Massive Swarm of Eels Is The Most Fish Ever Recorded at The Bottom of The Ocean

 Before we start mining for precious metals within the darkness of the deep sea, we would try switching on the sunshine first and observing our surroundings.

In this seemingly isolated abyss, at deeper than 3,000 metres (9,800 ft) below water level, scientists were able to coax an enormous swarm of 115 cutthroat eels (Ilyophis arx) out of the shadows and into the sunshine, and with only a comparatively small package of bait.

The footage represents the best number of deep-sea fish ever recorded at only once within the abyssal ocean, and it had been shot right near a world mining hotspot.

"Our observations truly surprised us," says biological oceanographer Astrid Leitner, who worked on the research at the University of Hawai'i at Mānoa. 

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

The Clarion-Clipperton Zone (CCZ) may be a massive expanse of seafloor that runs from Hawaii nearly to Mexico, and it contains a number of the rarest and most highly demanded metals and elements on our planet. 

Over the years, it's drawn increasing interest from the mining industry, which sees this new region as some way to chop down on human labour and therefore the destruction of precious land.

Sixteen contracts have already been issued for deep-sea mining in additional than 1 million square kilometers of this zone, and yet only a small portion of deep abyssal habitats are sampled, explored, or perhaps mapped by scientists.

It's decisions like this that have some scientists and environmentalists warning of a deep-sea "gold rush" that would cause unforeseen damage to ecosystems we all know little about.

The abyssal plains that blanket the underside of our oceans represent 70 per cent of our planet's seafloor and are considered the most important ecosystem on Earth. 

But while these depths are generally thought to contain sparse life, with particularly few fish, that generalisation won't apply to all or any the underwater mountain ranges – called seamounts – that border these valleys, emerging from the encompassing abyssal plain. 

Recent expeditions among submerged seamounts within the Galapagos and off the coast of Tasmania have revealed an unexpected abundance of life forms, many of which we've never seen before, and a few waver these submerged summits that are still deep enough to be considered 'abyssal'. 

To figure out what's down there before we start unearthing sediment and spreading plumes of fabric kilometres away, an expedition of ocean scientists taken off for 3 underwater seamounts within the CCZ and their surrounding plains.

All three of the summits analysed are in locations currently protected against mining activities, but they were chosen because they resemble nearby areas where industry operations have gotten able to tether their vehicles.

Deploying a remotely operated vehicle to every one of those mountains, researchers dropped a kilo of mackerel (around 2 lbs) before of a camera, filming two minutes on, eight minutes off, to provide the fish with a possibility from the sunshine.

The bait itself was purported to mimic a natural food fall, within which a whale or shark carcass drifts to the seafloor and brings together life at the underside.

On all three summits, scientists recorded large swarms of eels coming to feed, although none appeared on the deeper plains below, which sat roughly 1000 metres deeper.

Catching a number of the animals in traps, the team confirmed these were scavenging cutthroat eels, a little-known species with fewer than 10 specimens in captivity worldwide.

Combing through the present literature, the authors couldn't find the other study below 1,000 metres that turned up that number of fish per kilo of bait. Even larger food falls drew a smaller crowd.

Prior to this discovery, for example, a 29-kilogram shark carcass at a depth of 4,400 meters (14,435 ft) only drew in 68 deep-sea zoarcid fish.

It's hard to extrapolate from these brief encounters, but if the sheer number of eels observed during this study says anything about local abundance, then the density of life at these depths might be an order of magnitude more than even our highest predictions so far.

The problem is, fish at these depths are incredibly sensitive to the presence of ROVs, not simply because of their light and noise, but also due to the changes in pressure and electricity that ripple out from them within the water.

The current study tries to limit those influences by turning off the camera and light-weight every so often, but at the identical time, this also means researchers probably undercounted what number fish actually came to feed. 

"Thus," the authors write, "the number of eels observed during this study at abyssal depths is really unprecedented for both abyssal and bathyal depths." 

Whether these eels are just visiting the underwater summit or are permanent residents remains unclear, but this sort of eel has only ever been sampled on similar underwater ridges, suggesting they may be seamount feeding specialists.

It's doubtful that each one abyssal seamount will host similarly large populations of fish. Even within the current study, the northernmost summit had fewer feeders.

But if there are other summits out there with equally abundant kinds of life, scientists are worried what is going to happen to them within the future if suddenly their homes are swarmed with loud vehicles and also the water becomes thick with sediment.

"If this phenomenon isn't just isolated to those two seamounts within the CCZ, the implications on deep-sea ecology may be widespread," says Leitner, who now works at the bay Aquarium Research Institute.

"Our findings highlight what proportion there's still left to get within the deep sea, and the way much we all might lose if we don't manage to mine appropriately."