2.8 Million Eels Encircle an Undersea Seamount South of Tasmania — a Swarm Only Sonar Could Confuse

A CSIRO team, the Australian national science agency, quantified for the first time the eel swarm that forms around Mount Patience, south of Tasmania. At the peak of the season, nearly 2.8 million individuals converge there. The study sheds light on a deep-sea phenomenon with no known equal, and on the method that enabled its measurement.

Mount Patience, in Huon Marine Park, hosts a basket eel gathering filmed for the first time in 2007

The relief rises within the Huon Marine Park, a Commonwealth-protected area that encompasses several dozen submarine mountains. The eels move at depths of about 1,000 to 1,100 meters, in complete darkness. A camera towed by a CSIRO vessel filmed them in 2007, as a dense swarm over the seabed.

The species in question is the basket eel, Diastobranchus capensis, a fish in the Synaphobranchidae family, with no direct relation to the European eel. In 2015, specimens hauled up by longlines bore ripe gonads, according to a report published by the Australian Museum. Since then, the hypothesis of a spawning site has taken hold.

Between 2023 and 2025, researchers combined echosounders and tow-camera footage to separate eels from orange Hoplostethus

Counting posed a major technical challenge. Indeed, Mount Patience also hosts the orange Hoplostethus, Hoplostethus atlanticus, whose acoustic echo resembles that of the eels. Consequently, an echosounder alone sums the two species without distinguishing them.

The team led by Ben Scoulding therefore fused several acoustic campaigns, tow-camera imagery, and physical sampling. These images showed that eels occupy deeper and flatter terrains, while the Hoplostethus prefers the steep slopes at the summit. The authors subsequently bounded the Hoplostethus biomass at between 200 and 850 tonnes, based on historical stock assessments.

From March to May, biomass climbs to around 3,550 tonnes, a peak that would very likely correspond to spawning

According to the article published on October 7, 2026 in Royal Society Open Science, the aggregation remains present year-round but rises markedly during the austral autumn. In May, the biomass reaches between 3,547 and 3,557 tonnes, representing roughly 2.8 million eels. The authors attribute this maximum to reproduction, with due caution.

Without these constraints on Hoplostethus, the calculations would yield figures incompatible with the regional stock assessments. Thus, the study stresses that a discrimination of species remains essential in any acoustic inventory of the deep sea. However, the reason why the eels specifically choose this seabed relief remains unknown.

Ancient deep-sea trawling zone, the protected relief becomes a rare laboratory to monitor abyssal life

Mount Patience was once subjected to bottom trawling, notably for the orange Hoplostethus, a slow-growing fish that can surpass a century in age. Its current protection and signs of recovery observed, according to the authors, offer a rare opportunity to study these ecosystems over the long term.

Moreover, the method is designed to be transferable to other protected marine areas. It relies on non-invasive tools that avoid mass animal sampling for counting. This annual eel gathering could thus become a valuable indicator of the health of the submarine mounts of the Southern Ocean.