2.8 million eels gather near an Australian mountain every year. Here's why

Millions of eels are gathering in the deep, dark waters off southeast Australia near a mountain south of Tasmania. Patience Seamount has hosted millions of eels for decades, at one time at a particular time every year. Scientists believe basketwork eels, or Diastobranchus capensis, do so for reproduction. For the first time, a study has established exactly how many eels congregate in this region.

According to a study led by marine scientist Ben Scoulding of Australia's CSIRO, a total of 2.8 million eels gather at the site at the peak of the congregation. The slithery marine creatures weigh nearly 3,860 tons in total.

The researchers also analysed why the eels choose Patience Seamount, where there are around 120 seamounts in the Huon and Tasman Fracture marine parks south of Tasmania. "Although seamounts are well known as hotspots where fish can aggregate, it's unusual to find millions of fish concentrated around a single feature," Scoulding said.

The discovery of eel swarm

The eel congregation was spotted by commercial fisheries in the 1980s and 90s. In 2007, a deep-sea camera survey captured the gathering for the first time. It was caught on camera attached to a research vessel during an expedition to survey the seabed and marine life. They came across something completely unexpected.

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The dark waters were filled with eels. While the creatures reside in this seamount all throughout the year, from March to May their numbers see a sharp spike. Both male and female eels have been found in advanced spawning condition during this time, suggesting this is reproductive season.

Scoulding says "the scale, persistence, and likely reproductive nature of the aggregation" makes it "one of the most remarkable fish aggregations documented in the deep ocean."

To find out how many eels were present during an April 2015 gathering, scientists used hydroacoustic measurements and on-camera eel counts. A 2021 report stated that there were tens of thousands of them there that year. Scoulding's team used sonar to map the massive concentrations of fish between 2023 and 2025.

The eel swarm was several kilometres long

But the sonar signal was possibly also coming from other animals. So the team also used an underwater camera to understand how much each species was contributing to it. They found that the eel swarm spanned roughly 3 kilometres long, 2 kilometres wide, and 150 metres above the seafloor.

Scoulding called the sheer size "remarkable". He further said that "estimating that there could be up to 2.8 million eels and more than 3,500 tons of biomass concentrated around a single seamount really highlights just how extraordinary Patience Seamount is."

Scientists say the findings show it is important to safeguard these habitats to ensure the species grow and thrive. Patience Seamount was once subjected to bottom trawling but is now showing signs of recovery.