Near the Filchner Ice Shelf in the south of the Antarctic Weddell Sea, a research team has found the world's largest fish breeding area known to date, German marine research organisation the Alfred Wegener Institute (AWI) confirmed earlier this month.
A towed camera system photographed and filmed thousands of nests of icefish of the species Neopagetopsis ionah on the seabed. The density of the nests and the size of the entire breeding area suggest a total number of about 60 million icefish breeding at the time of observation.
These findings provide support for the establishment of a Marine Protected Area in the Atlantic sector of the Southern Ocean.
A team led by Autun Purser from AWI have published their results in the current issue of the scientific journal Current Biology.
The discovery was first made in February 2021, when researchers viewed numerous fish nests on the monitors aboard the German research vessel Polarstern. The vessel's towed camera system transmitted a live video feed to the vessel from the seabed some 535 to 420 metres below the ship, from the seafloor of the Antarctic Weddell Sea.
Later precise evaluation showed that there was on average one breeding site per three square metres, with the team even finding a maximum of one to two active nests per square metre.
The mapping of the area suggests a total extent of 240 square kilometres, which is roughly the size of the island of Malta. Extrapolated to this area size, the total number of fish nests was estimated to be about 60 million.
AWI has been exploring the area with its icebreaker Polarstern since the early 1980s. So far, only individual Neopagetopsis ionah or small clusters of nests have been detected in this area.
The unique observations were made with a so-called Ocean Floor Observation and Bathymetry System (OFOBS). It is a camera sledge built to survey the seafloor of extreme environments, like ice-covered seas.
OFOBS is towed on a special fibre-optic and power cable normally at a speed of about one half to one knot, about one and half metres above the seafloor.
Based on the images, the team was able to clearly identify the round fish nests, about 15 centimetres deep and 75 centimetres in diameter, which were made distinct from the otherwise muddy seabed by a round central area of small stones.
Several types of fish nests were distinguished: "active" nests, containing between 1,500 and 2,500 eggs and guarded in three-quarters of the cases by an adult icefish of the species Neopagetopsis ionah, or nests which contained only eggs; there were also unused nests, in the vicinity of which either only a fish without eggs could be seen, or a dead fish.
The researchers mapped the distribution and density of the nests using OFOBS's longer-range but lower-resolution side scan sonars, which recorded over 100,000 nests.
The scientists combined their results with oceanographic and biological data. The result is that the breeding area corresponds spatially with the inflow of warmer deep water from the Weddell Sea onto the higher shelf.
With the help of transmitter equipped seals, the multidisciplinary team was also able to prove that the region is also a popular destination for Weddell seals. Around 90 per cent of the seals' diving activities took place within the region of active fish nests, where they presumably go in search of food.
The researchers have estimated the biomass of the ice fish colony there at 60,000 tonnes.
