A glass eel is a juvenile stage of freshwater eels, named for the nearly transparent body it carries as it arrives at coastlines after hatching thousands of kilometers away in the open ocean. These tiny, finger-length creatures are among the most expensive wildlife products on Earth by weight because they are the only way to stock eel farms. No one has figured out how to breed eels commercially at scale, so the entire global eel-farming industry depends on catching wild glass eels and raising them in captivity. That dependency, combined with crashing wild populations, has created a strange and often illegal economy around an animal most people have never heard of.
The Eel Life Cycle and Where Glass Eels Fit
Eels have one of the most extraordinary life cycles of any fish. European eels hatch in the Sargasso Sea, a region of the western Atlantic, as tiny leaf-shaped larvae called leptocephali. These larvae drift on ocean currents for one to three years, crossing the Atlantic toward European and North African coasts. As they approach the continental shelf, they undergo a transformation, shrinking in body length and becoming the slender, translucent creatures known as glass eels. At this point they are roughly six to eight centimeters long and weigh under a gram.
Glass eels enter estuaries and begin moving upstream into freshwater rivers and lakes. As they settle into freshwater habitats, they develop pigmentation and become “elvers,” then eventually grow into yellow eels, the feeding stage that can last anywhere from five to twenty years depending on conditions. When they finally mature, they transform once more into silver eels, their eyes enlarge, their skin darkens, and they begin the long return migration to the Sargasso Sea to spawn and die. The entire cycle is one-way for each generation: adults spawn once and never return.
American eels follow a similar pattern within the western Atlantic, and Japanese eels spawn in the western Pacific near the Mariana Islands before arriving as glass eels along the coasts of East Asia. All temperate eel species share this basic catadromous life history, meaning they grow in freshwater but reproduce in the sea. The glass eel stage is the brief window when they are accessible near shore and small enough to capture in large numbers.
Why Nobody Can Farm Eels from Scratch
The value of glass eels comes down to a single bottleneck: the eel-farming industry cannot close the breeding cycle. Despite decades of research, no one has developed a commercially viable method for breeding eels in captivity and rearing the larvae to the glass eel stage. Japanese researchers have managed to produce eel larvae in laboratories, but the process is painstaking and survival rates are extremely low. The larvae require specific food sources that are difficult to provide at scale, and the conditions needed to mimic the deep-ocean spawning environment remain poorly understood.
This means that every eel you eat in a sushi restaurant, whether as unagi in Japan or jellied eel in London, began its life as a wild-caught glass eel. Eel aquaculture in China has grown over the past five decades from experimental pond culture into the dominant production and processing center of the global eel industry, driven by strong East Asian demand and technological improvements in grow-out operations. But the sector remains dependent on scarce wild glass eels as its only source of seed stock.1Reviews in Aquaculture. Fifty Years of Eel Aquaculture in China: Historical Trajectories, Sustainability Trade‐Offs and Future Pathways
What Glass Eels Actually Cost
Prices for glass eels fluctuate wildly depending on species, region, and the size of each year’s run, but they consistently rank among the most expensive wildlife commodities by weight. European glass eels have fetched prices well above a thousand euros per kilogram in some seasons, occasionally approaching or exceeding the per-kilo price of gold during years of especially low recruitment. Japanese glass eels command similarly staggering prices in East Asia, sometimes reaching several million yen per kilogram during poor catch years.
American glass eels, harvested primarily in Maine (the only U.S. state that still allows a significant glass eel fishery), have sold for anywhere from roughly $1,000 to over $2,000 per pound in recent years. Maine’s glass eel fishery is tightly regulated with individual quotas, and the total annual harvest is capped, which keeps supply limited and prices elevated. For the fishers who hold licenses, a good season can be worth tens of thousands of dollars from a catch measured in buckets.
These prices exist because demand from East Asian aquaculture operations is enormous, supply is limited by biology, and there is no synthetic alternative. When wild glass eel runs are poor, prices spike. When they are slightly better, prices come down but remain extraordinary for a creature that individually weighs less than a paperclip.
