Rahul SomvanshiIn the pitch-black water below the Tasman Sea, 2.8 million basketwork eels gather on a single underwater ridge off Tasmania in what researchers believe is a reproductive aggregation. Australian scientists just mapped the event for the first time using deep-towed acoustic and camera systems—and found what the lead researcher describes as an eel aggregation on a previously undocumented scale.
A CSIRO-led study published this week in the peer-reviewed journal Royal Society Open Science documents the discovery of a massive aggregation of approximately 2.8 million basketwork eels (Diastobranchus capensis). Lead author Ben Scoulding, a CSIRO fisheries acoustics researcher, and his team located the congregation along a deep-sea ridge next to Patience Seamount inside the Huon Marine Park, south of Tasmania. Scoulding called it “the only known aggregation of … any deep-sea eel anywhere.” Unlike the better-known European or short-finned eels that migrate from rivers to the open ocean to spawn, basketwork eels are a purely oceanic species—making this ridge an aggregation site not previously documented for any purely deep-sea eel species globally.
Because this ridge beside Patience Seamount appears to host the largest recorded aggregation of any deep-sea eel species, a single disturbance from deep-sea bottom trawling or seabed mining could affect a key biological concentration point for millions of animals. Located inside Australia’s Huon Marine Park, the discovery provides hard acoustic proof that banning bottom trawling on Tasmanian seamounts allows both deep-sea corals and threatened fish stocks to recover.
From Trawled Seabed to Thriving Deep-Sea Sanctuary
Patience Seamount carries a before-and-after history relevant to the discovery. The seamount was historically subject to commercial bottom trawling—industrial fishing that scrapes the seafloor and destroys cold-water coral habitat built over centuries. Now protected inside the Huon Marine Park, the newly mapped eel aggregation site sits alongside cold-water coral formations that have benefited from the trawling ban. The study authors suggest that continental shelf currents funnelling nutrient-rich water across the ridge likely draw eels to this specific point from across the south Tasman Sea. The find adds empirical weight to global arguments for protecting seamounts under the High Seas Treaty (BBNJ), and connects directly to prior CSIRO RV Investigator deep-sea survey findings and the case for global deep-sea bottom trawling bans. For Australians interested in local marine conservation, Tasmanian seamount coral recovery covers the broader restoration context.
When early media summaries of the CSIRO study hit social media, readers flagged an obvious mathematical problem: reports stated 2.8 million eels—each growing up to about 1.2 metres long—weighed just “3.5 tonnes”—which would make each eel weigh 1.25 grams, roughly the weight of a paperclip. The correct figure, based on standard eel density, is approximately 3,500 tonnes of deep-sea biomass—a dropped-thousand unit error that dramatically understated the biological scale of what is gathering at Patience Seamount.
*What species is in the 2.8 million eel ball off Tasmania? The giant deep-sea aggregation off Tasmania consists of approximately 2.8 million basketwork eels (Diastobranchus capensis*), an oceanic species that grows up to about 1.2 metres long. CSIRO scientists mapped the gathering along a deep-sea ridge next to Patience Seamount inside the Huon Marine Park, south of Tasmania.
Scientists are continuing to analyse the discovery to better understand what environmental or seasonal triggers cause basketwork eels to converge on this specific ridge.
