Biogeography of Arctic cod in a changing Arctic Ocean

Arctic Ocean faces extreme seasonality. During wintertime, light is limited, and the ocean is covered with ice. It is therefore extremely difficult to access the Arctic Ocean to study marine life. Animals cope with the extreme seasonality by developing life strategies, such as releasing lipid-rich eggs in winter under the ice where soon-to-hatch larvae will be protected from predators. This is the strategy of Arctic cod (Boreogadus saida), the most abundant fish in the Arctic and the main food source for seabirds, seals, and whales. Arctic cod larvae dwell under the ice cover until the end of summer, when the ice melts. The ability of larvae to efficiently grow under the ice at sub-zero temperatures until they transform into juvenile before the winter onset is critical to their survival. Transforming into juveniles is important because they have all swimming fins developed and improved abilities to catch prey and avoid predators, thus better facing harsh winter conditions.

Mathematical model can help us understand what happens with these animals under the sea ice and how they cope with winter conditions and climate change. Using a bioenergetic model that simulates larval Arctic cod growth, we were able to estimate how many fish successfully reach the juvenile stage by the end of summer in different Arctic regions. Realistic environmental conditions during early life development are simulated with an ocean and sea-ice model, provided through collaboration with researchers at Woods Hole Oceanographic Institution, US. Fish larvae are virtually dispersing with currents under the ice and develop along their drift. The model results showed us that in a first stage increasing temperature and early sea ice melt, will speed up development of Arctic cod early life stages and will lead to an increasing number of juveniles. In more southern regions, however, such as Svalbard, on-going increase in temperature will soon be detrimental to Arctic cod because the temperature will overpass their optimum for growth. Under such climate change trends, decline of Arctic cod population is imminent.