Although predators influence behavior of prey, analyses of electronic tracking data in marine environments rarely consider how predators affect the behavior of tracked animals. We collected an unprecedented dataset by synchronously tracking predator (killer whales, N = 1; representing a family group) and prey (narwhal, N = 7) via satellite telemetry in Admiralty Inlet, a large fjord in the Eastern Canadian Arctic. Analyzing the movement data with a switching-state space model and a series of mixed effects models, we show that the presence of killer whales strongly alters the behavior and distribution of narwhal. When killer whales were present (within about 100 km), narwhal moved closer to shore, where they were presumably less vulnerable. Under predation threat, narwhal movement patterns were more likely to be transiting, whereas in the absence of threat, more likely resident. Effects extended beyond discrete predatory events and persisted steadily for 10 d, the duration that killer whales remained in Admiralty Inlet. Our findings have two key consequences. First, given current reductions in sea ice and increases in Arctic killer whale sightings, killer whales have the potential to reshape Arctic marine mammal distributions and behavior. Second and of more general importance, predators have the potential to strongly affect movement behavior of tracked marine animals. Understanding predator effects may be as or more important than relating movement behavior to resource distribution or bottom-up drivers traditionally included in analyses of marine animal tracking data.predator-prey dynamics | sea ice | biologging | climate change | trait-mediated effects C onsumptive effects (alternatively termed "density-mediated effects") of predators on prey refer to the mortality incurred when predators kill and consume prey during predation events. They can control prey populations and in certain circumstances, restructure ecosystems through trophic cascades (1-3). Nonconsumptive effects (also termed "trait-mediated effects") can similarly affect prey populations by altering species' behavior and space use under perceived or real predation risk, which are associated with decreased fitness through loss of access to key foraging areas, disrupted social structure, increased energy expenditure and stress imposed by persistent vigilance and escape behaviors, and decreased reproductive success (3-7). Nonconsumptive effects are sublethal (8, 9), but because they can impact many individuals in a population simultaneously, the cumulative effect may exceed consumptive effects (8,(10)(11)(12).In terrestrial systems, movement data collected by electronic telemetry tracking tags have been used to clearly show that carnivores affect prey species' use of space and habitat selection (13-15), and these nonconsumptive effects can negatively impact population dynamics (10, 13, 11). When large enough, such effects have even been suggested to lead to trophic cascades (16,17,3). However, there is disagreement about whether nonconsumptive effects ...
