The eels you see in rivers and ponds don’t breed there. They reproduce once in their lives, far out in the open ocean, and their young travel back as drifting larvae carried by currents over thousands of kilometres.1PubMed. A century of research on the larval distributions of the Atlantic eels: a re-examination of the data This hidden ocean half of their lives is why, for more than two thousand years, people had no idea where eels came from.2Discover Wildlife. It lasted thousands of years and puzzled Aristotle and Freud – is this the biggest animal mystery of all time? Even now, nobody has watched wild eels spawn, so parts of the story are still worked out from clues.
Where Eels Go to Spawn
Both European and American eels swim to the Sargasso Sea, a calm stretch of the western Atlantic near Bermuda, and mate in a large, overlapping area.3PubMed Central. Introgressive hybridization and latitudinal admixture clines in North Atlantic eels The Sargasso Sea has no coastline. It is bounded by ocean currents instead, and it is known for floating mats of golden seaweed.
Satellite tags that pop off eels and report their position have tracked European eels from Europe into the Sargasso Sea itself.4PubMed Central. First direct evidence of adult European eels migrating to their breeding place in the Sargasso Sea That makes the journey more than 5,000 km each way between their home rivers and the breeding grounds.5PubMed. All roads lead to home: panmixia of European eel in the Sargasso Sea European eels from every coast form a single, randomly mating population, mixing at the spawning site regardless of which river they came from.6PubMed. All roads lead to home: panmixia of European eel in the Sargasso Sea
Scientists still call these “presumed” breeding areas, because no one has yet filmed or observed wild eels mating. Tags prove that adults arrive, not the moment of spawning. The location itself was pinned down by working backward from larvae. Researchers towed fine nets across the Atlantic and found that the smallest larvae turned up in the southwestern Sargasso Sea. Because larvae drift, the smallest ones sit closest to where they were born, so the size of the larvae in each net worked like a map pointing toward the source.
Pacific eels have their own grounds. Japanese eels spawn along the southern part of the West Mariana Ridge, an undersea mountain chain in the western Pacific.7PubMed Central. Tracking Anguilla japonica Silver Eels Along the West Marina Ridge Using Pop-up Archival Transmitting Tags The same larva-hunting approach worked there, first with newly hatched larvae and later with eggs. Eels from islands such as Vanuatu head to a spawning area roughly 870 km away from where they were tagged.8Marine Ecology Progress Series. Tracking the marine migration routes of South Pacific silver eels
How Eels Spawn in the Deep Ocean
Spawning appears to happen in water of about 16 to 24°C, where a warm layer dips deeper than usual.9PubMed Central. The hydrographic features of anguillid spawning areas: potential signposts for migrating eels Japanese eel eggs have been found only at depths of 150 and 180 m, near the top of the layer where water temperature drops sharply.10PubMed Central. Spawning Sites of the Japanese Eel in Relation to Oceanographic Structure and the West Mariana Ridge Adults ready to spawn swim at about 230 m at night, and swimming at the same depth may help mates find each other in a huge ocean.11PubMed Central. Estimation of the spawning time of Japanese eels in the open ocean Light levels steer how deep eels swim, so dark, moonless nights may line them up at the same layer.12PubMed Central. Estimation of the spawning time of Japanese eels in the open ocean
Once eggs and sperm are released, they meet in open water, so fertilisation happens in the sea rather than inside a parent’s body. The eggs float freely and nobody guards them. Within a few days they hatch into tiny larvae that carry a small yolk supply for their first feeding. The parents have spent their energy by then and never see their offspring.
Freshwater eels spawn only once and die afterward. The two Atlantic species mate side by side in the same sea and rarely mix, though hybrids do occur, mostly found in Iceland.13PubMed Central. Genomic Footprints of Hybridisation in North Atlantic Eels (Anguilla anguilla and A. rostrata)
The Long Drift Home as Larvae
Eel larvae are flat, transparent and shaped like a willow leaf, looking nothing like an adult eel. Ocean currents carry them the same thousands of kilometres back toward the coasts their parents left.14PubMed. A century of research on the larval distributions of the Atlantic eels: a re-examination of the data European eel larvae drift with the Gulf Stream and other currents for more than a year.15PubMed Central. Glass eels (Anguilla anguilla) have a magnetic compass linked to the tidal cycle Early on, they mostly drift passively rather than swimming toward a destination.16Dana. Spatial and temporal distribution of American eel larvae in relation to North Atlantic Ocean current systems
Naturalists who found the larvae in the sea described them as a separate kind of fish and gave them their own name, which stuck as the word for the larval stage. The link to eels became clear only after researchers watched the larvae change shape into small eels.
