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Deep-sea explorers have found the largest collection of fish ever seen in the abyssal zone of the ocean, which stretches from 3000 to 6000 metres deep. Footage of more than 100 cutthroat eels on top of an underwater mountain in the Pacific suggests that these areas may be less empty than previously believed.

鈥淭his was totally unprecedented for the deep ocean,鈥� says Astrid Leitner from the Monterey Bay Aquarium Research Institute in California.

Various countries have plans to mine minerals from the underwater plains in the Clarion-Clipperton zone of the Pacific, an area roughly the size of the continental US. This could lead to marine life seeking refuge on nearby underwater mountains.

To learn more about these areas, Leitner and her colleagues dropped autonomous underwater vehicles equipped with baited cameras on several seamounts just south-east of Hawaii.

They started with a seamount 3100 metres below the surface. When they retrieved the lander, the first images they saw were initially disappointing as they seemed to show a black screen. But a closer look revealed the frame was so full of eels that it just appeared black.

鈥淲e basically landed on top of eels, then they just swarmed at us,鈥� says Leitner.

The video contained at least 115 individual eels. This is the biggest known aggregation of eels, or indeed any fish, ever seen deeper than 1000 metres, including around sunken whale carcasses, which often attract scavengers.

The largest previous example the researchers could find record of in the deep sea was a group of 68 eelpouts, an eel-like fish, on the abyssal plains of the Arabian Sea, gathered around a large shark carcass that was submerged as part of an experiment.

A few specimens captured by the underwater vehicles confirmed them to be cutthroat eels (Ilyophis arx), which are up to a metre long. Only a handful of these had ever been captured before.

Leitner says the team鈥檚 subsequent seamount surveys turned up a second, slightly smaller aggregation on a different seamount, but the team hasn鈥檛 found anything similar on underwater plains. The finding is especially exciting since the deep sea was previously thought to be relatively void of such large groups of fish.

鈥淔or us, it was really an example of how much we have left to learn about these habitats,鈥� says Leitner, but she worries about the effects that planned mining projects may have on these little-known ecosystems. 鈥淚t鈥檚 a little scary to think of those impacts now and what could be lost.鈥�

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