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The Arctic cod is facing problems as the ice disappears

polartorsk Peter Leopold

Arctic cod eggs are totally dependent on the ice in order to develop successfully.

Photo: Peter Leopod (by agreement)

The Arctic cod, the Arctic cousin of the Atlantic cod, is a key species in the northern Barents Sea. The whole ecosystem may therefore be destabilised by the spawning grounds of the Arctic cod shrinking as a result of declining sea ice cover.

This bony fish, weighing barely 100 grams, is not sought-after for human consumption. But over 20 fish species, from the small Parr’s snailfish to the large Greenland shark, are more than happy to tuck into Arctic cod. As are seabirds, seals and whales.

Declining population

But now the situation is looking bad for the Arctic cod. This century, the amount of Arctic cod living in the Barents Sea has fallen from 2 million to 0.3 million tonnes. The quantity of short-lived species like the Arctic cod can vary greatly from year to year, but in this case the dramatic drop in the population is not just a case of natural fluctuations: the reduction in sea ice cover is probably also responsible. That’s because Arctic cod eggs are totally dependent on the ice in order to develop successfully.

Main spawning ground found

“We now know that the south-eastern Barents Sea – one of two spawning grounds of the Arctic cod – supplies larvae and juveniles to most of the areas where the species lives. The sea ice cover has shrunk dramatically in that area, affecting the Arctic cod throughout the Barents Sea,” explains Elena Eriksen of the Institute of Marine Research. She is one of the authors behind two new studies that have looked more closely at the status of the Arctic cod, a species that has not previously been the object of much research.

Need ice during the egg stage

Unlike the Atlantic cod, which can spawn up to 5 million eggs per year, the Arctic cod produces a relatively modest number of eggs. It compensates for this by spawning big eggs.

“The eggs of the Arctic cod have quite a thin membrane, and they are dependent on good ice conditions to protect them against damage from waves, whirlpools and ocean currents. In addition, the eggs need enough time to develop, which means that they need the ice to stay as long as possible. The temperature is also more stable below the ice, which increases their chances of survival”, explains Elena Eriksen.

The right food of the right size at the right time

But you don’t just need ice to produce an abundant year-class of Arctic cod. As soon as they hatch from the eggs, the larvae need easily-available, nutritious food. As with other newly-hatched fish larvae, that means zooplankton. Similarly, the zooplankton need the right food at the right time – they graze on the phytoplankton that bloom when the sunlight returns after the dark polar night. If the Arctic cod eggs hatch too early or too late in relation to the period with most zooplankton of the right size, they may experience a catastrophic lack of food.

Squeezed from several directions

Climate change is squeezing the Arctic cod from several directions – both directly and indirectly”, says Elena Eriksen.

“Lower sea ice cover is making its spawning grounds smaller, and the eggs, larvae and juveniles are also becoming more concentrated. The centre of gravity of the spawning grounds is moving north, which may result in the Arctic cod abandoning the Barents Sea and entering the Kara Sea. That would be bad news for all of the species in the Barents Sea that feed on Arctic cod.

Read the article here:

Eriksen, E., Huserbråten, M., Gjøsæter, H., Vikebø, F., & Albretsen, J. (2019). Polar cod egg and larval drift patterns in the Svalbard archipelago. Polar Biology, 1-14. LENKE: https://doi.org/10.1007/s00300-019-02549-6