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<\/p>\nMillions of eels gather like clockwork each year to make out along a ridge at the bottom of a deep-sea mountain off Tasmania.<\/p>\n
About 2.8 million basketwork eels (Diastobranchus capensis) \u2014 or 3,500 tonnes’ worth \u2014 make the pilgrimage to a site next to Patience Seamount in the Huon Marine Park, according to a CSIRO-led study published in Royal Society Open Science.<\/p>\n
CSIRO fisheries acoustics researcher and study lead author Ben Scoulding said finding a spawning site for any eel species was unique.<\/p>\n
“It’s the only known aggregation of \u2026 any deep-sea eel anywhere,” he said.<\/p>\n
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The densest aggregations of eels were found along a ridge line north-west of the Patience Seamount peak. (Supplied: CSIRO)<\/p>\n
Basketwork eels are apex scavengers that grow to about 1.8m in length and get their name from a crosshatch-like pattern on their skin.<\/p>\n
“It’s like those old woven baskets,” Dr Scoulding said.<\/p>\n
How do you find a big ball of eels?<\/p>\n
Patience Seamount has been a known hot spot for basketwork eels since the 1980s.<\/p>\n
Dr Scoulding said fishers had nicknamed the site Eel Hill because they regularly caught the species as bycatch while targeting orange roughy (Hoplostethus atlanticus).<\/p>\n
“Then CSIRO conducted some work on the RV Southern Surveyor in the mid 2000s and put a camera down \u2026 to confirm the presence of the eels,” he said.<\/p>\n
But it took until more recent surveys between 2023 and 2025 to determine how many eels were there.<\/p>\n
Dr Scoulding said researchers used a combination of techniques to calculate the size of the aggregation and what the eels were up to.<\/p>\n
These included hydro-acoustic surveys, which Dr Scoulding described as a “really expensive scientific fish finder”, from Australia’s flagship ocean science ship the RV Investigator.<\/p>\n
“We’re essentially pinging sound down into the water column,” Dr Scoulding said.<\/p>\n
That sound then bounces off different objects and back to the scientists with unique signatures, depending on what is hit.<\/p>\n
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Fine lines on the skin that look similar to weaving patterns give the basketwork eel its name. (Supplied: CSIRO)<\/p>\n
Dr Scoulding said basketwork eels reflected a lot of sound because they had a big swim bladder (an organ filled with gas to regulate a fish’s buoyancy).<\/p>\n
“Air is a really good reflector of sound,” he said.<\/p>\n
Knowing how much sound is reflected by a single eel, and using camera surveys, the researchers estimated the eel aggregation was about 1,000 metres deep and covered an area of about 2 by 3 kilometres.<\/p>\n
“The eels are almost touching each other in that centre part,” Dr Scoulding said.<\/p>\n
“And then the further you get out, it’s almost like having one eel in a living room, so every 20 metres cubed.”<\/p>\n
What are the eels up to?<\/p>\n
The eels start gathering at Patience Seamount in December every year.<\/p>\n
Dr Scoulding said numbers peaked around May to August before the eels disappeared in October.<\/p>\n
“One of the interesting things is the persistence of the aggregation \u2026 it is there for the majority of the year,” he said.<\/p>\n
The researchers caught several eels and found all of them had empty stomachs.<\/p>\n
Dr Scoulding said one female had spawned and all the others were close to spawning.<\/p>\n
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Female basketwork eels caught by CSIRO had empty stomachs and unfertilised eggs. (Supplied: CSIRO)<\/p>\n
Why exactly the eels chose to gather at Patience specifically was unknown.<\/p>\n
“It’s one of 120 seamounts within Huon Marine Park \u2026 so it’s quite unique that all the eels chose Patience to aggregate at,” Dr Scoulding said.<\/p>\n
Research is ongoing into the connectivity of basketwork eel populations around Australia and New Zealand and whether they all gather at Patience.<\/p>\n
Why does it matter how many eels we have?<\/p>\n