Surface and underwater observations of cooperatively feeding killer whales in northern Norway

Abstract

Killer whales (Orcinus orca) feeding on herring (Clupea harengus) were observed both from the surface and underwater. We refer to one of the feeding techniques used by killer whales as the carousel method, whereby whales cooperatively herded herring into a tight ball close to the surface. During herding and feeding, whales swam around and under a school of herring, performing much lobtailing and porpoising. When the herring were gathered into a tight ball whales often swam with the white underside of their body towards the fish and emitted large bubbles close to the surface. While feeding, whales spent more time circling around the ball of fish than eating. Ball formation is a known defence mechanism used by schooling fish, and the effort by killer whales appeared to be directed towards keeping the ball very dense and close to the surface. The whales stunned their prey by slapping the edge of the school with the underside of their flukes and then ate the stunned fish one by one. The tail slaps created a loud banging sound which could have been either a by-product or an aid to stunning the prey.