Scientists aboard an Antarctic research company have discovered that deep beneath the ice of the Weddell Sea lies the largest and most populous breeding colony of fish in the world. Covered in icefish nests, 240 square kilometers east of the Antarctic Peninsula have amazed marine ecologists. “We had no idea it would be on such a scale and I think it’s the most fantastic thing,” says Mark Belchier, a fisher biologist with the British Antarctic Survey and the government of South Georgia and the South Sandwich Islands, who was not involved in the new work.
In February 2021, the large German research vessel Polarstern broke through the sea ice in the Weddell Sea to study marine life. While towing video cameras and other instruments at a depth of half a kilometer on the seabed, the ship came across thousands of nests about 75 centimeters wide, each of which was occupied by one adult icefish, and up to 2,100 eggs. It was truly an amazing sight,” says biologist Autun Purser of the Alfred Wegener Institute, who led the underwater survey.
Sonar found nests stretching for several hundred meters, similar to a World War I battlefield covered in miniature sinkholes. High-definition video and cameras captured over 12,000 adult icefish (Neopagetopsis ionah).
Fish that grow up to about 60 centimeters are adapted to life in conditions of extreme cold. They produce antifreeze-like compounds and, thanks to the region's oxygen-rich waters, are among the few vertebrates to have colorless blood without hemoglobin.
Adult icefish build their circular nests by scraping gravel and sand with their pelvic fins. But before the study, only a few scattered nests were observed, not numerous and remote from each other. “I certainly didn't know they were just being built on such a huge scale — more like seabirds and penguins on land or in ice,” says Belchier, who has been studying Antarctic fish for over two decades.
In the process of observations, it was possible to count 16160 closely spaced fish nests, 76% of which were guarded by single males. Assuming a similar density of nests in unexplored areas along the course of the ship, it is likely that about 60 million nests occupy about 240 square kilometers, scientists report in the journal Current Biology. Due to their sheer numbers, icefish and their descendants are likely to be key players in the local ecosystem.
Adult icefish can use currents to find spawning grounds rich in zooplankton that their young feed on. In addition, dense nesting may help protect individuals from predators.
The extensive colony, researchers say, is a new reason for establishing a marine protected area in the Weddell Sea, an idea that has been put forward five of the last 6 years by the relevant fisheries regulator there. Such a proposal requires the unanimous approval of member countries and has not yet been adopted. The Weddell Sea, a unique and largely untouched ecosystem, is already protected from a destructive fishing practice called bottom trawling, Belcher notes, but he would like to see more safeguards for this environmental hotspot.
In the meantime, the researchers focused on one of the most densely populated parts of the colony to learn more about the fish's breeding behavior and nesting patterns by installing cameras and other equipment in those areas. They hope to return them in 2023. “There is a general consensus that the deeper you go into the ocean, the rarer life becomes,” says Belchier. I think that there will be other areas outside of Antarctica where fantastic zoological finds will be found.