Researchers record the largest aggregation of fish in the abyssal sea depths

The largest aggregation of fish ever recorded in the depths of the abyssal sea was discovered by a team of oceanographers from the University of Hawai’i at Mānoa (UH, USA), the Monterey Bay Aquarium Research Institute (MBARI, USA) and the National Oceanography Center (NOC, UK). Their findings were recently published in Deep-Sea Research.

“Our observations really surprised us,” said Astrid Leitner, lead author of the study, who led this work as a graduate researcher at the UH Mānoa School of Ocean and Earth Science and Technology (SOEST). “We have never seen reports of such a large number of fish in the sparsely populated deep sea with limited food.”

The researchers, including Leitner, Jennifer Durden (NOC), and professors Jeffrey Drazen (Leitner’s doctoral research consultant) and Craig Smith, carried out the observation during an expedition to the Clarion Clipperton (CCZ) area. The CCZ is a vast region extending almost from Hawai’i to Mexico, which is being explored for deep-sea extraction of nodules containing metals such as copper, cobalt, zinc and manganese.

Abyssal submarine mountains, deep submarine mountains whose peaks lie 3,000m below the sea surface, dot the deep seascape and are some of the least explored habitats on the planet. During the expedition, the research team sampled three of these seamounts and their surrounding plains as part of an effort to establish an ecological baseline prior to mining.

On top of one of three previously unmapped and completely unexplored seamounts, the team captured on video a swarm of 115 merciless eels (Synaphobranchidae family) in a small bait package containing about two pounds (1 kg) of mackerel. Some eels have been caught in a bait trap and identified as Ilyophis arx, a little-known species with fewer than 10 specimens in fish collections around the world.

These eels have been observed on top of all seamounts, but not on the surrounding abyssal plain. The findings provide evidence for an effect of abyssal seamounts (where these mountains can support a much higher number of animals than other surrounding habitats) and also indicate that these eels are likely seamount specialists.

Upon returning from the expedition, the team determined they had documented the largest number of fish ever recorded in the abyssal ocean at one time, nearly double the previous record.

“If this phenomenon is not just isolated to these two seamounts in the CCZ, the implications for deep sea ecology could be widespread,” said Leitner, who is now a postdoctoral fellow at the Monterey Bay Aquarium Research Institute. “Our findings highlight how much there is still to be discovered in the deep sea and how much we could lose if we don’t manage the mining business properly.”