of reproductive preparation. But extended swimming also suppressed later stages of egg maturation in females, suggesting that the body delays full reproductive development until the eel is closer to the spawning grounds. Males, by contrast, continued maturing during the simulated swim.9PubMed Central. Swimming physiology of European silver eels (Anguilla anguilla L.): energetic costs and effects on sexual maturation and reproduction The implication is that the migration itself is a timed biological program: swimming cues the body to prepare for spawning, but full maturation is held in check until the journey nears its end. This sequence is quite different from what happens when researchers try to mature eels artificially with hormone injections in the lab, which may explain why captive breeding has proven so difficult.
Why the Population Has Collapsed
European eel numbers have fallen dramatically since the early 1980s. Glass eel recruitment in some monitored rivers dropped to a fraction of historical levels, and the species was listed as Critically Endangered by the IUCN. No single cause explains the decline; instead, a combination of pressures is responsible, and researchers continue to debate the relative importance of each.
One line of evidence implicates oceanographic change. A close negative relationship has been documented over the past several decades between long-term fluctuations in glass eel recruitment and sea surface temperature in the North Atlantic. A regime shift in sea temperature preceded the start of the recruitment decline in the early 1980s, suggesting that changing ocean conditions may have reduced larval survival or disrupted the currents that carry leptocephali to Europe.10Fisheries Oceanography. Fluctuations in European eel (Anguilla anguilla) recruitment resulting from environmental changes in the Sargasso Sea Viability assessments using population models are consistent with a broad pattern of stock decline over recent decades, though the models necessarily rely on simplifying assumptions.11PubMed. A global viability assessment of the European eel
Overfishing has also played a part, particularly of glass eels. Historically, commercial fisheries for European eel are surprisingly recent. Large-scale exploitation only took off in the late 1800s and early 1900s as new markets and products developed across Europe. That expansion continued until around 1950, when the current multi-decadal decline began. As one historical analysis put it, eel fisheries have never experienced a period of stable, sustainable exploitation.12Fisheries Management and Ecology. The history of commercial fisheries for European eel commenced only a century ago
Hydropower and the Gauntlet of Dams
Rivers across Europe are lined with hydroelectric plants, weirs, and dams, and each one is a potential death trap for migrating silver eels heading downstream. A telemetry study at a Swedish hydropower plant found that eels passing through the turbines had only about a 40% chance of surviving to reach the next plant 24 kilometers downstream. For eels that had to pass through both the intake rack and the turbines, survival dropped to roughly 26%. The overall passage success rate for eels released in the reservoir was just 30%.13Freshwater Biology. Size-dependent mortality of migratory silver eels at a hydropower plant, and implications for escapement to the sea
Modeling studies have broken down how turbines kill eels. For a Kaplan bulb turbine, calculated mortality from blade collisions ranged from 22% to 37%, from shear events (violent water pressure changes) from 7% to 14%, and from barotrauma (rapid pressure drops) from 0% to 18%, depending on operating conditions. Running the turbines at moderate discharge levels produced lower mortality, supporting the idea that “fish-friendly” operating schedules could help.14Renewable Energy. Mortality assessment for adult European eels (Anguilla Anguilla) during turbine passage using CFD modelling But the damage is not only about immediate kills. Eels that survive passage through hydropower plants show elevated stress, disrupted immune function, and internal injuries that may cause delayed mortality further along the migration. Many European rivers have multiple dams in sequence, and the cumulative effect can be devastating.15PubMed. Impact of downstream passage through hydropower plants on the physiological and health status of a critically endangered species: The European eel Anguilla anguilla
A Parasite That Wrecks the Swim Bladder
As if ocean currents, predators, and dams were not enough, European eels also face a biological enemy that hitchhiked its way to Europe in the 1980s. The nematode Anguillicoloides crassus, originally a parasite of the Japanese eel, was introduced to European waters through the aquaculture trade and has since spread across the continent. The worm infects the swim bladder, the gas-filled organ eels use to regulate buoyancy during their daily vertical migrations in the deep ocean.
