Bats are a concern for proposed offshore wind energy development due to significant bat mortality occurring on terrestrial wind farms and documented observations of migratory bats offshore. This concern is further amplified by our limited understanding of bat populations, species distributions, migration behavior, and habitat use, particularly in the offshore environment. We sought to establish baseline information about bat migration along the California coast and offshore rocks and islands, creating the first systematic study on the West Coast. We established 20 sites, 10 coastal onshore sites and 10 sites on offshore rocks and islands spanning nearly the entire California coastline where we deployed acoustic bat detectors. We processed, auto-classified, and manually vetted the data to confirm species presence and activity. Both migratory and resident species are active in coastal and offshore environments throughout the year, with peaks in activity during spring and fall. Activity at offshore and island sites is significantly lower than coastal sites and northern sites have substantially higher activity than southern sites for most seasons and species. This research demonstrates coastal and offshore environments are frequently used by bats, which has implications for management of offshore wind energy development.

Bethany Schulze is the field and analysis lead for the Pacific Offshore Bat Project for U.S. Geological Survey - Western Ecological Research Center, along with a small team of dedicated bat ecologists. Her team’s research aims to answer questions about bat migration and roosting ecology of bats in the Western US. Bethany began studying bats in 2016, transforming her childhood curiosity of bats and ecology into passion for a career in research. She recently completed a master’s thesis on hoary bat and western red bat foraging activity across urban-natural habitat gradients through California State University, Monterey Bay.