Once the remote-controlled camera caught a glimpse of the bottom of the Weddell Sea, more than 300 meters below the surface ice ceiling, Lilian Boehringer, a student researcher at the Alfred Wegener Institute in Germany, spotted the ice fishing nests. The sandy craters dimpled the seafloor, each the size of a hula hoop and less than a foot apart. Each crater contained a single, stout kingfisher, dark pectoral fins spread like bat wings over a group of eggs.
Aptly named ice fish thrive in waters just above freezing with huge hearts and blood as clear as vodka. Their blood is transparent because they lack red blood cells and hemoglobin to carry oxygen around the body. Icefish’s loss of hemoglobin genes was less of an evolutionary adaptation than a happy coincidence, allowing them to absorb the oxygen-rich Antarctic waters through their skin.
The sighting took place in February 2021 in the camera room aboard a research vessel, the Polarstern, that had come to the Weddell Sea to study other things, not ice fishing. It was 3 a.m. near Antarctica, meaning the sun was shining, but most of the ship was asleep. To Ms. Boehringer’s surprise, the camera continued to broadcast images as it moved with the ship, revealing a continuous horizon of ice fishing nests every 20 seconds.
“It just didn’t stop,” said Mrs. Boehringer. “They were everywhere.”
Half an hour later, Autun Purser, a deep-sea biologist at the same institute, joined Mrs. Boehringer. Nothing left but nesting on the camera feed.
“We were like, is this ever going to end?” said Dr. purse. “How come no one has ever seen this before?”
The nests persisted for the entire four-hour dive, with a total of 16,160 recorded on camera. After two more dives through the camera, the scientists estimated that the Neopagetopsis ionah icefish colony stretched across 92 square miles of the serene Antarctic Sea, totaling 60 million active nests. The researchers described the site — the largest fish farming colony ever discovered — in a paper published Thursday in the journal Current Biology.
“Holy cow,” said C.-H. Christina Cheng, an evolutionary biologist at the University of Illinois-Urbana-Champaign, who was not involved in the study. “This is really unprecedented,” she says. “It’s insanely thick. It’s a great discovery.”
The paper provides “evidence of a complex and as yet undescribed benthic ecosystem in the Weddell Sea,” said Mario La Mesa, a biologist at the Institute of Polar Sciences in Bologna, Italy, who was not involved in the study. “I wouldn’t be surprised if I found other huge colonies of breeding fish elsewhere,” said Dr. La Mesa, who described the nest-guarding behavior of the same Antarctic icefish species last year from sites near the newly discovered colony.
Each of the newly discovered nests contained an average of 1,735 large, yolk-like eggs — low fertility for a fish. An unprotected clutch would be an easy snack for predators such as starfish, polychaete worms and sea spiders, said Dr. cheng. So the males are on watch to make sure their offspring aren’t devoured, at least not before they have a chance to hatch, and can clean the nests with their elongated lower jaws, according to Manuel Novillo, a researcher at the Bernardino Rivadavia Museum of Natural Science in Argentina, which was not involved in the study.
About three quarters of the colony’s nests were guarded by a single fish. The others had eggs but no fish, a fish carcass with a white coat of bacteria, or nothing at all. On the edges of the colony, many unused or abandoned nests cradled several ice fish carcasses, many with starfish and octopuses feasting on their eyes and soft parts.
“If you die in the fishing nest area, you rot out there,” said Dr. purse. “But when you die on the edges, it seems like everyone grabs you and starts eating you there.”
The researchers noted that the colony covered an unusually warm stretch of deep water, with temperatures reaching about 35 degrees Fahrenheit — practically warm compared to other Antarctic waters.
While the discovery of the nests contributes to scientists’ understanding of the ice fish life cycle, it raises even more questions. How often are the nests built and reused? Do the fish die after the eggs hatch? Or, perhaps most obviously, “Why there?” asked Dr. cheng.
The authors have no unequivocal answers, only speculation. Perhaps the warm deep currents will lead the fish to the site. Perhaps there is an abundance of zooplankton for the fry to devour. Or maybe it’s something else.
But there must be something special about the location of the active colony. About 50 kilometers to the west, the researchers found a patch of seafloor that was similarly littered with nests: all empty. These nests were abandoned, overtaken by sponges and corals — long-lived creatures that take years to grow, said Dr. purse.
Hungry foraging Weddell seals also live in the waters above the sprawling icefish settlement. When the researchers collected satellite tracking data from seals during the expedition and analyzed it with historical records, they found, unsurprisingly, that the seals mainly dive for the ice fishing nests. “They’re having a nice dinner,” Dr. Purser said.
Before the cruise’s end, the researchers deployed a camera that will photograph the site twice a day for two years, hopefully revealing even more about the icefish’s life cycle. dr. Novillo said he’s looking forward to what the camera captures. “It could be the first field sighting of courtship behavior and/or nest preparation,” he wrote in an email.
New insights into how ice fish reproduce and contribute to polar food webs could help manage and conserve populations. The authors argue that the new paper provides sufficient evidence to protect the Weddell Sea under the Convention on the Conservation of the Living Resources of the Antarctic.
“The seafloor isn’t just bare and dull,” said Dr. purse. “There are still such huge discoveries to be made, even today in the 21st century.”