Where eels spawn, and why nobody has watched
Explainer · Last reviewed 2026-08-14
The spawning ground was inferred from the sizes of larvae rather than seen, and more than a century after that inference nobody has observed a European eel spawning in the wild.
THE SPAWNING GROUND WAS FOUND BY ARITHMETIC RATHER THAN BY OBSERVATION, and the method is worth knowing because it is a lovely piece of scientific reasoning.
In the early twentieth century a systematic survey of the Atlantic sampled eel larvae — leptocephali — across the ocean and recorded where the SMALLEST ones were found. Reasoning that the smallest larvae must be closest to where they were spawned, the investigators traced the size gradient back to a region of the western Atlantic centred on the Sargasso Sea. That inference has stood for over a century and is the basis of everything written about eel spawning since.
WHAT HAS BEEN ADDED SINCE. Electronic tagging of silver eels leaving European rivers has tracked adults into the Atlantic and provided direct evidence about routes, depths and the extraordinary daily vertical migrations they perform. Genetic work has addressed the question of whether the European eel is a single panmictic population. Oceanographic modelling has connected larval drift to current systems.
WHAT HAS NOT BEEN ACHIEVED. Nobody has observed European eels spawning in the wild. No adult in spawning condition has been recovered from the presumed spawning area. Eggs have not been collected there. The reproduction of the species has been achieved in captivity only with substantial intervention, which tells us about the physiology and not about what happens in the ocean.
WHY IT IS SO HARD. The area is enormous, deep and remote. The event is presumed to be brief and to occur at depth. The animals are not aggregating anywhere anybody can predict precisely enough to be there. And the fish arrive having crossed an ocean without feeding, in a condition that makes them difficult to intercept.
WHY IT MATTERS RATHER THAN BEING A CURIOSITY. A species cannot be managed well when the location, timing and conditions of its reproduction are inferred rather than known. Recruitment — the arrival of glass eels at European coasts — has collapsed dramatically, and distinguishing between causes in the ocean, causes on the continental shelf and causes in fresh water is much harder when the first link in the chain has never been seen. That uncertainty is itself part of the conservation problem, and it is why the honest description of eel biology includes a large and specific gap.
Where to go next
Fish this applies to
What these connections rest on (1)
Scientific evidence — 1 connection
Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.
European eel
A mechanism edge between a record and the biology beneath it. The underlying claims are documented in the fisheries science and reference literature cited rather than restated from practice.
- ICES Advice on fishing opportunities, catch and effort — International Council for the Exploration of the Sea (Scientific institute)Relied on for: Scientific advice on the European eel including recruitment trends, the state of the stock and the basis for management advice.States this directlyCitation checked — the work exists and is described correctly
- The IUCN Red List of Threatened Species — International Union for Conservation of Nature (Conservation authority)Relied on for: Global extinction-risk assessment, population trend and the threats identified for the species concerned.States this directlyCitation checked — the work exists and is described correctly
Related reading
What these connections rest on (1)
Scientific evidence — 1 connection
Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.
Why fish migrate
A mechanism edge between a record and the biology beneath it. The underlying claims are documented in the fisheries science and reference literature cited rather than restated from practice.
- ICES Advice on fishing opportunities, catch and effort — International Council for the Exploration of the Sea (Scientific institute)Relied on for: Scientific advice on the European eel including recruitment trends, the state of the stock and the basis for management advice.States this directlyCitation checked — the work exists and is described correctly
- The IUCN Red List of Threatened Species — International Union for Conservation of Nature (Conservation authority)Relied on for: Global extinction-risk assessment, population trend and the threats identified for the species concerned.States this directlyCitation checked — the work exists and is described correctly
How this compares
What these connections rest on (1)
Scientific evidence — 1 connection
Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.
The eel life cycle
Two biological records a reader will hold together, joined where the contrast between them is the informative part.
- ICES Advice on fishing opportunities, catch and effort — International Council for the Exploration of the Sea (Scientific institute)Relied on for: Scientific advice on the European eel including recruitment trends, the state of the stock and the basis for management advice.States this directlyCitation checked — the work exists and is described correctly
- The IUCN Red List of Threatened Species — International Union for Conservation of Nature (Conservation authority)Relied on for: Global extinction-risk assessment, population trend and the threats identified for the species concerned.States this directlyCitation checked — the work exists and is described correctly
More explainer
Not related to this page — just the rest of the section, because this record connects to very little and a short page should still lead somewhere.
Sources for this page
- ICES Advice on fishing opportunities, catch and effort — International Council for the Exploration of the Sea (Scientific institute)Relied on for: Scientific advice on the state of north-east Atlantic stocks, including recruitment trends and the basis for management advice.States this directlyCitation checked — the work exists and is described correctly
- The IUCN Red List of Threatened Species — International Union for Conservation of Nature (Conservation authority)Relied on for: Global extinction-risk assessment, population trend and the threats identified for the species concerned.States this directlyCitation checked — the work exists and is described correctly
- Freshwater Fishes of the British Isles — HarperCollins (New Naturalist 75), 1992 (Reference work)Relied on for: Biology, life history, distribution and conservation status of British and Irish freshwater and diadromous fishes.States this directlyCitation checked — the work exists and is described correctly
How FishingHQ labels evidence is explained in the evidence policy.