the world's largest breeding colony of fish, representing a biomass of more than 135 million pounds, and covers an area roughly the size of birmingham.

The surprise discovery of about 60 million active nests was made by a team of biologists while collecting routine data at 1.5-2.5 metres above the seafloor of Antarctica's southern Weddell Sea.

Others contained only eggs.

The Alfred Wegener Institute ( AWI ) has been exploring the Weddell sea since the early 1980s but until this discovery had only ever found small clusters of nests.

"The idea that such a huge breeding area of icefish in the Weddell Sea was previously undiscovered is totally fascinating," says Dr Autun Purser, deep-sea biologist at the AWI.

he added : "after the spectacular discovery of the many fish nests, we thought about a strategy on board to find out how large the breeding area was- there was literally no end in sight."

Researchers were initially interested in the area because of a process called upwelling, in which wind and currents bring cold water to the surface, causing the water to be 2C warmer than the surrounding area.

The colony covers at least 150 square miles (Image: SWNS)

It was found to be a popular destination for Weddell seals.

The researchers tracked a number of seals and found that 90 per cent of their diving took place around the nests.

Dr Autun Purser, lead author of the findings, reported in journal Current Biology, said : "A great many seals spend much of their time in close proximity to the fish nests.

"we know this from historical tracking data and fresh tracking data from our cruise.

There were about 6 million active nests (Image: SWNS)

The nests are exactly where the warmer water is upwelling.

"These facts may be coincidence, and more work is needed, but the recorded seal data show seals do indeed dive to the depths of the fish nests, so may well be dining on these fish."

Researchers said the discovery of the breeding grounds showed that marine conservation in Antarctica needs to be improved.

the findings were published in current biology.

Each of the nests include about 1,700 eggs, with a number of fish carcasses found within, or near the site - as it is thought to be a feeding ground for seals

The ecosystem was found by accident using the ocean floor observation and bathymetry system, a large, towed camera device that records photos, videos and takes measurements of deep-sea habitats.

The researchers plan to return to the region in April 2022 to survey the surrounding waters and to see whether the fish breed again in the same nests.

Each of the nests was occupied by a single adult icefish, that would guard more than 1,700 eggs.

The researchers noted there were many fish carcasses found within and near the site, suggesting that the notothenioids, or icefish, play an important role in the wider food web.

Nest followed nest, the team said. Precise evaluations identified an average one breeding site per 33 square foot - with up to two per 10 square feet

Their bodily fluids contain antifreeze proteins that enable them to survive the very cold temperatures of the Southern Ocean.

As a result, blood is less thick and sticky- increasing supply of oxygen to organs.

"a few dozen nests have been observed elsewhere in the antarctic- but this find is orders of magnitude larger."

dr purser said : 'we did not know to expect any sort of fish nest ecosystem."

Antarctica breakthrough: World's largest colony of mystery creatures found beneath ice (Image: SWNS)

that part, he adds, came as a "total surprise."

It's likely to be the most spatially extensive contiguous fish breeding colony discovered worldwide to date.

The researchers have now deployed two camera systems to monitor the icefish nests until a research vessel returns.

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