Abstract
Since the 1970s, extensive vessel and aerial surveys have provided comprehensive data on seabird diversity, abundance and distribution across the California Current System (CCS), primarily focusing on horizontal (2D) characterizations of the community. While these surveys have supported identification of seabird ‘hotspots’ in 2D, there are currently no assessments that include the vertical (3D) distribution. Addressing knowledge gaps regarding seabird distribution patterns from a 3D perspective, however, will be required if California (CA) is to take advantage of its offshore wind (OSW) resources for purposes of reaching its 2045 renewable energy goals. Such assessments would allow the seabirds’ vertical distribution to be more explicitly considered in assessments of potential OSW impacts. For offshore wind turbines, the rotor swept zones (RSZ) typically start at 30 meters (m) and extend upwards by an additional 230 m. Collision vulnerability is greatest for birds flying at heights that overlap this zone, with the probability of collision influenced by seabird morphology, flight-style, and wind speed. This report presents a novel 3D Seabird Collision Vulnerability Framework (3D Framework) that integrates several decades of at-sea seabird observations and the offshore windscape to predict densities and vertical distribution of the 44 seabird species most widely encountered in at-sea surveys to allow a 3D perspective of what is expected below, versus at, RSZ-height. Predicting the proportion of seabirds moving at RSZ heights was achieved by quantifying: (1) flight height and wind speed relationships for seabirds grouped by similar morphologies and flight styles, (2) 2D density predictions for each species, and (3) the density of seabird species flying at RSZ heights based on the above factors combined with a comprehensive characterization of the windscape. The study region included all offshore waters capable of supporting current OSW mooring technologies (up to 1,300 m depth), covering the continental shelf and upper continental slope of CA north through southern Oregon (OR). Going forward, it will be possible to apply this 3D Framework to new data and new locations to help decide which areas to lease to the expanding OSW industry. The outcome supports the broader goals of the ‘3D Seabird Project’, to identify sites off CA that maximize energy generation while minimizing seabird exposure to RSZs.