Under the ice: the Arctic cod in a rapidly changing ocean

In the polar regions, there are big differences between summer and winter. Temperature, sea ice cover, salinity, food: everything fluctuates a great deal. "Many species are very well adapted to these fluctuations," says Schaafsma. "But that’s precisely why they are vulnerable if the sea ice disappears due to climate change. What will happen to the ecosystem and the food chain when the ice has gone?"

That is what Schaafsma and her colleagues are investigating in several research projects. This often involves collaborating with German partners at the Alfred Wegener Institute. "We often accompany them on their ship, the Polarstern. Recent projects have been focusing on fish in the Arctic Ocean beyond the continental shelf, so in deep water."

In the Arctic ecosystem, the Arctic cod is a cornerstone of the food web. Schaafsma: "Many animals depend on it. We know that they spawn close to shore and that the young fish like to hide under the sea ice. Some of the young fish float with the ice into the central ocean. Others remain on the continental shelf. These fish move to the bottom as they get older."

How the Arctic cod is managing is still largely a mystery. Reason: research under the ice is difficult. "We use a lot of hydroacoustics, but that method often misses the upper water layer. Echo sounders hang under the ship's keel at about 10 metres deep and don’t pick up fish near the surface. Moreover, the echo sounder identifies signals of swimming fish, but we aren’t sure which fish species they are."

The researchers therefore use the SUIT net, an innovative fishing net that allows fishing just below the ice (see box). Although it can catch quite a lot of cod, Schaafsma does not feel that the net is ideal because it was actually designed for smaller fish and zooplankton. During the MOSAiC expedition, when the Polarstern spent a year floating in the ice, the Wageningen researchers therefore experimented with under-ice pots and line fishing (longlines). Together with the SUIT net, the methods provided fish samples from the 'blind upper layer' under the ice for further analysis.

The SUIT (Surface and Under-Ice Trawl) net is a trawl specifically designed for the upper metres of water directly under the sea ice. Hydroacoustics don’t 'see' that shallow layer, while that’s where ice algae, plankton and many fish, including juvenile polar cod, are. A wide floating frame slides along the bottom of an ice floe and there are measuring instruments in the net that measure temperature, salinity and light, for example. The SUIT net is used for research in the Arctic and Antarctic.

Climate change produces losers and winners: species that thrive in cold conditions are expected to face increasingly tough conditions. Does that include Arctic cod? According to Schaafsma, this is possible: “Arctic cod eggs do not develop well above 2 degrees Celsius; they need cold. Furthermore, we know that young cod in the central Arctic Ocean like to feed on food close to the ice. But they have quite a wide diet and eat whatever is easiest to grab. That means that they may adapt as the food web changes."

Disappearing ice means more light and warmth underwater, and therefore likely competition from species advancing northwards. A northward expansion of habitat has already been observed for several species, such as capelin, haddock and Atlantic cod. "The Arctic cod loses its advantage as a result. We are studying what that means for birds and marine mammals that eat Arctic cod."

Understanding the changing ecosystem is also important because the melting ice brings opportunities for commercial fishing, raw material extraction and other human activities. "If you want something there, you need knowledge of the changing ecosystem to then manage it responsibly. This is exactly why countries in the Central Arctic Ocean Fisheries Agreement agreed to do research first and not start commercial fishing until enough knowledge was gathered."

"This is an ecosystem with great intrinsic value," she concludes. "As a marine biologist, I think we need to manage it carefully."

• Alfred Wegener Institut
• Dutch Research Council (NWO)
• Ministry of Agriculture, Fisheries, Food Security and Nature
• European Commission