The largest group of breeding fish ever found lives under the Antarctic ice

Five hundred meters below the ice covering Antarctica’s Weddell Sea is the world’s largest known colony of breeding fish, a new study finds.

An estimated 60 million active icefish nests extend over at least 150 square miles, nearly the size of Orlando, Florida. Many fish create nests, from freshwater cichlids to artistically inclined puffers (SN: 13-10-20). But so far, researchers have come across only a handful of ice fishing nests at a time, or maybe a few dozen. Even the most flock-like fish species that build nests were previously known to only gather in the hundreds.

The icefish are likely to have a substantial and previously unknown impact on the Antarctic food webs, researchers report Jan. Current Biology.

Deep-sea biologist Autun Purser of the Alfred Wegener Institute in Bremerhaven, Germany, and colleagues stumbled upon the massive colony in early 2021 during a research cruise in the Weddell Sea, which sits between the Antarctic Peninsula and the main continent.

The researchers studied chemical connections between surface water and the seabed. Part of the research involved examining life on the seafloor by slowly towing a device behind the scientists’ icebreaking research vessel. That device recorded video as it glided just above the ocean floor and used sound to map out the features of the seafloor.

At a location on the Filchner Ice Shelf in the Weddell Sea, one of Purser’s colleagues operated the camera tow and noted that he saw the circular Jonah’s icefish (Neopagetopsis ionah) nest below. Ice fish, of the family Channichthyidae, are found only in the Southern Ocean and Antarctic waters and have strange adaptations to the extreme cold, such as bright blood full of antifreeze compounds (SN: 19/09/98).

“When I came down half an hour later and saw nest after nest for the entire four hours of the first dive, I thought we were onto something unusual,” Purser said.

Purser and colleagues did three more surveys in the area, continuing to find nests of similar density mile after mile. Perhaps one of the closest comparisons to ice fishing is among the nest-spawning fish bluegills (Lepomis macrochirus), which form breeding colonies that can have hundreds of individuals, Purser says. But based on measurements showing about one ice fishing nest per four square meters across hundreds of miles of territory, the colony in the Weddell Sea is several orders of magnitude larger, the researchers say.

The colony is an “astonishing discovery,” said evolutionary biologist Thomas Desvignes of the University of Oregon at Eugene, who was not involved in the study. He especially noticed the extreme concentration of nests.

“It reminded me of bird nests,” Desvignes says. “When we see cormorants and other seabirds nesting like this, one next to the other, it’s almost like that.”

It’s not clear why so many ice fish gather in one place to breed. There appears to be good access to plankton at this location, which would be a vital food source for newly hatched fish. The team also found a zone of slightly warmer water in the area, which could help the ice fish find the nesting site.

The icefish can sustain Weddell seals, the researchers say. Previous studies have shown that the seals spend a lot of time diving in waters above the colony area.

Purser thinks there are smaller Jonah’s icefish colonies closer to shore where there is less ice. However, the Jonah’s ice fish may be disproportionately dependent on the huge breeding colony, effectively putting all their eggs in one basket. If so, “that would make the species extremely vulnerable” to extinction, Desvignes says. The discovery of the huge colony is another argument for providing environmental protection for the Weddell Sea, as has been done for the nearby Ross Sea, he says.

For now, Purser currently has two seafloor cameras at the colony site, where they will stay for a few years, taking pictures four times a day to see if the nests are reused over time.

“I would say [the massive colony] is almost a new ecosystem type on the seabed,” Purser says. “It’s really surprising that it’s never been seen before.”

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