Persistence of hotspots and variability of seabird species richness and abundance in the southern California Current

Aggregations of seabirds at sea may provide information on centers of enhanced trophic interactions and concentrating mechanisms, however, to date most studies lack quantification of persistence, a key hotspot characteristic. Persistence statistics may reduce uncertainty about seabird habitat use, improve understanding of the spatio-temporal scales of pelagic food web dynamics, and inform conservation planning. Using 26 years (1987-2012, 47 surveys) of shipboard surveys from a 300K km 2 study area within the southern California Current Ecosystem, we conduct a spatial assessment of the inter-annual and seasonal dynamics of the persistence of seabird hotspots and identify recurring sites of elevated seabird species richness and abundance. Previous studies document declines in abundance, but were based on broad spatial standardizations to assess where declines may have occurred. Here, we refine the hypothesis that seabird populations have declined off southern California by focusing on persistently used habitats in nearshore or offshore domains. We demonstrate that spatio-temporal variability of seabird distribution and abundance is characterized by anomalous events embedded within trends. In addition to identifying the locations of persistence of seabird aggregations, we found significant declines in species richness and the density of sooty shearwater (Puffinus griseus) and Leach's storm petrel (Oceanodroma leucorhoa); in contrast, black-footed albatross (Phoebastria nigripes) abundance appear to be increasing. This assessment provides a spatially-explicit framework for future evaluations of biophysical drivers of seabird hotspots and their associations and impacts on forage fish and zooplankton populations.