About 2.8 million eels gather every year along a ridge at the foot of an underwater mountain off Tasmania, in what researchers say is the only known spawning site for any deep-sea eel anywhere in the world.
The aggregation of basketwork eels (Diastobranchus capensis) sits next to Patience Seamount in the Huon Marine Park, according to a CSIRO-led study published in Royal Society Open Science. By weight it comes to about 3,500 tonnes.
"It's the only known aggregation of … any deep-sea eel anywhere," said the study's lead author, CSIRO fisheries acoustics researcher Ben Scoulding, who told ABC News that finding a spawning site for any eel species was unique.
Basketwork eels are apex scavengers that grow to about 1.8 metres and take their name from the crosshatched pattern on their skin. "It's like those old woven baskets," Dr Scoulding said.
The site itself is not a new discovery. Patience Seamount has been a known hot spot for the species since the 1980s, and fishers nicknamed it Eel Hill because they regularly caught basketwork eels as bycatch while targeting orange roughy. CSIRO lowered a camera there from the RV Southern Surveyor in the mid-2000s to confirm the eels were present.
What nobody had established was how many. That took surveys between 2023 and 2025 aboard the RV Investigator, Australia's flagship ocean science ship, using hydro-acoustic equipment that Dr Scoulding described as a "really expensive scientific fish finder".
"We're essentially pinging sound down into the water column," he said. The sound bounces off whatever it meets and returns with a signature that depends on what it hit.
Basketwork eels return a great deal of it, because each one carries a large swim bladder, the gas-filled organ a fish uses to regulate its buoyancy. "Air is a really good reflector of sound," Dr Scoulding said.
Knowing how much sound a single eel sends back, and checking the result against camera surveys, the researchers worked out that the aggregation sat about 1,000 metres down and covered an area roughly two kilometres by three. "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."
The eels begin arriving in December, peak between May and August, and are gone by October. "One of the interesting things is the persistence of the aggregation … it is there for the majority of the year," Dr Scoulding said. Of the eels the team caught, every stomach was empty; one female had spawned and the rest were close to it.
Why they choose Patience, which is one of 120 seamounts inside the Huon Marine Park, is not known. The species is listed as vulnerable to extinction globally, partly because of bycatch in orange roughy fisheries, and one New Zealand population on a continental slope has fallen 85 per cent in forty years. Patience Seamount was protected from bottom trawling under a management plan introduced last year, and Parks Australia's island and marine parks branch head, Shaun Barclay, said the study gave 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."
It is worth being precise about what has been counted. Nobody has seen 2.8 million eels. The survey heard them, and the figure rests on a single conversion: how much sound one eel sends back. An eel sends back sound because it is carrying a bladder of gas.
This newspaper regards that as the most important sentence in the study. The best-counted animal in the deep ocean is the one with the most air in it.
The consequence is a census weighted by buoyancy. A creature that is dense, compact and solid returns little, and is therefore scarce. A creature full of gas returns a great deal, and is therefore abundant. Nothing about the animals themselves need change; only the proportion of each one that is nothing.
This now becomes the baseline. Future change at Patience Seamount will be measured against the 2.8 million, which is to say against an echo, which is to say against the quantity of air that was present in 2023 to 2025 and arranged in the shape of eels.
The Department of Unnecessary Research accordingly declines to describe the basketwork eels as the most numerous animals on the seamount. They are merely the loudest.