After more than a century of research, scientists still have not observed a European eel spawning in the wild. The leading hypothesis, first proposed by Danish biologist Johannes Schmidt in the 1920s, places the breeding grounds of the European eel in the Sargasso Sea, a warm stretch of the western Atlantic bounded by ocean currents rather than coastlines. Yet no eggs, no mating adults, and no direct spawning behavior have ever been documented there for this species. A 2022 satellite-tagging study finally tracked adult European eels into the Sargasso Sea itself, but what they do once they arrive remains invisible to science. The eel’s reproductive life is one of the last great zoological mysteries, and the story of how far we have come in solving it is stranger than most people realize.
What Johannes Schmidt Proposed and What Is Still Missing
In the early twentieth century, Schmidt spent years trawling the Atlantic for the tiny, transparent, leaf-shaped larvae of the European eel. By mapping where progressively smaller larvae turned up, he traced them back to a region in the western Atlantic between roughly 24°N and 31°N latitude and 50°W and 70°W longitude. His conclusion was that all European eels, despite living in rivers and coastal waters from North Africa to Scandinavia, must travel thousands of kilometers to this single patch of ocean to breed and die. It was a bold claim, and the broad outlines have held up. But nearly a hundred years later, no one has confirmed it by finding spawning adults or fertilized eggs there.
A 2022 study using pop-up satellite tags finally delivered the first direct evidence that adult European eels actually reach the Sargasso Sea. Researchers tagged eels leaving European waters and tracked them as they swam southwest. Five of the tag pop-up locations fell within the Sargasso Sea boundaries, and one eel was located inside Schmidt’s presumed breeding zone. Tags recorded straight-line distances of up to about 2,275 km, with average migration speeds between roughly 3 and 12 km per day. Those numbers represent minimum distances, since the eels almost certainly do not swim in straight lines.1Nature. First direct evidence of adult European eels migrating to their breeding place in the Sargasso Sea Still, confirming that eels arrive is not the same as confirming that they spawn. The tags detach before the animals reach the final stage of their journey, and the deep, open ocean is an extraordinarily difficult place to observe mating behavior.
Japanese Eels Offer a Revealing Comparison
While the European eel’s spawning remains unwitnessed, researchers working on its Pacific cousin, the Japanese eel, have had better luck. Intensive collection efforts in the western Pacific determined that Japanese eels spawn along the western side of the West Mariana Ridge during new moon periods. Crucially, scientists have collected every oceanic life stage there, including eggs and spawning adults, something that has never been achieved for the European or American species.2PubMed Central. Spawning sites of the Japanese eel in relation to oceanographic structure and the West Mariana Ridge
The Japanese eel’s spawning site lies roughly 2,000 to 3,000 km from the rivers where adult eels grow, which is far but considerably shorter than the European eel’s estimated journey of more than 5,000 km across the Atlantic. That distance difference partly explains why researchers were able to close the loop for the Japanese eel first. Having the complete picture for one species gives scientists a working model for what might be happening with the European eel, even though the Atlantic presents its own logistical nightmares for field work.
A Life Cycle Built Around Transformation
One reason eels were so mysterious for so long is that they look like entirely different animals at different life stages. People have been eating eels in European rivers for millennia without any inkling that the tiny transparent “glass eels” arriving at the coast each spring were the same species as the thick, dark adults living upstream. The confusion is understandable: eel larvae, called leptocephali, are flat, leaf-shaped, and translucent. They look nothing like a fish, let alone the serpentine adults they will become.
Detailed histological work on Japanese eels has mapped the transformation from leptocephalus to juvenile. During the first phase of metamorphosis, the body undergoes a drastic shape change from leaf-like to eel-like, with gills and lateral muscles forming rapidly. At the same time, some structures actually regress, including esophageal muscle and the pancreas. After the animal reaches the glass eel stage, new structures continue to develop: cone photoreceptor cells appear, stomach and intestinal muscles begin forming, and the swim bladder starts to inflate. The process is not finished until the yellow eel stage, when nearly all organ structures are in place.3PubMed. Developmental features of Japanese eels, Anguilla japonica, from the late leptocephalus to the yellow eel stages: an early metamorphosis to the eel-like form and a prolonged transition to the juvenile In other words, the animal that arrives at the coast is still under construction. It will spend years in freshwater or coastal habitats growing and maturing before a final metamorphosis into the silver eel stage triggers its one-way migration back to the open ocean.
