This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Patience Seamount is an underwater mountain in Australia's Huon Marine Park off the coast of Tasmania. Every year, it is the site of an enormous gathering of basketwork eels (Diastobranchus capensis) for reasons unknown, although most likely to spawn.
The numbers are huge, but they have never been quantified. Now a team led by marine scientist Ben Scoulding of Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO) has estimated how many are down there. According to their study published in the journal Royal Society Open Science, the gatherings could contain as many as 2.8 million eels with a collective biomass of around 3,550 metric tons (about 3,900 U.S. tons).
The team reached these figures after carrying out nine acoustic surveys between 2023 and 2025. They also used underwater cameras towed behind a research vessel and collected a handful of eels. So how did they get numbers and biomass from this?
First, sonar equipment sent sound waves into the deep water to measure the echoes bouncing off schools of fish. These tell you something about the density of fish aggregations, but they cannot reliably distinguish one species from another. One major challenge of this research was that deep-sea fish called orange roughy (Hoplostethus atlanticus) also gather at Patience Seamount, making it impossible for acoustic equipment alone to separate the two populations, as the research team notes.
"Even with calibrated multi-frequency acoustics, camera imagery and physical sampling remain essential for species identification, size estimation and bioinformatics analysis." To calculate the eel population's size and total weight, the study authors combined visual observations with acoustic measurements and physical samples. In addition to working out the numbers, the scientists uncovered a few other details about the eels. Examination of female eels revealed that most had eggs at an advanced stage of development, indicating they were ready to spawn.
And all had empty stomachs. This supports the idea that they had gathered to reproduce rather than feed. Instead of spreading out across the underwater mountain, the eels cram into dense swarms at a specific depth along the western side.
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