Background/Question/Methods The scale and frequency of animal mass mortality events (MMEs) have increased over the last half a century, a trend that is likely to continue commensurate with increased climate variability and climatic extremes. Recent research has highlighted how changing atmospheric and ocean conditions impact upper-trophic marine species, including seabirds. Seabird MMEs are one response to a changing climate, as species contend with the direct and indirect effects of climate variability. Since 2014, seabird MMEs have occurred at least annually in the North Pacific, affecting a breadth of species and large marine ecosystems from the California Current system north to the Bering and Chukchi Seas. We use data from a citizen science program focused on beached birds (COASST), as well as information from coastal communities, the U.S. Fish and Wildlife Service, and the National Park Service, to document a shift in the frequency, magnitude, and taxonomic breadth of seabird MMEs throughout the northeast Pacific and Alaskan coastlines within the last 35 years.
Results/Conclusions Although MMEs have been a consistent feature of coastal ecosystems, a nonlinear increase in frequency and magnitude (measured as maximum monthly regional encounter rate, or carcasses/km) has been associated with the transition from predominantly cool to warm ocean conditions including the northeast Pacific marine heatwave, a weak El Niño event, and severe reductions in sea ice extent in the Bering Sea, all of which have contributed to warmer sea surface temperatures (SST) compared to climatological averages. Starvation coincident with large-scale shifts in epipelagic and nearshore ecosystem structure - and specifically shifts in prey distribution, quality, and abundance - is an obvious mechanism. However, several features of the broadscale pattern suggest synergistic effects of multiple stressors, including natural history and physiological pinchpoints (e.g., post-breeding mass minima, post-breeding migration, flight feather molt); as well as stressors collectively associated with persistent warming (e.g., harmful algal blooms, incidence of disease, intensification of ectotherm competition).