Which creatures swim through the water masses under the ice of the Arctic Ocean? Not only plankton and small fish – larger predators are apparently also on the move there: cod and squid are found surprisingly far north, as discoveries made as part of the MOSAiC expedition show. Presumably, these larger fish provide food for marine mammals and ultimately for the polar bears of the extreme north. However, the researchers do not see any potential for fishing and emphasize the importance of protecting the Arctic Ocean, which is severely affected by climate change.
It was the largest Arctic expedition of all time: starting in autumn 2019, the German research icebreaker Polarstern drifted through the Arctic Ocean for a year, frozen in sea ice. On board were scientists who were investigating questions about the climatic role and development of the far north of the earth, but the biology of the icy ocean was also the focus of research projects. One of them was dedicated to exploring the fish stocks in the Central Arctic: During the 3170-kilometer drift, researchers from the EFICA consortium (European Fisheries Inventory in the Central Arctic Ocean) first recorded information using the Polarstern’s fishery echo sounder system.
The hydroacoustic data showed that zooplankton and small fish are found in abundance in an Atlantic water layer of the Amundsen Basin that is 100 to 600 meters deep. Apparently, the comparatively low biomass production in the cold and dark sea can serve as a basis for these creatures there. However, with a deep-sea camera installed under the sea ice, the scientists then discovered that there are not only small inhabitants: They found large squid (Gonatus fabricii) and sardines (Benthosema glaciale) much further north than previously known.
Ice fishing surprise
But the biggest surprise came when fishing with long lines at holes in the ice: the scientists caught four large fish. They were amazed to find that three of them were Atlantic cod. This species, known as a food fish, was not expected so far north, and certainly not as an inshore fish in a deep sea basin more than 500 kilometers from any coast, the scientists point out.
In order to find out more about the fish, tissue samples were later subjected to various analysis methods. Genetic comparisons revealed that the Atlantic cod caught on the MOSAiC expedition in the Central Arctic came from Norwegian spawning grounds. Specific isotope signatures in the body tissue of one of the specimens revealed that it spent up to six years in arctic water temperatures of minus one to two degrees Celsius. “Even if the Atlantic cod does not have its own central arctic stock, this study shows that it can live there. A small number of individuals appear to be able to find enough food to survive for long periods of time,” says lead author Pauline Snoeijs Leijonmalm from Stockholm University.
A new link in the food chain of the Central Arctic ecosystem has now become known – the level of the large predatory fish and squid, the scientists say. As they explain, in addition to the smaller fish, the continuous immigration of larger Atlantic fish also contributes to the food base of the mammals there. “The availability of small and even some larger fish in the Atlantic water layer could explain why seals, walruses and polar bears can be found even at the North Pole. Both fish and mammals are very rare, but they are there,” says co-author Hauke Flores from the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research in Bremerhaven.
No usable fish stocks
When it comes to cod, the fishing industry naturally pricks its ears. However, the researchers emphasize that the Arctic Ocean does not offer an alternative to the stressed fishing grounds in the North Atlantic. “This was to be expected given that the central Arctic Ocean has very low nutrient concentrations and very low biological productivity. Even if more Atlantic fish and their prey were introduced with the water inflow from the Atlantic in the future, the capacity of the Central Arctic ecosystem to support larger fish stocks is undoubtedly very limited,” says Snoeijs Leijonmalm. According to her, the opposite of use is therefore necessary: It is of great importance that this sensitive but fully functional ecosystem, like Antarctica, receives robust international protection.
In this regard, for a change, there are encouraging developments to report. “Typically, the exploitation of newly accessible natural resources precedes scientific research and the establishment of sustainable management, and internationally shared fish stocks at sea are particularly vulnerable to overfishing,” says Snoeijs Leijonmalm. However, in line with the precautionary principle, a multinational agreement came into force on June 25, 2021: “It prevents all commercial fishing for at least 16 years and puts science first by providing scientific assessments of the status and distribution of possible fish stocks in the central Arctic Ocean and the ecosystem that supports it – a wise political decision and a good start on the way to comprehensive protection,” says Snoeijs Leijonmalm.
Source: Stockholm University, Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research, specialist article: Science Advances, doi: 10.1126/sciadv.abj7536