Scientists count more than 2 million eels on one underwater mountain

October 28, 2026 report Scientists count more than 2 million eels on one underwater mountain Paul Arnold Author Gaby Clark Scientific Editor Robert Egan Senior Editor 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). Eel counting 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. Secrets of the deep 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. Of around 120 underwater mountains in the area, Patience Seamount is the only location where researchers have recorded these massive gatherings, raising the still-unanswered question of why. "Patience Seamount emerges as a rare and ecologically significant site, offering a unique opportunity for long-term monitoring of basketwork eels, orange roughy and deep-sea habitats." Written for you by our author Paul Arnold, edited by Gaby Clark, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly). You'll get an ad-free account as a thank-you. Publication details Ben Scoulding et al., A unique ecological phenomenon quantified by hydroacoustics: the deep-sea eel spawning aggregation at Patience Seamount, Royal Society Open Science (2026). DOI: 10.1098/rsos.260397 Journal information: Royal Society Open Science © 2026 Science X Network