Cod in Arctic Have Multiple Predators; Will Climate Change Alter Food Chain?

What do ringed seals, beluga whales, narwhals and seabirds living in the Arctic all have in common? They all prefer to dine on pacific cod that thrive in frigid habitats underneath sheets of sea ice, and with climate change rapidly melting the ice, competition for this seafood delicacy may increase among these predators.

Using a new, specialized net, a team of researchers recently collected polar cod (Boreogadus saida) from their icy homes in order to better understand the fishes' large-scale distribution and origin. It turns out that only juvenile cod live under the ice, according to a study recently published in the journal Polar Biology.  

"For the first time, we've been able to use a special net directly below the sea ice to catch a large number of polar cod, and therefore to estimate their prevalence over a large area," Carmen David, a biologist from Alfred Wegener Institute (AWI) and first author of the study, said in a news release.  "If you extrapolate these findings, there could be more than nine billion polar cod living under the ice in the Eastern Arctic. What's more, we have also collected fundamental biological and physical data."

The fact that so many large marine predators depend on polar cod as a food source makes them one of the Arctic Ocean's most ecologically important animals but researchers were unsure how abundant polar cod populations actually are. That's where researchers from AWI, the Helmholtz Centre for Polar and Marine Research, the Universität Hamburg and the Dutch research institute IMARES entered the picture.

Using the research icebreaker ship Polarstern, researchers gathered valuable information during an Arctic expedition in the summer of 2012. For their study, a specialized under-ice trawl net, also known as SUIT (Surface and Under Ice Trawl), was dragged next to the ship at 13 locations between Greenland, Svalbard and Russia. The net, designed by IMARES, has a large frame that quickly sinks below the sea ice when it's cast. Then, floats propel the net toward the water's surface and polar cod living directly beneath the ice are swept up. This net is also equipped with a camera and equipment for measuring ice thickness, temperature and salinity.

"Up until our expedition, catches had only been made at specific points and observations made on individual polar cod caught beneath the ice by divers," David explained in the release. "Now we know: there are mainly one- or two-year-old juvenile fish living directly below the ice, and these feed on, among other things, amphipod crustaceans. Since some of the polar cod live in overhangs and cracks under the ice, it's likely that we didn't manage to catch all of them with our net -- which means that the polar cod population beneath the ice may be even bigger than our figures suggest."

The researchers then aimed to find out where the young polar cod came from. For this they used satellite data and computer model that allowed them to retrace the slow movement of the drifting sea ice. They concluded it forms in the coastal waters of the Laptev and Kara Seas in northern Siberia and is later blown northwards into open waters. Since polar cod often spawn under drifting ice, it is assumed that juvenile fish travel northwards as the ice moves. Therefore, the fish caught in the west would have come from the Kara Sea, while those caught in the east are more likely to have come from the Laptev Sea.

"We analyzed the satellite data to determine how far the ice in that particular area has travelled," Hauke Flores, co-author of the study, said in a statement. "It took the ice between 240 and 340 days to travel from the coast to our measurement stations in the sea. These figures correspond with the age and size of the juvenile polar cod that we caught."

Researchers also concluded that the fish were very well-fed and in top conditions after having been born in these sea ice nursery grounds. Teh point of the study is to help scientists assess how polar cod populations and the animals that feed on them may be affected by future climate change.

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