Beneath the ice, fish have been shielded by the darkness. What happens now that the ice is melting?

Under the Arctic sea ice, fish and plankton live in complete darkness, even in the middle of summer. Ice floes stop the sun's rays, especially if they are covered by snow. As the ocean heats up, the sea ice melts and new regions are exposed.

In a new study, researchers warn that climate change will create trouble for cold-water fish species in this century. Paired with higher temperatures, changes in light conditions will make less food accessible when the fish need it the most.

After 2060, polar cod may have difficulties finding suitable living spaces in the Barents Sea, as well as in the northern parts of the Bering and Chukchi Seas. Other species will find untouched feeding grounds in regions they have so far not grazed in.

Light controls growth, and as so often in nature, life comes down to a simple balance: to see or be seen, to eat or be eaten.

Larger feeding grounds where the ice disappears

“The cards are being dealt again,” says Øystein Varpe.

He is a professor of marine ecology at the University of Bergen's Bjerknes Centre and the Department of Biological Sciences. There he studies how variations in light affect life in the ocean, from day to night, from summer to winter, and when the climate changes.

In the new study, led by Trond Kristiansen at the Fallaron Institute in California, the research team has considered the future of fish in a warmer ocean with less sea ice in the Arctic. 

The researchers have used climate models to simulate how decreasing sea ice will affect light conditions in the northern parts of the Bering, Chukchi, and Barents Seas. 

Historically, these regions have been covered by sea ice well into the summer, such as late June, when the sun is at its highest. When the ice disappears, the sun penetrates far down into the ocean around the clock.

The model simulations indicate that the amount of light will be 75 to 160 per cent higher by the year 2100 compared to the period 1980–2000.

Under the ice, fish and other creatures have been protected by darkness. Now there is light. For some, this increases the risk of being eaten; for others, the chance of finding something to eat.

Feasts are served at the wrong time

The spring bloom is the plants' annual gala performance, in the ocean as on land. Light returns after winter and sets off an explosion of growth. Phytoplankton are eaten by zooplankton, which are eaten by small fish, which are eaten by bigger fish, which are eaten by even bigger fish or seals.

But what happens when the light is switched on earlier than before?

During winter, a fish like the polar cod may rest while waiting for spring and food. It hardly grazes, has antifreeze chemicals in its blood, and can endure long, dark winters as long as they are cold enough.

"When the temperature of the water increases, the metabolism of fish increases," Trond Kristiansen writes in an email.

Because its machinery runs faster in warmer water, the fish requires more energy. Meanwhile, the serving hours shift. When the winter ice melts earlier, the spring bloom also comes earlier. 

In summer, when the fish needs the most food, there will be less to find.

Trond Kristiansen is particularly worried about the fish's access to food in July and August, when temperatures rise drastically and the availability of food decreases. In the Barents Sea it will be much harder for a polar cod to survive the first year after hatching. Fewer fish will gain enough weight to make it through the following winter.

Kristiansen emphasises that the development will vary from region to region. The polar cod in the Barents Sea is particularly vulnerable, while smaller changes are expected in the Chukchi Sea.

Moving is no simple solution

Could the polar cod just move? Swim northwards into regions still covered by sheltering ice? Steer toward the eternal darkness in the abyss of the North Pole?

"We do not expect the Arctic Ocean to become particularly productive. There may not be enough nutrients, and therefore no basis for large populations of fish," says Øystein Varpe.

One reason why fish thrive in the Barents Sea is that it is shallow enough for light to reach all the way to the sea floor. 

In other places, copepods may dive to a thousand metres below the surface and wait in hiding until the next spring. In the Barents Sea, such organisms hit the bottom after a couple of hundred metres. When the light returns, a copepod will be visible to any hungry cod or herring.

The Arctic Ocean is deep and offers fewer items on the menu. The limited food supply sets a boundary for how far north polar cod and other Arctic fish species may venture.

An advantage for one species may be a disadvantage for another.

Some still go

Fish that normally live farther south are now being observed in the Arctic. In Svalbard, species preferring warmer waters have become more common, while the local polar species are thinning out. 

Atlantic cod is taking over; polar cod is becoming rarer.

In regions that used to be covered by ice, species from the south now find food more easily. But the Arctic has characteristics even climate change will not alter.

"The polar night in the Arctic will be very dark, also when the ice is gone," says Varpe.

A strong swimmer like the mackerel can visit the Arctic in summer and withdraw south before winter comes. But if it or other species were to stay all year, they would have to tolerate unfamiliar surroundings: floodlighted waters around the clock in summer and pitch black darkness throughout the winter.

"In the Barents Sea, species dwelling near the bottom – like cod, haddock, and flounder – seem to move northwards," says Varpe.

These fish live different lives than mobile herring or mackerel. When there is no more plankton to eat, the mackerel may swim hundreds of kilometres south and simply live there instead.

Polar cod is a key species, Trond Kristiansen explains, as food for birds and whales. 

An important question is whether capelin or other fish species can move north and take over the polar cod's role as a link between species that are low and high in the food chain.

Neither Varpe nor Kristiansen is willing to predict whether there will be more or less fish overall in the northern regions in the years to come.

References:

Kristiansen et al. Climate change impacts on ocean light in Arctic ecosystems, Nature Communications, 2025. DOI: 10.1038/s41467-025-64790-4

Varpe et al. A fish-eye view on the new Arctic lightscape, ICES Journal of Marine Science, 2025. DOI: 10.1093/icesjms/fsv129

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