MEPS 392:295-305 (2009)  -  doi:10.3354/meps08152

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

Kelly M. Newton1,*, Donald A. Croll1, Hannahrose M. Nevins2, Scott R. Benson3, James T. Harvey2, Bernie R. Tershy1

1Center for Ocean Health, University of California Santa Cruz, 100 Shaffer Rd. Santa Cruz, California 95060, USA
2Moss Landing Marine Laboratories, 8272 Moss Landing Rd., Moss Landing, California 95039, USA
3NOAA Fisheries, 7544 Sandholdt Rd., Moss Landing, California 95039, USA

ABSTRACT: Although seabird population biology is relatively well studied, little data exists on mortality at-sea. Beached bird surveys are used to identify patterns of seabird mortality, but resulting patterns 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 relatively 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 anthropogenic 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 mortality of common murres Uria aalge cannot be explained by the derived principal components, relative 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.


KEY WORDS: Seabirds at-sea · Beached bird survey · Density · Mortality · Sooty shearwater · Common murre


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Cite this article as: Newton KM, Croll DA, Nevins HM, Benson SR, Harvey JT, Tershy BR (2009) At-sea mortality of seabirds based on beachcast and offshore surveys. Mar Ecol Prog Ser 392:295-305

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