With the find, researchers have proven that extensive breeding icefish colonies actually exist.

You may think that birds in particular are often busy building nests. But some fish can do it too. Researchers are in the Weddell Sea near Antarctica on no fewer than 60 million nests of ice fish (Notothenioidei) stopped. “It came as a total surprise,” said researcher Autun Purser.

Filchner Ice Shelf

Scientists had actually traveled to the area to inspect the Filchner-Ronne Ice Shelf — the second largest ice shelf on Earth. The researchers recorded the water under the plateau using the Ocean Floor Observation and Bathymetry System (OFOBS) under scrutiny; an ingenious camera/sonar device with which one can study the deep sea. “This is a big thing that weighs a ton and we’re towing behind our icebreaker at a speed of one to four kilometers per hour,” Purser says. “The device is located at a height of about 1.5 to 2.5 meters above the seabed. Then we can make videos and collect acoustic data.”

Warmer water

The researchers were particularly interested in a particular part of the Weddell Sea where warm water is welling up. For example, the water at that location near the seabed is no less than 2 degrees Celsius warmer than the surrounding bottom water. But while surveying the area with their camera, they were surprised to see countless ice fish nests. “We didn’t expect to find fish nests here,” Purser says.

Colony

It means the researchers have discovered an immense breeding colony of ice fish. They estimate that the colony covers at least 150 square miles and contains about 60 million (!) nests. The unprecedentedly large colony represents a fish biomass of more than 60,000 tons. “Our main finding is that we have now demonstrated the existence of such an extensive breeding icefish colony,” Purser said. “A few dozen nests have been observed elsewhere in Antarctica, but our find is many times larger.”

Ice fish found in the Weddell Sea. Image: PS118, AWI OFOBS team

Most of the nests the researchers saw with their own eyes were guarded by a single heroic adult fish. Single-handedly, he supervised as many as 1,700 eggs.

carcasses

In addition to this bizarre find, the researchers also found numerous carcasses of deceased fish in and near the breeding colony. This suggests that the fish play an important role in the larger food web. Although more research is needed, the team suspects that hungry predators may be targeting the colony.

seals

Seals, among others, might occasionally pay the colony a visit during mealtime. “A lot of Weddell seals spend a lot of their time near the fishing nests,” Purser said. “We can determine that from newly collected and historical tracking data. It may be a coincidence, but it seems that seals dive to the depths of the fishing nests. And so it is quite possible that they feast on the fish.”

The findings reveal a globally unique ecosystem, the researchers underline. And so they advocate that the region should become a marine protected area. In the meantime, they have deployed two camera systems to monitor the ice fishing nests. The hope is that the photos taken several times a day will provide new insights into how this newly discovered ecosystem works.