Cooperative foraging between dolphins and fish-eating killer whales

Interspecies associations during foraging can range from competition to cooperation but are often complex and difficult to interpret. Using aerial drone and biologgers (n=9) equipped with video, acoustic and inertial sensors, we recorded interactions between fish-eating northern resident killer whales (Orcinus orca) and Pacific white-sided dolphins (Lagenorhynchus obliquidens) in the presence of adult Chinook salmon (Oncorhynchus tshawytscha). We observed frequent co-occurrence and coordinated movements, with the killer whales orienting towards the dolphins and following them to depth. We also recorded reduced echolocation and rolling movements by the killer whales in the presence of dolphins, suggesting that the whales may eavesdrop on dolphin echolocations to scan broader areas to locate large Chinook salmon-prey that are too big for the dolphins to capture and swallow whole. Captured fish were brought to the surface by the killer whales and broken apart for sharing with other matrilineally related pod members-while the accompanying dolphins scavenged scraps. No antagonistic interactions or avoidance behaviours were observed between the killer whales and dolphins. The spatially and temporally coordinated foraging behaviours we recorded suggest northern resident killer whales and dolphins opportunistically engage in cooperative foraging, which may improve the ability of killer whales to detect Chinook, while dolphins benefit by scavenging prey scraps-highlighting the need for further investigation into the ecological implications of these interspecific encounters.