Near the Filchner Ice Shelf in the south of the Antarctic Weddell Sea, scientists have discovered the largest known fish-farming area in the world. Using towed cameras that take pictures of the seabed, they documented thousands of ice fish nests. Based on the breeding area of 240 square kilometers, they estimate the total number of nests at around 60 million. Fish therefore make a key contribution to the food web in the Antarctic ecosystem. Based on their results, scientists support the creation of a marine protection area in the Atlantic sector of the Southern Ocean.
Ice fish live in regions that are actually too cold to survive. However, thanks to special evolutionary adaptations, they can inhabit polar waters with a water temperature in the range of minus two degrees Celsius. Due to inaccessible conditions, their habitat has so far been investigated to a limited extent. For example, the German icebreaker Polarstern has been on regular expeditions to Antarctica since the early 1980s. Scientists are also using seals equipped with measuring devices as research assistants to study areas that the research vessel cannot penetrate.
Expedition in the Antarctic Ocean
During the expedition from Polarstern in February 2021, a team led by Autun Purser from the Alfred Wegener Institute, Polar and Marine Research Center Helmholtz in Bremerhaven made a remarkable discovery: “In the southern Weddell Sea, we have a fish colony of unprecedented size worldwide, with millions of Neopagetopsis ionah ice fish actively guarding egg-filled nests on many square kilometers of seabed,” the researchers report. “Dozens of nests have been observed elsewhere in Antarctica, but this find is orders of magnitude larger,” said Purser.
During the expedition, the scientists originally wanted to map the seabed in the area of the Filchner Ice Shelf. To this end, they used a towed camera system that they towed behind their research vessel at a height of about 1.5 to 2.5 meters above the seabed at a speed of one to four kilometers per hour. On the one hand, cameras record images and videos, and on the other, they measure the topography of the seabed. The area near the Filchner Ice Shelf was of particular interest to scientists because it has an ocean current which, at temperatures down to zero degrees Celsius, is about two degrees warmer than the surrounding water. “We didn’t know there was a kind of fish nest ecosystem waiting for us there,” says Purser. “It was a total surprise for us.”
million active sockets
At a depth of 535 to 420 meters, the cameras recorded numerous fish nests. “Most of the nests were occupied by adult fish that kept around 1,700 eggs,” the researchers said. That’s about 25 percent more eggs than in previously known ice-fish nests. “This indicates that the conditions around the Filchner Ice Shelf are particularly favorable for ice fish farming,” the researchers write. The nest density is also unique: Purser and his colleagues found up to 1.5 active nests per square meter, an average of one nest per three square meters. In addition to active nests, which accounted for 79 percent of all discovered breeding sites, scientists also found nests empty or containing dead fish.
“After the spectacular discovery of many fish nests, we developed a strategy on board to find out how large the breeding area was – there was literally no end in sight,” says Purser. “The nests are three-quarters of a meter in diameter – so much larger than the sometimes only centimeter structures and creatures we usually record with a towed camera system. This allowed us to increase the height above the ground to about three meters and the towing speed to 5.5 km / h, thereby multiplying the area under study. We covered an area of 45,600 square meters and we counted an unbelievable 16,160 fish nests in the photos and videos. “
Further research planned
By further mapping the area with sonar, which has a greater range but low resolution, scientists estimate that the entire breeding range covers an area of 240 square kilometers and contains about 60 million nests. “The idea that such a large breeding area for ice fish in the Weddell Sea has remained unexplored is absolutely fascinating,” says Purser. According to the current state of research, the newly discovered breeding area is the largest neighboring fish breeding colony in the world.
Earlier studies with seals as research assistants have already shown that the fish eaters enjoyed being in the region where Purser and his colleagues now discovered a breeding colony. For the purposes of the current expedition, scientists re-equipped the seals with measuring devices – and indeed: about 90 percent. seal dives took place in the area of active fish nests. “These fish provide a rich source of food for higher-order predators such as the Weddell seal,” the researchers write.
To protect a unique ecosystem, scientists are campaigning to classify the region as a marine reserve. To monitor ice fish nests, they have now installed two camera systems that take pictures several times a day. On future research expeditions, they want to further explore the breeding colony.
Source: Autun Purser (Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research, Bremerhaven) et al., Current Biology, doi: 10.1016 / j.cub.2021.12.022