CSIRO study counts 2.8 million basketwork eels gathering at Patience Seamount off Tasmania

Left · 2A CSIRO-led study published in Royal Society Open Science estimates that about 2.8 million basketwork eels (Diastobranchus capensis) gather at a ridge next to Patience Seamount in the Huon Marine Park, south of Tasmania. 1 2 Left · 2ABC News gives the aggregation's weight as 3.5 tonnes; Metro says the eels weigh nearly 3.5 million kilograms, a figure that does not match ABC's. 1 2 Left · 1Lead author Ben Scoulding, a CSIRO fisheries acoustics researcher, said it is the only known aggregation of any deep-sea eel anywhere, and ABC reports it as the only known spawning site globally for purely oceanic eels. 1

Left · 1Fishers have known the site as "Eel Hill" because they caught the eels as bycatch while targeting orange roughy, and CSIRO confirmed the eels' presence with a camera in the mid-2000s. 1 Left · 2Surveys between 2023 and 2025 used hydro-acoustic sonar from the research ship RV Investigator, which Scoulding called a "really expensive scientific fish finder", along with camera surveys; the eels reflect sound strongly because of their large swim bladders. 1 2 Left · 2The researchers estimated the aggregation lies at about 1,000 metres depth and covers roughly 2 by 3 kilometres, densest at its centre where Scoulding said the eels are "almost touching each other". 1 2

Left · 2Scoulding said the aggregation persists "for the majority of the year". ABC says the eels start gathering in December, peak from May to August and are gone by October, while Metro says the peak is March to May before they disperse in October. 1 2 Left · 2The researchers caught several eels and all had empty stomachs. ABC reports one female had already spawned and the others were close to spawning, while Metro says most females were close to laying eggs. Metro adds that earlier catches found males and females in peak spawning condition, and that a 2015 estimate was tens of thousands of eels. 1 2 Left · 1Why the eels choose Patience, one of 120 seamounts in the marine park, is unknown. Further research is looking at how eel populations around Australia and New Zealand are connected, and where the eels come from and their offspring go. 1

Left · 2The species is listed as vulnerable to extinction, partly because it is caught as bycatch in orange roughy fisheries, and populations on a New Zealand slope have declined 85 per cent in 40 years. Whether Patience Seamount has seen similar declines is unclear. 1 2 Left · 1Parks Australia's Shaun Barclay said the study gives a baseline for measuring future change and that Patience Seamount was protected from bottom trawling through a new management plan last year. 1