2000 kilometres south of the southernmost tip of Antarctica lies the Weddell Sea. These cold, iceberg-filled waters are home to krill, Emperor Penguins – and a massive breeding colony of icefish.
Researchers from the Alfred Wegener Institute in Bremerhaven, Germany, have just discovered 60 million active fish nests across 240 square kilometres of the Weddell Sea. This is the largest fish breeding colony yet discovered, and appears to be a globally unique ecosystem.
“A few dozen nests have been observed elsewhere in the Antarctic, but this find is orders of magnitude larger,” says deep-sea biologist Autun Purser, lead author of the paper published in Current Biology.
“The idea that such a huge breeding area of icefish in the Weddell Sea was previously undiscovered is totally fascinating.”
Known as Jonah’s icefish (Neopagetopsis ionah), these fish are found exclusively in the Southern Ocean and live between 20 and 900 metres below the surface, preying on other fish and krill.
The vast breeding colony was discovered while Purser and colleagues were surveying the Filchner ice shelf in the German research vessel, Polarstern. They were towing a camera “sled” called the Ocean Floor Observation and Bathymetry System (OFOBS), designed to survey the seafloor of extreme environments.
“Basically this is a large, towed device, weighing one ton, which we tow behind the icebreaker RV Polarstern at a speed of one to four kilometres per hour,” Purser explains. “We tow this at a height of about 1.5 to 2.5 metres above the seafloor, recording videos and acoustic bathymetry data.”
The team knew that this area of the seafloor has an upwelling of warm water so they thought they might see something interesting, but they certainly weren’t prepared to see thousands upon thousands of fish nests.
That, Purser says, came as a “total surprise”.
“After the spectacular discovery of the many fish nests, we thought about a strategy on board to find out how large the breeding area was – there was literally no end in sight,” he adds.
The team decided to tow the camera a little higher off the sea bed and increase their towing speed, allowing them to survey an area of 45,600 square metres.
In the photo and video footage, they directly observed more than 16,000 nests, each about 75 centimetres in diameter and 15 centimetres deep. Most of the nests they found were guarded by a single adult fish, protecting (on average) more than 1700 eggs.
The consistent density – about 0.26 nests per square metre – allowed the team to estimate that approximately 60 million nests span across 240 square kilometres.
“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 processes,” Purser and colleagues write in their paper.
In fact, they suspect Weddell seals may take advantage of this colony.
“A great many Weddell seals spend much of their time in close proximity to the fish nests,” Purser says. “We know this from historical tracking data and fresh tracking data from our cruise. The nests are exactly where the warmer water is upwelling.
“These facts may be coincidence, and more work is needed, but the recorded seal data show seals do indeed dive to the depths of the fish nests, so may well be dining on these fish.”
When they left, the research team deployed two camera systems to monitor the nests, taking several photographs each day to track what the fish are up to.
Purser and colleagues plan to return in April 2022 to survey other areas of the Weddell Sea.
In the meantime, they say that this colony highlights the need for the area to be protected.
“We believe our discovery provides support for endeavours to protect the Weddell Sea from anthropogenic impacts by establishing a regional marine protected area,” they conclude.