The rise of killer whales as a major arctic predator

Abstract

Anecdotal evidence, sighting reports, Inuit traditional knowledge, and photographic identification indicate that killer whale (Orcinus orca) occurrence in Hudson Bay is increasing. Killer whales were not known to be present in the region prior to the mid-1900s but have since shown an exponential increase in sightings. More sightings from Foxe Basin, Nunavut in the north to Churchill, Manitoba in the south appear to be related to a decrease in summer sea ice in Hudson Strait. Killer whale activity during the open water season has been concentrated in the northwest Hudson Bay region that includes the Repulse Bay and northern Foxe Basin areas. Here, prey items are diverse and abundant. Killer whales are reported in western Hudson Bay on an annual basis with sighting reports and anecdotal evidence suggesting they are first observed heading through Hudson Strait in July and returning to the northwest Atlantic in September. However, arrival, occupancy, and departure times are likely related to yearly ice conditions and prey availability. Killer whales have been observed preying on a number of marine mammal species in Hudson Bay. Of particular concern is predation on bowhead whales in Foxe Basin, narwhal in northwest Hudson Bay, and beluga in southwest Hudson Bay. The impact of killer whale predation on marine mammal species is unknown without long-term studies and direct observation of killer whale hunting behaviour. However, by defining population energetic requirement and considering population demography of prey, we can begin to assess the basic requirements of predator-prey dynamics in Hudson Bay marine ecosystem. To estimate predation impact we used a simple mass-balanced marine mammal model that includes age structure, population size, and predation rate inputs. Estimates of killer whale population size were variable, with the majority of information suggesting that at least 25 whales use the area each summer. Results suggest that the Northern Hudson Bay narwhal population may be negatively impacted by continued killer whale predation. We conclude that conservation of marine mammals in Hudson Bay should consider killer whale effects since they have the potential to regulate population growth of prey populations. © 2010 Springer Netherlands.