The same currents raise a puzzle. European and American eels spawn in the same sea but end up on opposite sides of the Atlantic. Currents and timing sort them, not choice, and their different drift times likely help send each species to its own coast.
When the larvae reach the shallow continental shelf, they change shape into glass eels, small see-through eels.17PubMed Central. Glass eels (Anguilla anguilla) have a magnetic compass linked to the tidal cycle Glass eels find their way using smells, the saltiness of the water, tides, the moon and Earth’s magnetic field.18PubMed. A comprehensive hypothesis on the migration of European glass eels (Anguilla anguilla) They avoid light and change depth with the tides to ride incoming water into estuaries.19PubMed. A comprehensive hypothesis on the migration of European glass eels (Anguilla anguilla) They also seem to remember the magnetic direction of the incoming tide in their home estuary and swim along it.20PubMed Central. Glass eels (Anguilla anguilla) imprint the magnetic direction of tidal currents from their juvenile estuaries
Swimming Upstream and Growing Up
Young eels moving up a river can travel about 0.7 km a day on average, and up to about 1.8 km a day.21PubMed Central. Glass eel migration in an urbanized catchment: an integral bottleneck assessment using mark-recapture They migrate most in water between 12 and 20°C, best around 16.5°C.22Springer. Water temperature and upstream migration of glass eels in New Zealand: implications of climate change Water around 22°C almost stops upstream migration, so warmer rivers could reduce how many young eels arrive.23Springer. Water temperature and upstream migration of glass eels in New Zealand: implications of climate change
In spring, where young eels arrive in numbers, they can pile up below weirs, small dams and waterfalls in dense, wriggling clusters of tiny clear or greyish eels. They can crawl over wet rock, moss and damp concrete, which lets them get past small barriers that would stop many fish. Tall, dry or vertical barriers still stop them.
Not every young eel goes far upstream. Some settle in brackish coastal water and still return to the same ocean breeding site later.
In fresh water, eels darken into yellow eels, the stage most people see in rivers and ponds. In one small Danish stream, yellow eels grew about 3 cm a year and were up to 10 years old.24Preprints. Food, Growth and Biological Production of European Eel Anguilla anguilla in a Small Stream in Jutland, Denmark They eat mainly insect larvae and small crustaceans found on the stream bed.25Preprints. Food, Growth and Biological Production of European Eel Anguilla anguilla in a Small Stream in Jutland, Denmark
Silvering, the Change Before Breeding
Before leaving, a yellow eel becomes a silver eel, with a silvery belly and darker back. Its eyes grow larger and its gut shrinks.26PubMed. Endocrine evidence that silvering, a secondary metamorphosis in the eel, is a pubertal rather than a metamorphic event Its body fat and protein content also change markedly compared with yellow eels.27Animal Biology. Silvering of European eel (Anguilla anguilla L.): seasonal changes of morphological and metabolic parameters
Hormone studies suggest silvering behaves more like puberty than like a change of body form.28PubMed. Endocrine evidence that silvering, a secondary metamorphosis in the eel, is a pubertal rather than a metamorphic event The sex organs begin developing during silvering, and the same changes in gonads and eyes happen in eels from different places.29Journal of Fish Biology. The silvering process of Anguilla anguilla: a new classification from the yellow resident to the silver migrating stage A rise in the stress hormone cortisol in autumn helps release stored energy for swimming and egg or sperm growth.30Animal Biology. Silvering of European eel (Anguilla anguilla L.): seasonal changes of morphological and metabolic parameters
Silvering is tied to autumn, which is when most eels start to move out of rivers toward the sea. Many head downstream on dark, rainy nights when the water is high and cloud hides the moon, which probably gives cover. An eel may spend several years as a yellow eel before it silvers, so only part of the eels in a river leave in any one year.
