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Massive nursery for icefish found underneath Weddell Sea in Antarctica

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Scientists on an icebreaker in Antarctica were astounded to discover a trove of 60 million icefish nests dotting the Weddell Seafloor. The swarm of nurseries, each guarded by a ghostly-looking parent, is the world's largest known breeding colony of fish.


Autun Purser of the Alfred Wegener Institute was on the bridge of the German icebreaker RV Polarstern, keeping an eye out for whales, when his graduate student, Lilian Böhringer, who was watching the camera feed, called up to the bridge. Böhringer was watching a live video feed from the Ocean Floor Observation and Bathymetry System (OFOBS), a one-ton camera towed behind the ship, as part of the ship's mission to monitor the seafloor of the Weddell Sea.


Böhringer could see fish nests pockmarking the seafloor every 10 inches (25 centimetres) in all directions and covering an area of 93 square miles on the video feed (240 square kilometres). "The camera was moving [across the seafloor] and didn't seem to want to stop. They were all over "Böhringer explained to Live Science.


The nests were simple bowls carved into the mud on the seafloor by notothenioid icefish (Neopagetopsis ionah), which live in the cold southern oceans. They are the only vertebrates known to have no haemoglobin in their blood. As a result, icefish are referred to as "white-blooded."


"We realised the next day after calling the home institute that we had discovered something spectacular," Purser said.


Following the initial discovery, the team returned to the site, towing the camera at a shallower depth to get a better view of the colony.


Icefish nest in clusters, but "the most ever seen before was forty nests or something like that," Purser said. Following extensive surveying, this nesting site is estimated to have 60 million nests. "This is the first time we've seen anything like it," Purser added.


The majority of those nests had one adult fish watching over an average of 1,700 eggs.


The researchers were in the area to study an upwelling of water that was 3.6 degrees Fahrenheit (2 degrees Celsius) warmer than the surrounding water. Purser explained, "Our goal was to see how carbon moves from the surface to the seafloor and what communities are in the water column."


They discovered microscopic zooplankton near the surface of the upwelling column of water, where young icefish swim after hatching to feast on the floating buffet before returning to the seafloor to breed. The presence of icefish in the upwelling was expected due to the food. A breeding colony many orders of magnitude larger than had ever been seen before, on the other hand, was not.


In addition to living fish guarding nests, the team discovered fish carcasses, implying that this massive icefish colony is an important part of the local ecosystem, most likely serving as prey for Weddell seals.


The discovery of the colony prompted an international effort to designate it as a Marine Protected Area under the auspices of the International Commission for the Conservation of Antarctic Marine Living Resources.


Surprisingly, the icefish colony appears to have a distinct border. "[The colony] went from very, very dense to nothing, similar to penguin colonies," Purser explained. "It was like a sandcastle."


They discovered that the "line in the sand" was the outer edge of the warm upwelling. While more research is needed to determine whether this is a coincidence, the upwelling appears to create a unique and ideal environment for icefish breeding.


Before departing, the crew of the Polarstern left two cameras to document the inner workings of this unique ecosystem. Purser intends to visit the Weddell Sea again in April 2022.


"There's definitely a lot to discover," Purser said.

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