The effects are severe. Infected eels show lower cruising speeds and higher energy costs per kilometer. Nearly half of the eels with damaged swim bladders in one study stopped swimming entirely at low aerobic speeds. Simulated migration trials confirmed that heavily parasitized eels, or those with damaged bladders, fail before reaching 1,000 kilometers. Since silver eels tend to carry far heavier parasite burdens than yellow eels, the implication is clear: many migrating adults may be physically incapable of completing the trip to the Sargasso Sea.16Journal of Experimental Marine Biology and Ecology. Swimming performance of silver eels is severely impaired by the swim-bladder parasite Anguillicola crassus Mechanical analyses of severely infected swim bladders confirm that their structural integrity can degrade past a tipping point where normal function becomes impossible.17PubMed. A mechanical approach to understanding the impact of the nematode Anguillicoloides crassus on the European eel swimbladder
Pollution in the Fat Stores
Because European eels accumulate fat over years or decades in often heavily polluted rivers and estuaries, they are walking repositories of environmental contaminants. Metals, PCBs, and organochlorine pesticides build up in their tissues, and this matters enormously for a fish that relies entirely on stored energy for reproduction. Cadmium exposure, for example, impairs lipid storage itself, potentially reducing the fuel available for migration before the eel even begins its journey.18PubMed. Impairment of lipid storage by cadmium in the European eel (Anguilla anguilla)
Field studies of resident yellow eels have found that tissue pollution levels are strongly and negatively associated with body condition indices, including lipid reserves. In other words, eels in more contaminated habitats tend to be in worse physical shape.19PubMed. Gene transcription reflects poor health status of resident European eel chronically exposed to environmental pollutants The concern is that when a pollutant-loaded silver eel begins burning its fat reserves during migration, those stored toxins are released back into the bloodstream at the worst possible time, precisely when the eel needs to be in peak physiological condition to mature and spawn.
Predators in the Open Sea
Satellite tagging has revealed that predation takes a substantial toll on migrating eels even after they leave coastal waters. In one study of eels tagged and released from Ireland and France, roughly a quarter of the recovered datasets showed depth and temperature profiles consistent with the eel having been eaten. Five predation events occurred on the continental shelf and appeared to involve large predatory fish, while three occurred in deep water over the continental slope and were attributed to marine mammals.20Deep Sea Research Part I: Oceanographic Research Papers. Evidence of marine mammal predation of the European eel (Anguilla anguilla L.) on its marine migration A Mediterranean tagging study found a similar pattern, with eight of ten prematurely surfaced tags showing temperature spikes above 30°C and dive patterns consistent with a warm-blooded predator’s stomach.21Journal of Experimental Marine Biology and Ecology. Predation on migrating eels (Anguilla anguilla L.) from the Western Mediterranean These predation rates are measured in tagged eels, which carry an added burden that might make them more conspicuous or less agile. Still, the findings suggest that natural predation is a real bottleneck, and one that conservation managers have no way to control.
The Illegal Glass Eel Trade
International trade in European eels has been regulated since 2009, when the species was listed under CITES Appendix II, and the European Union banned exports of all life stages outside Europe. Despite this, a large and well-organized smuggling network continues to move glass eels from Europe to East Asia, where they are fattened for the lucrative Japanese unagi market. Genetic testing confirmed European eel DNA in seizures at international airports across Europe and in Hong Kong between 2010 and 2016. Transit routes ran through countries including Romania, Bulgaria, Greece, and Morocco, with glass eels often hidden in shipments of other fishery products or mislabeled. In the first few months of 2016 alone, 13 seizures were reported, nearly all destined for Hong Kong.22Conservation Genetics Resources. First genetic evidence of illegal trade in endangered European eel (Anguilla anguilla) from Europe to Asia Europol has called the illegal eel trade one of the most significant wildlife crimes in Europe by value, and enforcement has struggled to keep up with the shifting routes and methods.
Why Captive Breeding Has Not Solved the Problem
For most endangered fish species, aquaculture offers a fallback: breed the animals in captivity and restock wild populations. European eels have resisted this approach more stubbornly than almost any other species. The problem starts with reproduction. No European eel has ever spawned naturally in captivity. Researchers can force maturation using months of hormone injections, and they have produced fertilized eggs and larvae. But the larvae, once hatched, barely survive. A breakthrough came when researchers succeeded in getting captive-bred larvae to begin feeding, which was described as a critical step toward closing the life cycle.23Aquaculture. First-feeding by European eel larvae: A step towards closing the life cycle in captivity Even so, raising leptocephali through their full development to the glass eel stage has not yet been achieved. The larvae require specific, still poorly understood dietary and environmental conditions during a developmental period that in the wild takes two or more years.