The Recruitment Collapse
Glass eel prices would not be what they are if populations were healthy, and they are not. European eel recruitment has plummeted since the 1980s. The steep decline observed during that period remains largely unexplained, though ocean current changes, habitat loss, overfishing, pollution, and disease have all been implicated.2Current Biology. Recruitment Collapse and Population Structure of the European Eel Shaped by Local Ocean Current Dynamics Current glass eel arrivals in European rivers are estimated to be a small fraction of what they were in the early 1980s, and the species is listed as critically endangered by the International Union for Conservation of Nature.
The causes are probably stacked. Eels face threats at every stage of their lives. Dams and other barriers block glass eels from reaching upstream habitat. Estuary dams are often the first obstacles encountered during colonization of rivers, and research has explored managing sluice gates to improve passage based on the timing of glass eel migration behavior.3Internationale Revue der gesamten Hydrobiologie und Hydrographie. Gestion des barrages estuariens et migration d’anguilles Hydropower turbines kill adult silver eels trying to migrate back to the sea. Pollution accumulates in their fatty tissues over years. And on top of all that, commercial and illegal fishing remove glass eels from the population before they ever have a chance to grow up and spawn.
Japanese eel populations have experienced similar declines, and the species is also classified as endangered. American eel populations are in better shape overall but still face significant pressures and are considered depleted in parts of their range. The pattern is global: temperate eel species are in trouble nearly everywhere.
A Parasite That Came Along for the Ride
One of the more insidious threats to European eels is a parasitic nematode called Anguillicoloides crassus, which originally evolved with Japanese eels in East Asia. It was introduced to Europe through the importation of live eels for aquaculture in the 1980s and has since spread to eel populations across the continent. The parasite infects the swim bladder, causing inflammation, hemorrhages, dilation of blood vessels, and tissue erosion.4PubMed Central. The impact of Anguillicoloides crassus (Nematoda) on European eel swimbladder: histopathology and relationship between neuroendocrine and immune cells
The concern is that damaged swim bladders could prevent silver eels from completing their spawning migration. Eels need functional swim bladders to regulate buoyancy during the long journey across the Atlantic to the Sargasso Sea. The nematode can severely impair swim bladder function and has caused mortalities in both farmed and wild populations when other stressors are present, though direct evidence that it prevents eels from completing the spawning migration is still lacking.5Fisheries Management and Ecology. The impact of Anguillicola crassus on European eels Interestingly, not all eel populations suffer equally. A long-term study in a Mediterranean lagoon in northern Italy found minimal impact of the parasite on eel body condition, liver health, or swim bladder integrity, suggesting the parasite’s effects may depend on local environmental conditions and the density of infection.6Estuarine, Coastal and Shelf Science. Long-term ecological analysis of Anguillicola crassus occurrence and impact on the European eel population in a Mediterranean lagoon (North Italy)
This disagreement in the literature is worth noting because it illustrates a broader problem in eel science: almost everything about these animals is hard to study. Their spawning grounds are remote, their migrations are poorly tracked, and their population dynamics involve time lags measured in decades. Researchers often find themselves working with indirect evidence and conflicting results.
Smuggling and the Black Market
The combination of extreme value and declining supply has made glass eels a target for organized crime. Following the 2007 decision by the Convention on International Trade in Endangered Species (CITES) to list European eels on Appendix II, and a subsequent EU ban on all eel exports in 2010, a significant illegal trade developed.7Marine Policy. CITES-listings, EU eel trade bans and the increase of export of tropical eels out of Indonesia European glass eels are caught legally in countries like France, Spain, and Portugal, then smuggled out of the EU in suitcases, sometimes packed in water-filled plastic bags inside checked luggage bound for East Asia.
