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Before starting the mining of precious metals in the darkness of the deep sea, we could try to turn on the light first and observe our surroundings.
In this seemingly isolated abyss, more than 3,000 meters (9,800 feet) below sea level, scientists were able to persuade a huge swarm of 115 merciless eels (Ilyophis arx) out of the shadows and into the light, and with only a relatively small package of bait.
The footage represents the largest number of deep-sea fish ever recorded in the abyssal ocean at one time, and was shot right next to an international mining hotspot.
“Our observations really surprised us,” says biological oceanographer Astrid Leitner, who worked on the research at the University of Hawai’i in Mānoa.
“We have never seen reports of such a large number of fish in the sparsely populated deep sea with limited food.”
The Clarion-Clipperton (CCZ) area is a huge expanse of seabed that stretches from Hawaii almost to Mexico and contains some of the rarest and most sought-after metals and elements on our planet.
Over the years, it has aroused growing interest from the mining industry, which sees this new region as a way to reduce human labor and the destruction of precious lands.
Sixteen contracts have already been issued for deep-sea mining in more than 1 million square kilometers of this area, yet only a small portion of the deep abyssal habitat has been sampled, explored or even mapped by scientists.
It’s decisions like this that have prompted some scientists and conservationists to warn of a deep-water “gold rush” that could cause unexpected damage to ecosystems that we know very little about.
The abyssal plains that cover the bottom of our oceans account for 70% of our planet’s seabed and are considered the largest ecosystem on Earth.
But while these depths are generally thought to contain sparse life, with particularly few fish, that generalization may not apply to all of the underwater mountain ranges – called seamounts – that border these valleys, emerging from the surrounding abyssal plain.
Recent expeditions into the submerged seamounts of the Galapagos and off the coast of Tasmania have revealed an unexpected abundance of life forms, many of which we have never seen before, and some linger on these submerged peaks which are still deep enough to be considered ‘abysmal’.
To figure out what’s down there before starting to dig up sediments and spread plumes of material miles away, an expedition of ocean scientists set out for three seamounts in the CCZ and surrounding lowlands.
All three summits analyzed are located in locations currently protected from mining, but were chosen because they resemble nearby areas where industrial operations are preparing to tie up their vehicles.
By deploying a remote-controlled vehicle over each of these mountains, the researchers dropped a pound of mackerel (about 2 pounds) in front of a camera, filming two minutes later, eight minutes outside, to give the fish a break from the light.
The bait itself was supposed to mimic a natural food drop, in which a whale or shark carcass slides to the bottom of the sea and gathers life at the bottom.
On all three peaks, scientists recorded large swarms of eels coming to feed, though none appeared in the deeper plains below, which were about a thousand meters deeper.
By capturing some of the animals in traps, the team confirmed that they were ruthless eels, a little-known species with fewer than 10 in captivity worldwide.
Sifting through the current literature, the authors could not find any other studies below 1,000 meters that found that number of fish per pound of bait. The biggest food spills also attracted a smaller crowd.
Prior to this discovery, for example, a 29-kilogram shark carcass at a depth of 4,400 meters (14,435 ft) attracted only 68 deep-sea zoarcid fish.
It’s hard to extrapolate from these brief encounters, but if the number of eels observed in this study says anything about local abundance, then the density of life at these depths could be an order of magnitude higher than even our highest predictions so far.
The problem is that fish at these depths are incredibly sensitive to the presence of ROVs, not only because of their light and noise, but also because of the resulting changes in pressure and electricity in the water.
The current study tries to limit these influences by occasionally turning off the camera and the light, but at the same time, this also means that the researchers likely underestimated how many fish actually came to feed.
“Thus,” the authors write, “the number of eels observed in this study at abyssal depths is truly unprecedented for both abyssal and bathyal depths.”
It is still unclear whether these eels are only visiting the underwater peak or are permanent residents, but this type of eel has only been sampled on similar underwater ridges, suggesting that they may be seamount feeding specialists.
It is doubtful that all abyssal seamounts will be home to equally large fish populations. Even in the current study, the northernmost peak had fewer feeders.
But if there are other peaks out there with equally abundant life forms, scientists are concerned about what will happen to them in the future if suddenly their homes are flooded with noisy vehicles and the water becomes thick with sediment.
“If this phenomenon is not just isolated to these two seamounts in the CCZ, the implications for deep sea ecology could be widespread,” says Leitner, who now works at the Monterey Bay Aquarium Research Institute.
“Our results highlight what remains to be discovered in the deep sea and how much we could lose if we do not manage the mining activity properly.”
The study was published in Deep sea research.
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