Fishers had a name for it long before scientists did: Eel Hill. Now Australian researchers have finally put a number on the vast gathering of basketwork eels at Patience Seamount south of Tasmania, and it is staggering. At its peak the crowd holds an estimated 2.8 million animals, together weighing about 3,500 tonnes.
The study, led by CSIRO fisheries acoustics scientist Ben Scoulding and published in Royal Society Open Science, describes the site as the only known spawning ground anywhere for a purely oceanic deep-sea eel. The species, Diastobranchus capensis, is a scavenger reaching about 1.8 metres, named for the woven, crosshatch look of its skin.
Counting animals in permanent darkness took some ingenuity. Between 2023 and 2025 the team ran sonar surveys, including from the research vessel RV Investigator, and paired them with camera footage and sampling. That combination was needed because orange roughy share the seamount and look much the same as eels on an echo sounder. The eels are present all year, but numbers swell in the southern autumn, and both males and females caught then were ready to spawn.
What puzzles the researchers most is the choice of venue. Patience covers only about two square kilometres and is one of roughly 120 seamounts in the surrounding marine parks, many of them similar, yet none hosts anything comparable. Scoulding says the team has promising ideas but no firm explanation so far.
Next, scientists hope to learn where the eels travel from and where their larvae drift after the spawning season ends, questions that matter for protecting the Huon Marine Park.
