Nearly 3 Million Eels Gather at an Underwater Mountain—But What For?

Every year, a massive gathering of basketwork eels occurs at Patience Seamount, an underwater mountain off Tasmania's coast. For the first time, researchers have tallied the precise number of eels that arrive. According to their findings, published in Royal Society Open Science, there were approximately 2.8 million eels in the region during May 2023.

Numbers were notably lower in March 2024 and April 2025, likely due to seasonal variations rather than a dwindling eel population. The congregation appears to increase throughout the year, peaking in May. Rather than counting eels individually, researchers employed underwater cameras and sonar equipment to estimate the biomass of eels at Patience Seamount.

This led to a figure of 3,900 U.S. tons (3,550 metric tons) of eel flesh, or roughly 2.8 million eels. The next inquiry was: Why are so many eels congregating there? While the researchers do not have a definitive answer, the most plausible explanation is that Patience Seamount is a favored spawning site for the eels. Observations suggest the eels cluster together at a specific depth, indicating the location and topography play a significant role.

The seamount's proximity to the continental shelf may act as a physical boundary for the eels' migration or impact local circulation. Additionally, female eels captured during the study were found to have empty stomachs and late-stage eggs, which further supports the spawning theory. However, the researchers did not directly observe the eels spawning or find eggs released into the water, leaving the mystery of why Patience Seamount is their preferred spawning ground unresolved.