Europol has described the illegal glass eel trade as one of the most significant wildlife trafficking operations in Europe. The trade involves interactions between legal and illegal actors all along the supply chain, from the rivers where glass eels are caught to the consumers who eat them. Legal businesses sometimes provide cover, financing, or laundering services that sustain the illegal pipeline.8Trends in Organized Crime. The illegal trade in European eels: outsourcing, funding, and complex symbiotic-antithetical relationships Seizures at European airports have intercepted shipments worth millions of euros, but enforcement is difficult because the product is small, lightweight, and perishable.
The trade ban also shifted demand toward tropical eel species in Southeast Asia. After the CITES listing and EU export ban, exports of tropical Asian eel species to East Asia increased as buyers sought alternative sources of seed stock.7Marine Policy. CITES-listings, EU eel trade bans and the increase of export of tropical eels out of Indonesia This raised new conservation concerns for species that had previously faced little commercial pressure.
Disease Risks from Illegal Shipments
Smuggled glass eels pose another risk beyond population depletion: disease spread. Glass eels of unknown origin bypass the health screening and quarantine protocols that apply to legal aquaculture shipments. Researchers examining confiscated glass eels have used genetic barcoding and viral sequencing to trace their origins. In one case, phylogenetic analysis of an eel virus found in seized glass eels linked the strain most closely to isolates from Denmark and France, demonstrating how illegal trade can spread pathogens across borders.9PubMed Central. Is the Illegal Trade of Glass Eels (Anguilla anguilla) Increasing the Spread of Disease? A Case of EVEX
If smuggled European glass eels carrying European viruses are released into Asian aquaculture systems alongside Asian eel species, the potential for novel disease outbreaks increases. The swim bladder parasite that devastated European eel populations was itself introduced through a similar pathway of live eel importation decades ago, so the precedent for disease transfer through the eel trade is already well established.
How Glass Eels Navigate
One of the more fascinating aspects of glass eel biology is how these tiny creatures find their way from the open ocean into specific rivers and estuaries. Research has revealed that glass eels use a surprisingly sophisticated toolkit of sensory cues. A study testing European glass eels in controlled magnetic fields found that they oriented to the magnetic direction of the prevailing tidal flow at their recruitment estuaries, suggesting they imprint on local magnetic signatures much the way salmon imprint on chemical signatures of their home streams.10PubMed Central. Glass eels (Anguilla anguilla) imprint the magnetic direction of tidal currents from their juvenile estuaries
Their navigation appears to rely on a hierarchy of cues that shifts depending on where they are. In open pelagic water, they use lunar and magnetic cues. As they approach the coast, chemical signals and the magnetic field become more important. Once they enter estuaries, they add salinity gradients, water currents, and odors to their orientation toolkit, while also avoiding light and adjusting their depth according to tidal cycles.11Biological Reviews of the Cambridge Philosophical Society. A comprehensive hypothesis on the migration of European glass eels (Anguilla anguilla) The fact that an animal weighing under a gram, with a brain smaller than a grain of rice, can process and integrate this many environmental signals is remarkable. It also helps explain why barriers like dams and altered tidal flows can so thoroughly disrupt eel colonization of river systems.
What Glass Eels Grow Up to Do in an Ecosystem
Once glass eels settle into estuarine and freshwater habitats and grow into yellow eels, they become important generalist predators. In estuaries, their diet is dominated by crustaceans and fish. Gut content analysis of European eels in brackish water found that crustaceans and small fish made up the bulk of their diet, with amphipods and shore crabs being the most frequently consumed prey, along with smaller eels and juvenile flounder.12PubMed Central. Role of estuarine habitats for the feeding ecology of the European eel (Anguilla anguilla L.)
Eels occupy an unusual ecological niche because they use such a wide range of habitats over their lifetimes. A single eel might start in an estuary as a glass eel, migrate hundreds of kilometers upstream to a lake, feed there for a decade, then return through the entire river system to the ocean. That means eels connect freshwater and marine food webs in a way that few other species do. Their decline has knock-on effects that are difficult to measure but potentially significant for the ecosystems they once populated in large numbers.