What Leptocephali Eat in the Open Ocean
Leptocephali drift in ocean currents for months or even years, yet for a long time nobody could figure out what they were eating. Their guts seemed nearly empty under the microscope, and they refused to feed on standard prey items in captivity. The answer turned out to be marine snow, the diffuse rain of organic particles, mucus, dead plankton, and bacterial aggregates that drifts through the water column. Leptocephali appear to consume types of marine snow rich in easily assimilated carbohydrates and fatty acids, converting those materials into glycosaminoglycans for energy storage and growth.4Fisheries Science. The mysterious feeding ecology of leptocephali: a unique strategy of consuming marine snow materials
This feeding strategy is unusual among fish larvae and may help explain how leptocephali survive such long drift periods. Marine snow is ubiquitous in the ocean, so the larvae do not need to find concentrated patches of prey. The trade-off is that it is low-quality food, which likely contributes to the slow growth rates and extended larval periods seen in Atlantic species whose larvae must cross an entire ocean basin before reaching the continental shelf.
How Glass Eels Find Their Way
When leptocephali finally reach coastal waters and metamorphose into glass eels, they face a navigation challenge: finding their way into estuaries and rivers. A striking experimental finding is that glass eels possess a magnetic compass linked to the tidal cycle. In field tests, nearly all glass eels exhibited a preferred orientation that shifted with the tide. When the same eels were tested in a laboratory under a manipulated simulated magnetic field with no other navigational cues available, about 71% maintained the same directional preference they had shown during ebb tide in the sea.5PubMed Central. Glass eels (Anguilla anguilla) have a magnetic compass linked to the tidal cycle
Linking a magnetic sense to tidal rhythms is a clever solution for an animal that needs to ride tides into estuaries. By knowing both the direction of the magnetic field and the phase of the tide, a glass eel can selectively swim or hold position to move upstream. Whether adult silver eels use a similar system for their return trip across the Atlantic is still debated, and testing it is far harder since those animals migrate through the deep ocean where experimental manipulation is essentially impossible.
Deep-Diving Behavior During the Spawning Migration
Pop-up satellite tags have revealed that migrating eels do not simply swim at a comfortable depth across the ocean. Instead, they perform dramatic daily vertical migrations, swimming hundreds of meters deeper during the day and rising at night. Tracked Japanese eels showed stable daily vertical movements for most of their tracking periods, descending to depths between roughly 590 and 940 meters during the day and rising to as shallow as 12 meters at night. Daytime minimum water temperatures hovered around 5 to 6°C regardless of location, suggesting that the eels stop descending when they hit a thermal floor rather than a specific depth.6Journal of Experimental Marine Biology and Ecology. Horizontal and vertical migration behavior of silver-phase Japanese eels in coastal, pelagic and spawning areas observed by pop-up satellite archival tags Separate tracking work confirmed that eels from different coastal regions generally avoid water colder than about 4°C, regardless of the local depth profile.7Animal Biotelemetry. Regional differences in oceanic migratory behavior of Japanese silver eel in waters with different vertical temperature gradients
European eels appear to follow similar patterns. Early tagging work using miniaturized satellite tags documented comparable depth oscillations during the Atlantic migration.8PubMed. Oceanic spawning migration of the European eel (Anguilla anguilla) Why they do this is not entirely clear. The prevailing idea is predator avoidance: by retreating to dark, cold water during the day when visual predators are most effective, eels reduce their odds of being eaten. There may also be thermoregulatory benefits, as alternating between warm and cold water could influence metabolic rate and help the eels conserve energy over a journey lasting months.
