Spatio-temporal variability in prey harvest and reproductive ecology of a piscivorous seabird, Cerorhinca monocerata, in an upwelling system

Ocean climate affects the life history and demography of top marine predators through changes in local prey availability. In the California Current System, abundance and distribution of mid trophic-level forage fish may be affected by seasonal and interannual variability in upwelling. We tested the hypothesis that upwelling influences forage fish availability and response of a seabird, but that the effects differ spatially within a region. We examined the availability of multiple forage species and the reproductive ecology of rhinoceros auklets Cerorhinca monocerata relative to ocean climate over 11 yr (1993 to 2003) for 2 colonies off central California: one near the shelf break and another at the coastline. The upwelling index that we used increased through time, while sea surface temperature (SST) generally decreased. Abundance indices of juvenile rockfish Sebastes spp. fluctuated, while northern anchovy Engraulis mordax decreased. Diet of the auklet reflected availability of prey in the environment. Auklet reproduction was affected by both marine climate and prey availability. Seabird breeding as well as harvest of anchovy, rockfish and Pacific saury Cololabis saira were linked to offspring growth, but growth was not necessarily related to offspring survival. Offspring survival was inversely correlated with SST and positively correlated with mass of prey rather than diet composition. Auklet reproduction was more variable offshore than inshore, possibly reflecting variation in the upwelling cells affecting prey availability within the foraging range of birds from each colony.