Using the ice net SUIT (Surface and Under Ice Trawl) which was used before in Antarctica, polar cod was caught for the first time underneath the Arctic sea ice, allowing the determination of large-scale distribution and origin. This information is of fundamental importance, as polar cod are a major source of food for seals, whales and seabirds in the Arctic.
This study was recently published in the journal Polar Biology. Carmen David from the Alfred Wegener Institute (AWI) shows the ice net SUIT (Photo Sina Löschke). With this gear, the top layer of the water under the ice can be examined. AWI is partner of IMARES in the research project Iceflux-NL.
Collecting data about Polar cod
Beluga whales, narwhals, ringed seals and numerous Arctic seabirds have one thing in common: their preferred food is polar cod, Boreogadus saida. As such, the fish is one of the Arctic Ocean's most ecologically important animals. Despite its importance, there are still gaps in our understanding of it. For example, biologists have known for years that juvenile polar cod live under the sea ice. However, to date it was not known just how many live there. "For the first time, we've been able to catch a large number of polar cod using the SUIT, and therefore to estimate their prevalence over a large area. 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," explains Carmen David.
The data was gathered in summer 2012, during an Arctic expedition on board the research icebreaker Polarstern. "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”, says David. “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”.
Juvinile fish travels along with drifting ice
It has long been surmised that the young fish reach the Central Arctic from their spawning grounds under drifting ice. These spawning grounds are found in the coastal waters of the Laptev and Kara Seas in northern Siberia. In autumn new sea ice forms there, and is gradually pushed northwards into open waters by the wind. The juvenile fish are assumed to travel along under the ice. "We analysed satellite data to determine how far the ice in that particular area has travelled," says Hauke Flores, former IMARES colleague and current Iceflux group leader at AWI. "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." Tissue analysis performed in the laboratory showed that the fish were in top condition, which suggests that there was enough food under the ice, making the sea ice a true nursery ground for polar cod.
Climate change and juvinile fish
The new insights into the juvenile fish under the ice are important because it's still impossible to say how polar cod populations will change in the face of climate change. The largest and most important population lives in the Barents Sea, to the north of Norway. As climate change causes the Barents Sea to grow warmer, for some years now other fish species like capelin (Mallotus villosus) and Atlantic cod (Gadus morhua) have moved further northward, creating new competition that could reduce the polar cod population. In fact, Norwegian researchers recently confirmed that, in a regularly monitored fjord on the island Spitsbergen, for the first time ever there were no polar cod found - but plenty of Atlantic cod.
If the polar cod population in the Barents Sea actually does shrink, the juvenile fish under the ice of the Eastern Arctic could become even more important - especially in order to make up for losses elsewhere. "We want to determine whether or not the young fish under the ice serve as a form of 'polar cod reserve', increasing the overall chances of survival for the coastal populations through genetic exchanges with populations in Siberia and elsewhere," explains Flores.
Carmen David, Benjamin Lange, Thomas Krumpen, Fokje Schaafsma, Jan Andries van Franeker, Hauke Flores (2015). Under-ice distribution of polar cod Boreogadus saida in the central Arctic Ocean and their association with sea-ice habitat properties. Polar Biology. DOI 10.1007/s00300-015-1774-0
Source: Billions of juvenile fish under the Arctic sea ice - AWI