Proximate composition and energy density of forage fish delivered to Rhinoceros Auklet Cerorhinca monocerata nestlings at Triangle Island, British Columbia

Abstract

We measured the proximate composition of the four main prey species delivered by adult Rhinoceros Auklets Cerorhinca monocerata to nestlings at Triangle Island, British Columbia, Canada: Pacific sand lance Ammodytes hexapterus, Pacific saury Cololabis saira, juvenile rockfish Sebastes spp., and juvenile salmon Oncorhynchus spp. We also assessed within-species variability in energy density and the variation between two years (2003 and 2004). Both the lipid content and the protein content varied significantly among prey species, causing significant interspecific variation in energy density. Energy density was consistently high in Pacific sand lance, the main prey species at Triangle Island in successful breeding years for the auklets, but variably lower in other prey types such as Pacific saury and salmon. Energy density was lower in 2003 than in 2004 in Pacific saury and, to a lesser extent, in juvenile Pacific sand lance. Our results constitute one of the few assessments of the energetic value of forage fish important in northeast Pacific marine systems and indicate that the choice of prey species can be important for predators including seabirds.