Stock composition of juvenile salmon taken by an abundant, pursuit-diving seabird in the coastal waters of British Columbia, Canada

We determined the stock composition of juvenile salmon taken by a pursuit-diving seabird, the Rhinoceros Auklet ( Cerorhinca monocerata), on five breeding colonies in British Columbia in 2012–2014. Our goal was to test the hypothesis that these birds take salmon in proportion to their abundance, with specific focus on first, the biennial cycle of pink salmon; and second, the enormous out-migration of juvenile sockeye from BC's Fraser River in 2012. We obtained 772 meals in 11 colony-years of sampling, of which 198 (26%) included one or more salmon, and determined stock composition for 126 pink, 101 sockeye, and 113 chum. As expected, the biennial cycle of pink salmon was evident in the birds’ diets: individuals from Puget Sound were common on colonies in 2012 and 2014 but absent in 2013 and replaced by pinks from northerly stocks. Also as expected, Fraser River sockeye were unusually abundant in the birds’ diets in 2012. Our results show that these seabirds can respond to pulses of increased abundance of juvenile salmon, although they are not usually primary prey types.