As prey changes, how bowhead whales will adapt
The bowhead whale (Balaena mysticetus) or Arvik holds a cultural, spiritual and subsistence importance for Canadian Inuit and has been a key component of Arctic indigenous peoples for hundreds of years. But more recently, the disproportional warming of the Arctic from climate change has resulted in the reduction of the sea ice cover, the increased presence of killer whales, and large-scale changes in zooplankton composition and abundance that all make us question the ability of bowhead whales to continue thriving in the Arctic.
Bowhead whales are well adapted to survive in the harsh Arctic marine environment with their large head that they use to break the sea ice to breathe and their thick layer of blubber that keeps them well insulated. They use their long baleen to filter zooplankton in the water column, tiny organisms rich in fat on which they feed exclusively. They rely on huge quantities of zooplankton to meet their large energetic requirements. But the shifts in abundance and composition of zooplankton resulting from changing marine conditions could prevent bowhead whales from meeting those requirements.
One way that they can cope with changes in prey is by having diet variation within the population: different individuals having different diets by feeding on distinct prey. In that way, if a certain prey changes its abundance or distribution in response to climate change, only a small portion of bowhead whales individuals will be negatively affected by the change and the impact at the population level will be dampened.
To study the inter-individual diet variation in bowhead whales, I used diet and movement data from the Eastern Canada – West Greenland (ECWG) bowhead whale population that were collected by Fisheries and Oceans Canada (DFO) in collaboration with local Inuit in Igloolik and Pangnirtung between 2012 and 2016. I followed the whales in their horizontal movements between regions and vertical movements underwater as they dive to feed with satellite and time-depth recorder tags to understand their feeding behaviour. I also investigated the diet variation between the individuals by measuring different types of dietary biomarkers (i.e., stable isotopes of nitrogen and carbon, and fatty acids) from the biopsy samples of skin and blubber collected from the same tagged whales.
Results show that differences in diet were associated with differences in where and how the individuals acquire their prey during the summer and fall, which is the period when most of their feeding occurs during the year. The individuals showed important variation in diet and behaviour but grouped together in a way that the whales tagged in one region all had similar diets and feeding behaviours but differed from the whales tagged in another region. Foxe Basin whales fed mostly on mysids, euphausiids and chaetognaths, had shallower feeding dives, travelled long distances between regions and increased their dive depth seasonally. On the other hand, Cumberland Sound whales fed mainly on calanoid copepods, had deeper feeding dives, stayed stationary in the region and did not increase their dive depth seasonally.
Overall, results show some group specialization where individuals frequenting distinct summer and fall feeding habitats display diet and behavioural differences. This could be a sign that a subpopulation structure exists within the ECWG population. We cannot conclude for sure that the feeding behaviour differences observed in my study are driving the diet variation at the population level, but we can say that the variation in both diet and feeding behaviour among individuals will likely allow the population to cope with current and future changes in prey.
Tommy Pontbriand is an M.Sc. student at the University of Manitoba. email@example.com