Killer whale abundance and predicted narwhal consumption in the Canadian Arctic.

Range expansions and increases in the frequency of killer whale (Orcinus orca) sightings have been documented in the eastern Canadian Arctic, presumably the result of climate change-related sea-ice declines. However, the effects of increased predator occurrence on this marine ecosystem remain largely unknown. We explore the consequences of climate change-related range expansions by a top predator by estimating killer whale abundance and their possible consumptive effects on narwhal (Monodon monoceros) in the Canadian Arctic. Individual killer whales can be identified using characteristics such as acquired scars and variation in the shape and size of their dorsal fins. Capture-mark-recapture analysis of 63 individually identifiable killer whales photographed between 2009 and 2018 suggests a population size of 163 +/- 27. This number of killer whales could consume >1,000 narwhal during their seasonal residency in Arctic waters. The effects of such mortality at the ecosystem level are uncertain, but trophic cascades caused by top predators, including killer whales, have been documented elsewhere. These findings illustrate the magnitude of ecosystem-level modifications that can occur with climate change-related shifts in predator distributions.