The most extensive and densely populated breeding colony of fish anywhere lurks deep underneath the ice of the Weddell Sea, scientists aboard an Antarctic research cruise have discovered. The 240 square kilometers of regularly spaced icefish nests, east of the Antarctic Peninsula, has astonished marine ecologists. “We had no idea that it would be just on this scale, and I think that’s the most fantastic thing,” says Mark Belchier, a fish biologist with the British Antarctic Survey and the government of South Georgia & the South Sandwich Islands, who was not involved in the new work.
In February 2021, the RV Polarstern—a large German research ship—was breaking through sea ice in the Weddell Sea to study marine life. While towing video cameras and other instruments half a kilometer down, near the sea floor, the ship came upon thousands of 75-centimeter-wide nests, each occupied by a single adult icefish—and up to 2100 eggs. “It was really an amazing sight,” says deep-sea biologist Autun Purser of the Alfred Wegener Institute, who led the ship’s underwater imaging.
Sonar revealed nests extending for several hundred meters, like a World War I battlefield scarred by miniature craters. High-resolution video and cameras captured more than 12,000 adult icefish (Neopagetopsis ionah). The fish, which grow to about 60 centimeters, are adapted to life in the extreme cold. They produce antifreezelike compounds, and—thanks to the region’s oxygen-rich waters—are among the only vertebrates to have colorless, hemoglobin-free blood.
Adult icefish build their circular nests by scraping away gravel and sand with their pelvic fins. But only a handful of scattered nests, few and far between, had been observed prior to the cruise. “I certainly was unaware of the fact that they just build across this huge scale—more like seabirds and penguins on land or ice,” says Belchier, who has studied Antarctic fish for more than 2 decades.
Including three subsequent tows, the team on the RV Polarstern saw 16,160 closely packed fish nests, 76% of which were guarded by solitary males. Assuming a similar density of nests in the areas between the ship’s transects, the researchers estimate that about 60 million nests cover roughly 240 square kilometers, they report today in Current Biology. Because of their sheer numbers, the icefish and their eggs are likely key players in the local ecosystem.
Purser says adult icefish may use currents to find the spawning grounds, whose waters are rich in the zooplankton their offspring eat. In addition, the dense collection of nests could help protect individuals from predators.
The vast colony, the researchers say, is a new reason to create a marine protected area in the Weddell Sea, an idea has been proposed five out of the past 6 years to the intergovernmental treaty organization that regulates fisheries there. Such a proposal requires unanimous approval by member nations and has not passed. The Weddell Sea—a unique and largely undisturbed ecosystem—is already protected from a destructive fishing practice called bottom trawling, Belchier notes, but he’d like to see more safeguards for this ecological hot spot.
Meanwhile, the researchers have left cameras and lights moored above one of the densest parts of the colony to learn more about breeding and nesting behaviors; they hope to retrieve them in 2023. There’s a larger lesson: “The popular belief is that the deeper you go in the ocean, the more life becomes sparse,” Belchier says. “I think there are going to be other areas beyond the Antarctic that are going to hold fantastic zoological finds.”