At around 1,000 metres beneath the sea, millions of eels gather around the same seamount. And no one knows exactly why they choose this location.
This scene takes place off the coast of Tasmania, Australia, near the seamount "Patience". Every year, a deep-sea eel species, Diastobranchus capensis, gathers there in exceptionally large numbers. These animals can grow up to 1.80 metres long, but remain very difficult to observe in their natural habitat.
Several Diastobranchus capensis eels near the seabed of Mount Patience, filmed during a 2025 oceanographic expedition. Credit: CSIRO.
Australian researchers attempted to measure the scale of this gathering. To do so, they used sonar devices, which send sounds underwater and analyse their echoes. They also filmed the fish using cameras lowered into the depths, in order to distinguish the eels from the other species present.
Their calculations produced an impressive result: up to 2.8 million eels, weighing around 3,500 tonnes in total. The group can extend for nearly three kilometres in length and two kilometres in width.
The eels begin arriving around December. Their numbers increase over the following months, reach a peak during the year, and then decline until they disappear from the site around October. They therefore seem to follow an annual rendezvous that lasts for much of the year.
Why do they gather there? Researchers believe they come to reproduce. The females examined were carrying highly developed eggs, and the captured animals had empty stomachs. They therefore seem to devote this period to reproduction rather than searching for food.
However, no one has yet directly observed spawning during this gathering. It is also unknown where the eels come from and where their young go. Even more strangely, the region has many seamounts, but none of the others appears to host such a large concentration.
This discovery is also a good reason to protect the deep sea. These eels are considered vulnerable and can be caught accidentally in fishing nets. Surveys conducted around Mount Patience will now make it possible to monitor their population and see whether it changes over the years.