World’s Largest Fish Breeding Colony Discovered

ANTARCTICA— A new study from Jan. 13 has found the world’s largest known colony of breeding fish 500 meters below the ice covering Antarctica’s Weddell Sea. The fish, Notothenioid icefish, are found in the Southern Ocean around Antarctica and are the only known vertebrates that lack hemoglobin in their blood as adults. The study was conducted under Current Biology and written by Autun Purser, Laura Hehemann, Lillian Boehringer, Andreas Rogge, Moritz Holtappels, and Frank Wenzhoefer.


An estimated 60 million active nests of this icefish stretch across at least 240 square kilometers, nearly the size of Orlando, Florida. The unique colony represents a fish biomass (the whole body, wet-weight, of the in-water part of a fishable population) of more than 60,000 tons, equivalent to 135 million pounds. It is known that many fish will make nests, but until now, researchers have encountered only a handful of icefish nests. Even the most generous of nest building fish were known to gather on in the hundreds.


Most of the nests were found occupied by one adult fish guarding about 1,735 eggs. The bottom water temperatures measured across the nesting colony were two degrees Celsius warmer that the surrounding bottom waters.


“This indicates a spatial correlation between the modified Warm Deep Water (mWDW) upflow onto the Weddell Shelf and the active nesting area,” said the study.


The study also suggested that that congested areas of nesting may be being utilized by predators such as Weddell seals.


“Numerous degraded fish carcasses within and near the nesting colony suggest that, in death as well as life, these fish provide input for local food webs and influence local biogeochemical processing.,” said the study.


Deep-sea biologist Autun Purser of the Alfred Wegener Institute in Bremerhaven, Germany, and colleagues stumbled across the massive colony in early 2021 while on a research cruise in the Weddell Sea, located between the Antarctic Peninsula and the main continent. It is unclear why so many icefishes have gathered in one spot to breed. However, there appears to be suitable access to plankton in this location, which would be a crucial food source for newly hatched fish. The team also found a zone with slightly warmer water in the area, which could help the icefish locate the breeding ground.


As for now, Purser has two seafloor cameras at the colony site, where they will remain for the next few years to take photos four times a day to see if the nests are ever reused over time.


For more information on the discovery, visit the Alfred Wegener Institute website.

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