At Patience Seamount, an underwater peak off Tasmania, as many as 2.8 million basketwork eels may gather in a dense deep-sea swarm each year. The eels pack into a particular depth along the mountain’s western side, turning a small patch of the seafloor into one of the largest known gatherings of its kind.
Patience Seamount lies inside Australia’s Huon Marine Park. It is an extinct volcanic feature with a base area of about 3 square kilometers. Its peak is 857 meters below the surface, rising from a base at 1,250 meters. The annual eel gathering had been seen before, but its size had never been measured. Knowing how many animals use the site can help scientists understand and protect a rare part of the deep-sea ecosystem.
A team led by marine scientist Ben Scoulding of Australia’s Commonwealth Scientific and Industrial Research Organisation estimated the population through nine surveys conducted from 2023 to 2025. Researchers used sonar to send sound into the water and measure the echoes from groups of animals. They also towed underwater cameras behind a research vessel and collected a small number of eels for examination. Combining those visual checks, acoustic readings and physical samples allowed the team to estimate both the number of eels and their combined weight.
The study, published in Royal Society Open Science in 2026, puts the gathering at up to 2.8 million basketwork eels, with a total biomass of about 3,550 metric tons. Sonar alone could not tell which animals were eels. Orange roughy, another deep-sea fish, also gather at the seamount, and their echoes can overlap with those of the eels. The researchers therefore relied on camera images and collected specimens to identify species and assess their size.
The samples also offered clues about why the eels come together. Most of the females examined had eggs at an advanced stage of development, suggesting they were close to spawning. All the eels examined had empty stomachs. Together, those findings support the idea that the gathering is linked to reproduction rather than feeding, though scientists do not yet know for certain why the eels choose this mountain.
The concentration is striking: among roughly 120 underwater mountains in the area, Patience Seamount is the only place where researchers have recorded gatherings on this scale. The discovery gives scientists a clear place to return to and track changes in basketwork eels, orange roughy and the deep-sea habitat they share. Continued monitoring may help explain what makes this one mountain so important to the eels—and whether the gathering changes over time.