Prey-induced behavioural plasticity of herring-eating killer whales

Abstract

Behavioural plasticity of top marine predators to adapt to seasonal changes in the behaviour of their prey is little understood. Here, we used data from 16 multi-sensor archival tags attached to killer whales in 2005, 2006 and 2009 to evaluate whether and how their feeding behaviour varied according to seasonal life-stages (overwintering, feeding and spawning) of their herring prey. Feeding periods with tail slaps were consistently characterized by non-directional movement with the production of calls and echolocation clicks, but angular deviation in heading during feeding periods was lower during overwintering than spawning prey life-stages. Tail slaps were produced more often at depths >10 m in the feeding and spawning prey life-stages, while during overwintering, they often occurred at <10 m depth, indicating the depth of feeding changed with predicted positions of prey in the water column. Echolocation clicks were produced less often during feeding upon overwintering herring, suggesting that visual cues may be used more in winter during daytime due to better underwater visibility. Thus, while maintaining an encircling strategy to exploit the year-round anti-predator schooling behaviour of their herring prey, observed variations in details of killer whale feeding behaviour across prey life-stages are likely driven by the size and depth of herring schools, and the cost-benefit balance of bringing herring schools to the surface in different environmental contexts. However, observed variations do not appear strong enough to indicate reciprocal phenotypic plasticity by the herring prey which seem to rely primarily upon general anti-predator strategies.