Does Restocking Actually Help
One of the main conservation strategies used in Europe is restocking: catching glass eels in places where they arrive in relatively high numbers (like the French Atlantic coast) and transporting them to rivers and lakes elsewhere that have lost their eel populations. The idea sounds logical, but the evidence for its effectiveness is mixed at best.
A study tracking restocked eels in the Baltic region found troubling results. In Estonia, about three-quarters of sampled eels were natural recruits and only a quarter had been restocked. When researchers looked at eels collected from coastal waters, just one percent of those had originally been restocked into freshwater lakes in the region. The study concluded that restocking eels into freshwater systems may be futile as a conservation measure in some situations, and that better escapement of spawning adults is likely achieved by restocking directly into coastal waters or undammed freshwater systems with a direct connection to the sea.13ICES Journal of Marine Science. Conservation restocking of the imperilled European eel does not necessarily equal conservation
The problem is that restocking eels into dammed or landlocked waters may produce adult eels that never make it back to the ocean to spawn. If the goal is maintaining the breeding population, moving glass eels into systems where they cannot easily escape as adults may actually waste the resource. The finding highlights a frustrating tension in eel management: the same glass eels used for conservation restocking are also the ones in demand for aquaculture, so every glass eel placed in a poorly connected lake is one that neither contributed to farming nor to wild population recovery.
Glass Eels as a Fishery in the United States
In the U.S., the glass eel fishery is concentrated almost entirely in Maine, which issues a limited number of licenses and sets a quota for the entire state. South Carolina also allows some harvest, but Maine accounts for the vast majority of the American catch. The fishery is seasonal, running roughly from March through June when glass eels enter coastal rivers on incoming tides, typically at night.
Fishers use fine-mesh dip nets or fyke nets set in tidal streams, often working through the night during spring tides when glass eel runs tend to be strongest. The catch is sold to dealers who ship the live glass eels, packed in oxygenated water, to aquaculture operations in Asia. Maine has implemented increasingly strict management measures over the years, including individual harvester quotas, mandatory reporting, and swipe-card tracking systems at the point of sale. These measures were partly a response to poaching: when glass eels are worth more per pound than almost any other legal catch, the temptation to fish without a license or over quota is considerable.
The economics are unusual for a fishery. A single night of good fishing might yield a pound or two of glass eels, worth a few thousand dollars. For comparison, a lobster fisher might need to haul hundreds of traps over days to earn the same amount. But glass eel fishing is unpredictable: runs vary enormously from night to night and year to year, and the season is short. Many license holders treat it as supplemental income during a time of year when other fisheries are slow.
The Unagi Connection
For most people who encounter eels as food, the connection to glass eels is invisible. Unagi, the grilled freshwater eel served over rice in Japanese cuisine, is one of the most popular and culturally significant dishes in Japan. Consumption peaks during the summer, especially around the midsummer day of the ox, when eating eel is a centuries-old tradition believed to provide stamina during the hot months. Japan alone consumes tens of thousands of tonnes of eel annually.
Almost all of that eel is farmed, but “farmed” in the eel industry means something different than it does for salmon or tilapia. Eel farms receive wild-caught glass eels or elvers and raise them in ponds or recirculating systems for a year or more until they reach market size. China, Japan, South Korea, and Taiwan are the major producing countries, with China by far the largest.1Reviews in Aquaculture. Fifty Years of Eel Aquaculture in China: Historical Trajectories, Sustainability Trade‐Offs and Future Pathways The industry’s complete reliance on wild glass eel capture means that every fluctuation in wild recruitment directly affects the supply and price of farmed eel products reaching consumers. A bad glass eel season in Asia can push up the price of unagi in Tokyo restaurants months later.
European glass eels smuggled to Asia often end up feeding this same supply chain. A glass eel caught illegally in a French river might be grown out in a Chinese pond and served as grilled eel in a Japanese restaurant, its European origin untraceable by the time it reaches the plate. That traceability gap is one reason the illegal trade has been so persistent despite enforcement efforts.