Near the Filchner Ice Shelf in the south of the Antarctic Weddell Sea, researchers have discovered the largest known fish breeding area in the world. With the help of towed cameras that take pictures of the seabed, they documented thousands of icefish nests. Based on the size of the breeding area of 240 square kilometers, they estimate the total number of nests at around 60 million. The fish therefore make a crucial contribution to the food web in the Antarctic ecosystem. Based on their results, the researchers are in favor of establishing a marine protection area in the Atlantic sector of the Southern Ocean.
Icefish live in regions that are actually too cold to survive. However, due to special evolutionary adaptations, they can inhabit polar waters with water temperatures in the range of minus two degrees Celsius. Due to the inaccessible conditions, their habitat has so far only been explored to a limited extent. For example, the German icebreaker Polarstern has been undertaking regular expeditions to Antarctica since the early 1980s. In order to investigate areas that the research vessel cannot penetrate, scientists also use seals equipped with measuring devices as research assistants.
Expedition in the Antarctic Ocean
During an expedition with the Polarstern in February 2021, a team led by Autun Purser from the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research in Bremerhaven has now made an extraordinary discovery: “In the southern Weddell Sea we have a fish colony of unprecedented size worldwide discovered in which millions of icefish of the species Neopagetopsis ionah are actively guarding the egg-filled nests over many square kilometers of the sea floor,” the researchers report. “A few dozen nests have been observed elsewhere in Antarctica, but this find is orders of magnitude larger,” Purser said.
During the expedition, the researchers originally wanted to map the seabed in the area of the Filchner Ice Shelf. To do this, they used a towed camera system, which they towed behind their research vessel at a height of around 1.5 to 2.5 meters above the sea floor at a speed of one to four kilometers per hour. On the one hand, the cameras record images and videos, on the other hand they measure the topography of the seabed. The area near the Filchner Ice Shelf was of particular interest to the researchers because there is an ocean current there that, at up to zero degrees Celsius, is around two degrees Celsius warmer than the surrounding water. “We didn’t know that a kind of fish nest ecosystem would await us there,” says Purser. “It was a total surprise for us.”
million active nests
At a depth of 535 to 420 meters, the cameras recorded numerous fish nests. “Most of the nests were each occupied by an adult fish, which guarded around 1,700 eggs,” the researchers said. That is around 25 percent more eggs than in previously known ice fish nests. “This indicates that the conditions in the area of the Filchner Ice Shelf are particularly favorable for the breeding of ice fish,” the researchers write. The density of the nests is also unique: Purser and his colleagues found up to 1.5 active nests per square meter, on average there was one nest per three square meters. In addition to the active nests, which accounted for 79 percent of all discovered breeding sites, the researchers also discovered nests that were empty or contained dead fish.
“After the spectacular discovery of the many fish nests, we came up with a strategy on board to find out how big the breeding area is – there was literally no end in sight,” says Purser. “The nests have a diameter of three quarters of a meter – so they are much larger than the sometimes only centimeter-sized structures and creatures that we normally record with the towed camera system. This enabled us to increase the height above the ground to around three meters and the towing speed to up to 5.5 kilometers per hour, thus multiplying the area examined. We covered an area of 45,600 square meters and counted an incredible 16,160 fish nests on the photo and video recordings.”
Further research planned
By further mapping the area using sonars, which have a longer range but low resolution, the researchers 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 of icefish in the Weddell Sea has remained undiscovered is absolutely fascinating,” says Purser. According to the current state of research, the newly discovered breeding area is the largest contiguous fish breeding colony in the world.
Previous studies with seals as research assistants had already shown that the fish eaters liked to stay in the region where Purser and his colleagues have now discovered the breeding colony. For the current expedition, the researchers again equipped seals with measuring devices – and indeed: around 90 percent of the seals’ diving activities took place in the area of active fish nests. “These fish provide a rich food source for higher-order predators such as the Weddell seal,” the researchers write.
In order to protect the unique ecosystem, the researchers are campaigning for the region to be classified as a marine reserve. In order to monitor the ice fish nests, they have now installed two camera systems that take photos several times a day. On future research expeditions, they want to explore the breeding colony further.
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