Marine Researchers Discover Largest Aggregation of Fishes Ever Observed at Abyssal Depths
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Marine biologists from the United States and the United Kingdom have recorded over 100 deep water-dwelling cutthroat eels, identified as Ilyophis arx, at a 1 kg bait package deployed on an abyssal seamount summit in the southwestern Clarion-Clipperton Zone in the central Pacific Ocean. This is the highest number of fishes per kg of bait ever recorded below 1,000 m (3,281 feet), including observations from large organic falls such as cetacean and shark carcasses. It is also the highest number that has ever been recorded at carrion of any kind or size at abyssal depths.

Ilyophis arx eels swarm around a small bait package deployed on the summit of an unnamed abyssal seamount in the southwestern Clarion-Clipperton Zone in the central Pacific Ocean. Image credit: Deep Sea Fish Ecology Lab, University of Hawaii.

Abyssal seamounts are deep underwater mountains whose summits are 3,000 m (9,800 feet) below the sea surface. They dot the deep seascape and are some of the least explored habitats on Earth.

Abyssal fish communities are notoriously difficult to quantify. Trawls typically find lower abundances than visual methods, and some studies are biased by some species’ avoidance of noise and light.

Baited cameras provide an efficient and unobtrusive alternative method for observing large numbers of abyssal fishes, scavengers, and predators, as they mimic natural food falls where these animals naturally congregate and which make up a regular part of their diet.

This method overcomes some of the challenges of low densities, high mobility, and high sensitivity by using bait to attract individuals from a surrounding area for census before a camera.

“Our observations truly surprised us,” said lead author Dr. Astrid Leitner, a postdoctoral researcher at the Monterey Bay Aquarium Research Institute.

“We had never seen reports of such high numbers of fishes in the sparsely-populated, food-limited deep-sea.”

Dr. Leitner and colleagues sampled abyssal seamounts and their surrounding plains in the Clarion-Clipperton Zone, a large region stretching nearly from Hawai‘i to Mexico, which is being explored for deep sea mining of nodules containing metals such as copper, cobalt, zinc and manganese.

On the summit of one of the three previously unmapped and completely unexplored seamounts, they captured on video a swarm of 115 cutthroat eels at a small bait package containing 1 kg of mackerel.

A few eels were caught in a baited trap and identified to be of Ilyophis arx, a poorly known species with fewer than 10 specimens in fish collections worldwide.

These eels were observed at the top of all of the seamounts, but not on the surrounding abyssal plain.

The findings provide evidence for an abyssal seamount effect (where these mountains can support much higher numbers of animals than other surrounding habitats), and also indicate these eels are likely to be seamount specialists.

After returning from the expedition, the authors determined they had documented the highest number of fishes ever been recorded at one time in the abyssal ocean — almost double the previous record.

“Traditionally, the abyssal seafloor is considered a habitat of low megafaunal abundances with populations limited by challenging environmental conditions (low food availability, low temperatures, high pressures), but these generalizations may not apply to seamount summits at abyssal depths,” they said.

“How can such high numbers of active megafaunal predators be sustained in a relatively small and seemingly isolated area of the abyssal seafloor on a seamount summit?”

“Are these eels permanent seamount residents or were these ephemeral aggregations?”

“Abyssal seamounts may provide unusual laboratories in which to explore carbon flows and energy availability in abyssal food webs with a high abundance of top predators.”

The findings were published in the journal Deep Sea Research Part I: Oceanographic Research Papers.

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Astrid B. Leitner et al. Synaphobranchid eel swarms on abyssal seamounts: Largest aggregation of fishes ever observed at abyssal depths. Deep Sea Research Part I: Oceanographic Research Papers, published online November 17, 2020; doi: 10.1016/j.dsr.2020.103423

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