2.8 Million Eels Gather Around an Underwater Volcano Off Tasmania to Spawn, but Why They Choose This Spot Remains a Mystery

A study published in Royal Society Open Science revealed an astonishing gathering of 2.8 million basketwork eels around Patience Seamount, an extinct volcano approximately 69 miles southwest of Hobart, Tasmania. The eels, which grow up to 5 feet, were estimated to weigh around 3,900 U.S. tons in May 2023, compared to about 820 U.S. tons in March 2024.

The exact reason for their annual congregation remains a mystery, but researchers believe it is for spawning. The research ship's echo sounder struggled to distinguish eels from orange roughy, a fish species trawled from the mountain in large numbers, leading to an overestimation of the fish population. To differentiate between the two, the team used a camera sled to observe the eels' behavior and a camera sled to confirm the roughy estimate, which was based on fishing records from 1990 to 2003.

The footage suggested that eels prefer deeper, flatter areas, while roughy stay on steep slopes near the summit. The mix of species changed with the season, with eels likely gathering to breed, not feed. The team also noted that no males were caught, suggesting more sampling over different seasons would be necessary to confirm the spawning hypothesis.

Commercial trawling ceased in 2003, and the area now hosts marine park protections, leading to potential recovery of deep-sea coral and an increase in orange roughy population. The researchers propose the sonar-and-camera approach as a scalable, non-invasive method for monitoring deep-sea gatherings in marine parks.