At-sea mortality of seabirds based on beachcast and offshore surveys

Although seabird population biology is relatively well studied, little data exists on mor- tality at-sea. Beached bird surveys are used to identify patterns of seabird mortality, but resulting pat- terns are difficult to interpret without corresponding data on at-sea density. We examined seabird mortality relative to at-sea density in Monterey Bay, California over 10 yr by integrating data from monthly beachcast seabird and offshore seabird censuses. Beachcast seabird numbers were rela- tively constant (mean 2.82 ± 0.31 seabirds km -1 ) throughout the year. At-sea seabird density (mean 148.9 ± 16.12 seabirds km -2 ) peaked in the summer upwelling period and was least in the winter Davidson period. A principal components analysis of seasonal climatic, prey availability, and anthro- pogenic variables for Monterey Bay derived 3 significant principal components (PCs) (explaining 70% of variance) characterized by storm activity and low prey availability (PC1), river discharge and krill abundance (PC2), and oiling (PC3). These principal components were used in detailed analyses of the 2 most abundant seabird species and indicate that sooty shearwater Puffinus griseus relative mortality is greatest during decreased productivity and increased storm activity. While relative mor- tality of common murres Uria aalge cannot be explained by the derived principal components, rela- tive mortality increased in late winter when prey availability decreased concurrent with the annual increase in reproductive stress. Beachcast seabird data is difficult to interpret in isolation; however, when linked to at-sea densities of seabirds, it becomes a powerful tool to examine the relative impacts of oceanography and direct human perturbations on seabird demography.