Rhinoceros Auklets (Cerorhinca monocerata) were studied at Triangle Island, Cleland Island, and Seabird Rocks, British Columbia, in 1986-1989. Epipelagic schooling fish were consistently the most common prey delivered to auklet chicks at all three localities. Pacific sand lance (Ammodytes hexapterus), Pacific herring (Clupea harengus), juvenile salmon (Onchorhyncus spp.), Pacific saury (Cololabis saura), and juvenile rockfish (Sebastes spp.) were important prey. Bite marks on the fish showed that 73% were attacked from below. The mean deepest depth recorded for 16 auklets was 30 m (range 12-60 m). Eleven time-at-depth records showed that the auklets were epipelagic foragers: 90% of the mean underwater time was spent in the upper 10 m, although most birds had a few deeper dives of 20-60 m. Shipboard transects showed that Rhinoceros Auklets usually foraged in water considerably deeper than 15 m. A model of diving efficiency indicated that a relatively large proportion (>40%) of the average dive cycle was spent foraging rather than travelling or resting, and prolonged dives invoking anaerobic glycolysis were avoided. We discuss the implications of diving limitations on foraging behaviour and the use of Rhinoceros Auklets as indicators of prey availability.
