2.8 million eels gather off Tasmania in 'only known' deep-sea eel spawning aggregation

CSIRO-led study documents basketwork eels massing at Patience Seamount in the Huon Marine Park

By LineZotpaper AI staff

Published28 October 2026

Updated28 October 2026

Read Time3 min

Sources2 outlets

Illustration: 2.8 million eels gather off Tasmania in 'only known' deep-sea eel spawning aggregation

About 2.8 million basketwork eels, weighing around 3.5 tonnes, gather each year on a ridge beside a deep-sea mountain off Tasmania to spawn, in what researchers describe as the only known aggregation of any deep-sea eel anywhere in the world. The finding comes from a CSIRO-led study published in Royal Society Open Science.

The eels converge on a ridge line north west of the peak of Patience Seamount in the Huon Marine Park, according to the study. Hydro-acoustic and camera surveys conducted between 2023 and 2025 estimated the aggregation sits about 1,000 metres deep and covers an area of about 2 by 3 kilometres. "The eels are almost touching each other in that centre part," said Ben Scoulding, CSIRO fisheries acoustics researcher and lead author. "And then the further you get out, it's almost like having one eel in a living room, so every 20 metres cubed."

Basketwork eels (Diastobranchus capensis) are apex scavengers that grow to about 1.8 metres and take their name from a crosshatch pattern on their skin. "It's like those old woven baskets," Dr Scoulding said.

Patience Seamount has been a known hot spot for the species since the 1980s. Dr Scoulding said fishers nicknamed the site "Eel Hill" because they regularly caught basketwork eels as bycatch while targeting orange roughy. A camera dropped by CSIRO's RV Southern Surveyor in the mid 2000s confirmed the eels' presence, but it took surveys from 2023 to 2025 to count them. Researchers used hydro-acoustic surveys from the RV Investigator, which Dr Scoulding described as a "really expensive scientific fish finder". "We're essentially 'pinging' sound down into the water column," he said. The eels' large swim bladders made them strong sound reflectors. "Air is a really good reflector of sound."

The eels begin gathering in December, numbers peak around May to August, and they disappear in October. The researchers caught several eels and found all of them had empty stomachs; one female had already spawned and the others were close to spawning. Why the species chooses Patience is unknown. "It's one of 120 seamounts within Huon Marine Park … so it's quite unique that all the eels chose Patience to aggregate at," Dr Scoulding said. Research is continuing into whether basketwork eel populations around Australia and New Zealand are connected and whether they all gather at Patience.

Basketwork eels are considered vulnerable to extinction on a global list of threatened species, partly because of their status as bycatch in orange roughy fisheries. Populations of the eel on a particular continental slope in New Zealand have declined 85 per cent in the past 40 years; whether similar collapses happened at Patience Seamount in the past is unclear.

Shaun Barclay, Parks Australia island and marine parks branch head, said the study provided a baseline for measuring future change. "It's a great example of what marine parks are designed to do, protecting extraordinary places and to let us undertake the science to manage them in the future," he said. Patience Seamount was protected from bottom trawling through a new management plan last year.

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Analysis

Why This Matters

The aggregation is the first known spawning site for any deep-sea eel species, giving scientists a rare window into the life cycle of a poorly understood group of fish.
Basketwork eels are listed as vulnerable to extinction and are caught as bycatch in commercial fisheries, so knowing where and when they gather helps assess risks to the species.
The study establishes a baseline inside a marine park, allowing future management decisions to be measured against hard numbers.

Background

Patience Seamount is one of about 120 seamounts in the Huon Marine Park off Tasmania. Basketwork eels are deep-sea scavengers found in southern hemisphere waters. Seamounts, underwater mountains rising from the seafloor, are known to concentrate marine life, and many are protected from activities such as bottom trawling. The new study builds on decades of observations at Patience, from fishers' bycatch reports in the 1980s to camera surveys in the mid 2000s, and for the first time quantifies the size of the annual aggregation.

Key Perspectives

Ben Scoulding, CSIRO: says the Patience aggregation is "the only known aggregation of … any deep-sea eel anywhere", and that why the eels all chose Patience, one of 120 seamounts in the park, is unknown. Shaun Barclay, Parks Australia: says the study provides a baseline for measuring future change in the eel population and is an example of what marine parks are designed to do; he notes Patience Seamount is now protected from bottom trawling.

What to Watch

Whether surveys at Patience Seamount detect population declines like the 85 per cent fall recorded on a New Zealand continental slope over 40 years.
Findings from ongoing research into whether Australian and New Zealand basketwork eel populations are connected and whether all of them gather at Patience.
How the new management plan's protection from bottom trawling affects the aggregation in coming years, measured against the new baseline.

Sources

Giant ball of 2.8 million eels gathers off Tasmania's coast to spawn · ABC News
Millions of Mysterious Deep-Sea Eels Caught Gathering Off The Coast of Tasmania · ScienceAlert

Checked

12 claims in this story checked against 1 source: 10 supported.

Show each claim

SupportedAbout 2.8 million basketwork eels gather at Patience Seamount.Just In: “About 2.8 million basketwork eels (Diastobranchus capensis) — or 3.5 tonnes' worth — make the pilgrimage to a site next to Patience Seamount in the Huon Marine Park”
SupportedThis is the “only known” aggregation of any deep-sea eel anywhere.Just In: ““It's the only known aggregation of … any deep-sea eel anywhere.””
Reports differThe eels weigh around 3.5 tonnes.Just In: “About 2.8 million basketwork eels (Diastobranchus capensis) — or 3.5 tonnes' worth — make the pilgrimage”
SupportedThe eels gather each year to spawn.Just In: “Millions of eels gather like clockwork each year to make out along a ridge at the bottom of a deep-sea mountain off Tasmania.”
SupportedThe finding comes from a CSIRO-led study published in Royal Society Open Science.Just In: “according to a CSIRO-led study published in Royal Society Open Science.”
SupportedThe aggregation is on a ridge line north west of Patience Seamount’s peak.Just In: “The densest aggregations of eels were found along a ridge line north west of the Patience Seamount peak.”
SupportedThe aggregation is about 1,000 metres deep.Just In: “the researchers estimated the eel aggregation was about 1,000 metres deep and covered an area of about 2 by 3 kilometres.”
SupportedThe aggregation covers about 2 by 3 kilometres.Just In: “the researchers estimated the eel aggregation was about 1,000 metres deep and covered an area of about 2 by 3 kilometres.”
Reports differEel numbers peak around May to August.Just In: “Dr Scoulding said numbers then peaked around May to August before the eels disappeared in October.”
SupportedScoulding said eels are almost touching in the centre and become sparse farther out, with about one eel per 20 cubic metres.Just In: ““The eels are almost touching each other in that centre part,” Dr Scoulding said. “And then the further you get out, it's almost like having one eel in a living room, so every 20 metres cubed.””
SupportedScoulding described the basketwork pattern as like old woven baskets.Just In: ““It's like those old woven baskets,” Dr Scoulding said.”
SupportedBarclay said the study was a good example of what marine parks are designed to do.Just In: ““It's a great example of what marine parks are designed to do, protecting extraordinary places and to let us undertake the science to manage them in the future,” he said.”