Eel aquaculture in Europe does exist, but it depends entirely on wild-caught glass eels. Farm operators capture glass eels as they arrive on European coasts and grow them to market size in recirculating systems. This means the industry is itself a source of pressure on wild populations. Restocking programs, which release captured or farmed glass eels into rivers, remain controversial. Research suggests that a short quarantine period before release is feasible and does not harm the animals, which at least reduces the risk of spreading parasites or diseases into new waters.24Journal for Nature Conservation. 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 But there is no consensus on whether restocked eels actually contribute to the spawning population, since the key question is whether they successfully silver, migrate, and reproduce.
Adapting to Salt, Then Fresh, Then Salt Again
One of the European eel’s most impressive and underappreciated feats is physiological. Over its lifetime, it switches between saltwater and freshwater environments and must completely retool its internal salt-and-water balance each time. This ability has made it a favored model organism for studying how fish handle salinity changes. The molecular machinery involved is substantial: genes for ion transport proteins, water channel proteins, and stress response molecules all shift their activity as the eel moves between environments.25PubMed. Transcriptomic approach to the study of osmoregulation in the European eel Anguilla anguilla
Even at the larval stage, salinity matters. Laboratory experiments on pre-leptocephalus larvae found that those reared in conditions closer to the salt concentration of their own body fluids showed lower expression of stress-related and ion-transport genes, suggesting they spent less energy managing their internal chemistry. The result was better growth and survival compared to larvae raised in full-strength seawater.26PLoS ONE. Salinity reduction benefits European eel larvae: Insights at the morphological and molecular level This finding has practical implications for anyone attempting to rear eel larvae in captivity, since adjusting salinity could improve survival during the most fragile stage.
Sex, Density, and Flexible Biology
European eels do not have fixed sex chromosomes in the way that mammals do. Instead, sex appears to be determined environmentally, and the leading factor is population density. In crowded conditions, a higher proportion of eels develop as males; where densities are low and food is plentiful, more become females. Females are generally larger and older at the time of silvering, which makes sense given that a bigger body holds more fat and can produce more eggs for the long spawning migration. The exact mechanism is not fully pinned down. Reviews have pointed to early growth rate and social interactions among individuals as the proximate cues underpinning the density effect.27Fish and Fisheries. Sex differentiation and sex determination in eels: consequences for management This flexibility means that habitat quality and stocking density in managed waters can inadvertently skew sex ratios, with potential consequences for reproduction that conservation managers need to factor in.
What Eels Eat and Why It Matters to Ecosystems
During their long yellow eel phase, European eels are opportunistic predators in freshwater and estuarine habitats. Gut content analyses from estuarine eels found their diets dominated by crustaceans and fish, with amphipods and shore crabs making up the bulk of crustacean prey, and smaller eels of the same species plus juvenile flatfish being the most common fish eaten.28PubMed Central. Role of estuarine habitats for the feeding ecology of the European eel (Anguilla anguilla L.) Eels are nocturnal hunters with an exceptional sense of smell, and they occupy a niche that few other species fill in many European waterways. Their decline has left a gap in freshwater and estuarine food webs that is difficult to quantify but likely meaningful: they were once among the most abundant large predators in rivers from Scandinavia to the Mediterranean.
A Shared Ancestor and a Complicated Cousin
The European eel’s closest relative is the American eel (Anguilla rostrata), which also breeds in the Sargasso Sea but colonizes North American rivers. The two species overlap in their spawning range and occasionally hybridize, particularly around Iceland. Genomic analysis supports a model in which the two species split without gene flow roughly 150,000 to 160,000 generations ago, then made secondary contact around 87,000 to 92,000 generations ago, with about two-thirds of the genome experiencing restricted gene exchange. This points to a separation driven by ice-age changes in ocean currents rather than a gradual split within a single population.29PubMed. Speciation history of European (Anguilla anguilla) and American eel (A. rostrata), analysed using genomic data The existence of ongoing, low-level hybridization and gene flow between the two species adds another layer of complexity to conservation. Managing one species in isolation may not fully capture the genetic dynamics at play in the Sargasso Sea, where larvae of both species hatch side by side before heading off in opposite directions.