Physiological studies support the idea that eels are pre-adapted for these deep excursions. When eels metamorphose from the yellow (freshwater resident) stage to the silver (migrant) stage, their mitochondrial membranes become more fluid and cholesterol levels drop, changes that improve cellular function under the high hydrostatic pressures encountered at depth.9Comparative Biochemistry and Physiology. Hydrostatic pressure effects on eel mitochondrial functioning and membrane fluidity The silver eel is essentially a different animal from the yellow eel: bigger eyes for dim light, a darker back for camouflage in open water, and cells tuned for a pressure regime it has never experienced before.
One Species, One Breeding Population
You might expect that European eels living in, say, Scandinavian rivers would be genetically distinct from those in Mediterranean lagoons. They are not. Genetic studies have consistently found almost no geographic structure across the European eel’s enormous range. Differences between eels sampled at different locations are vanishingly small, and temporal variation within a single site actually exceeds the geographic component. The evidence supports panmixia, meaning the entire species functions as a single randomly mating population.10PubMed Central. Panmixia in the European eel: a matter of time…
The same pattern holds for the American eel. Genetic analyses have found a total lack of differentiation among samples from across the species’ range, from the Caribbean to Canada, providing what researchers describe as conclusive evidence for panmixia.11PubMed. Conclusive evidence for panmixia in the American eel More recent genomic work extended this finding into the tropical portions of the American eel’s range, confirming that no population structure exists even between temperate and tropical regions.12PubMed Central. Panmixia in the American eel extends to its tropical range of distribution: Biological implications and policymaking challenges
Panmixia has profound implications for conservation. If all European eels are one population, then a decline in glass eel recruitment to one river reflects the health of the entire species, not just a local stock. You cannot manage eel fisheries country by country as if each nation has its own population. A glass eel captured in England might be the offspring of an adult that grew up in a Moroccan lagoon. This complicates fisheries management enormously, because the reproductive contribution of any individual eel depends on whether it survives a migration that crosses multiple national jurisdictions and vast stretches of unregulated open ocean.
How the Marathon Migration Evolved
The 5,000-plus-kilometer Atlantic crossing seems like an absurd reproductive strategy until you look at it from an evolutionary perspective. Molecular studies suggest that the most ancestral anguillid eel species is a tropical eel from the Borneo region. Tropical eels today make very short migrations, sometimes less than 100 km, from their freshwater habitats to nearby ocean spawning sites.13Canadian Journal of Fisheries and Aquatic Sciences. Migration, speciation, and the evolution of diadromy in anguillid eels The prevailing evolutionary scenario is that the long-distance migrations of temperate eels evolved gradually as ocean currents carried larvae farther from tropical spawning areas, and the animals that colonized these distant habitats thrived. Over millions of years, the spawning location stayed put while the living range expanded.
The range of migration distances across living eel species illustrates this gradient. Tropical species in Southeast Asia still migrate less than 100 km, while the European eel travels more than 5,000 km across the Atlantic.14Heliyon. Ecology and evolution of migration in the freshwater eels of the genus Anguilla Schrank, 1798 The Japanese eel falls somewhere in between. Evidence from multiple tropical eel species confirms that short-distance, local spawning near freshwater habitats was likely the ancestral condition, and the epic ocean crossings are a derived trait.15PubMed Central. Evidence of local short-distance spawning migration of tropical freshwater eels, and implications for the evolution of freshwater eel migration The European eel did not choose to cross an ocean. Its ancestors were carried there by currents, and evolution cemented the route.
Why Captive Breeding Has Proved So Difficult
Given how precarious wild eel populations have become, the obvious question is whether we can breed them in captivity. The short answer is: barely, and not at scale. Eels do not mature sexually in freshwater under normal conditions. In the wild, the hormonal cascade that triggers gonad development appears to require the specific environmental cues of the oceanic migration, and in captivity, those cues are absent. Researchers have tried to force the issue with hormone injections, and the results are telling.