Eels leave rivers not yet fully ripe. Their sex organs finish developing during the ocean trip, which is why no one found ripe eels in rivers, feeding the old mystery.
The Journey Back to Sea
Migrating eels swim in deep cold water by day, about 600 to 940 m down, and rise to warmer water at night.31Journal 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 Their depth follows the moon and sun.32Journal 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 They stop diving when the water reaches about 5°C.33Journal 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 Routes vary, though. Some eels zigzag while others head almost straight for the spawning area.34Marine Ecology Progress Series. Tracking the marine migration routes of South Pacific silver eels The eel does all of this on stored fat, because it has stopped feeding.
Why the Eel Mystery Lasted So Long
Ancient Egyptians believed eels sprang to life when the sun warmed the Nile.35Hakai Magazine. The Mysterious, Vexing, and Utterly Engrossing Search for the Origin of Eels Aristotle thought eels arose from mud and rainwater and had no sex organs.36The New Yorker. Where Do Eels Come From? Pliny the Elder claimed rubbed-off eel skin came to life.37Hakai Magazine. The Mysterious, Vexing, and Utterly Engrossing Search for the Origin of Eels In the English countryside, many believed eels grew from horse tail hairs falling into water.38The New Yorker. Where Do Eels Come From? Eels also turn up in ponds and streams that seem cut off from the sea, which made an origin from mud or water look plausible. These ideas made sense because no one could see the ocean part of the life cycle.
What finally solved it was the trail of larvae. Finding ever-smaller ones further out at sea traced them back to the Sargasso Sea, showing that eels migrate thousands of kilometres to spawn. Satellite tags later confirmed that adults arrive there.
Can Eels Be Bred in Captivity
Captive eels don’t mature on their own, so scientists inject hormones to ripen them.39PubMed Central. The first success of glass eel production in the world: basic biology on fish reproduction advances new applied technology in aquaculture Females are treated until they are ready to release eggs, and then eggs and sperm are combined by hand. This can make almost all treated females release eggs, with about 60% fertilised and 40% hatching.40PubMed Central. The first success of glass eel production in the world: basic biology on fish reproduction advances new applied technology in aquaculture
Raising the larvae and scaling up remain major hurdles for Japanese eel farming.41Reviews in Aquaculture. From Barriers to Breakthroughs: Toward Sustainable Artificial Reproduction in the Japanese Eel (Anguilla japonica) Eel farms still depend on catching wild glass eels.
What Threatens Eel Reproduction
Freshwater eels, especially in cooler regions, are declining sharply.42Journal of Great Lakes Research. Distribution of American eels (Anguilla rostrata) and the influence of barriers in the Lake Champlain basin Overfishing, damaged habitat and dams that block migration are major causes.43Journal of Great Lakes Research. Distribution of American eels (Anguilla rostrata) and the influence of barriers in the Lake Champlain basin Pollution and changes in ocean temperature and in the Sargasso Sea add to the threat.44National Park Service. American Eels in the Potomac Watershed Dams can stop young eels moving upstream, and silver eels heading downstream can be injured or killed by turbines at hydropower plants.
A parasitic worm living in the swim bladder makes eels swim slower and use more energy.45Journal of Experimental Marine Biology and Ecology. Swimming performance of silver eels is severely impaired by the swim-bladder parasite Anguillicola crassus Heavily infected eels fail within about 1,000 km, long before reaching the spawning grounds.46Journal of Experimental Marine Biology and Ecology. Swimming performance of silver eels is severely impaired by the swim-bladder parasite Anguillicola crassus Silver eels carry far more of these worms than yellow eels.47Journal of Experimental Marine Biology and Ecology. Swimming performance of silver eels is severely impaired by the swim-bladder parasite Anguillicola crassus
Some measures help. Fish passes and eel ladders let young eels get past dams, and screens at hydropower plants protect silver eels heading out. Buying eel only from regulated sources reduces pressure on wild stocks.