A study testing controlled-release osmotic pumps loaded with hormones in European eels found mixed success. In males, the pumps induced maturation and sperm production, but not enough high-quality sperm to supply a commercial hatchery. In females, the method triggered early maturation in half of the fish tested, with maturation beginning four to ten weeks after treatment.16PubMed Central. Using Osmotic Pumps to Induce the Production of Gametes in Male and Female European Eels Getting eggs and sperm in a lab is a long way from producing larvae that survive, metamorphose, and grow into juveniles. Japanese researchers have achieved this for the Japanese eel, producing captive-bred glass eels, but the process remains extremely expensive and labor-intensive. No commercially viable hatchery exists for any anguillid eel species. This means the global eel supply still depends entirely on wild-caught glass eels, many of which are raised in aquaculture ponds but none of which were born in captivity.
Chemical Contamination and Its Consequences
Eels spend years, sometimes decades, in freshwater and coastal environments before migrating. That long residency makes them exceptionally good at accumulating pollutants. Because they are fatty fish, lipophilic contaminants like PCBs and certain pesticides concentrate in their tissues. A study of European eels in a Mediterranean coastal lagoon found that 20% of the eels sampled exceeded the EU’s maximum safe level for the sum of six indicator PCBs in wild-caught eel flesh. The pesticide chlorpyrifos was detected in virtually all samples, at concentrations up to about 20 ng per gram of wet tissue.17Scientific Reports. First direct evidence of adult European eels migrating to their breeding place in the Sargasso Sea
These contaminant loads are not just a food-safety concern for people eating eel. They may directly impair reproduction. Eels rely on their stored fat reserves to fuel the entire spawning migration, since they stop feeding when they leave freshwater. As fat is metabolized during the journey, lipophilic contaminants are released into the bloodstream, potentially reaching toxic concentrations at the worst possible time, right as the animal needs to produce viable eggs or sperm. This creates a grim feedback loop: the same fat stores that power the migration also deliver a concentrated dose of pollutants to the reproductive system.
The Commercial Fishery Is Younger Than You Think
People have eaten eels for thousands of years, but the large-scale commercial eel fishery is surprisingly recent. In the late 1800s, development programs in central Europe began transitioning from subsistence fishing to organized commercial exploitation, opening new markets and developing new products. By the early 1900s, expanding fisheries and trade were reported throughout northern Europe, and commercial development was spreading south.18Fisheries Management and Ecology. The history of commercial fisheries for European eel commenced only a century ago The modern eel industry, particularly the enormous demand from East Asian markets for species like the Japanese eel, is a twentieth-century phenomenon. That is worth keeping in mind when evaluating population declines: the European eel’s glass eel recruitment has dropped by more than 90% since the 1980s, but the baseline we are measuring from is itself a period of already-intensive exploitation. The pre-commercial abundance of eels is essentially unknown.
Eels as Ecosystem Engineers
Eels are not just interesting for their reproductive mystery. In many freshwater systems, they function as apex predators with outsized ecological influence. A study in New Zealand quantified how longfin eels and brown trout in a degraded agricultural stream were sustained by marine inputs. Small fish migrating upstream from a connected estuarine lagoon supported 60% to 80% of longfin eel biomass. The stream maintained exceptionally high predatory fish biomass, between 610 and 830 kg per kilometer, almost entirely because of its connectivity to a near-pristine estuary.19New Zealand Journal of Marine and Freshwater Research. Sea to the mountains: quantifying freshwater eel and trout diet reliance on marine subsidies from upstream migrating fish
The finding highlights something often overlooked in eel conservation: eels are conduits between marine and freshwater ecosystems. When they migrate inland as glass eels, they carry marine-derived nutrients upstream. When they leave as silver eels, they export freshwater-derived energy to the ocean. Blocking either direction of that exchange, with dams, weirs, or degraded estuaries, does not just harm eels. It restructures the food webs that depend on them. Restoring eel populations is not only about saving a charismatic species; it is about reconnecting ecosystems that evolved around their movement.