Seabirds in 3D: A framework to evaluate collision vulnerability with future offshore wind developments in the California current system

Abstract

Since the 1970s, numerous vessel and aerial surveys of marine birds, covering many thousands of square kilometers, have been conducted in the California Current System (CCS), providing insights into seabirds' horizontal (2D) diversity and abundance, including the identification of “hotspots.” Addressing knowledge gaps regarding seabird distribution patterns from a 3D (vertical) perspective, however, will be required if California (CA) is to use offshore wind (OSW) facilities to assist in reaching the state's 2045 renewable energy goals. Such an analysis will allow seabirds' vertical distribution to be considered when assessing potential OSW impacts, as collision vulnerability is greatest for birds flying at heights overlapping turbine rotor-swept zones (RSZ). This probability is determined by the interaction of species-specific morphology and flight-style with wind speed. Thus, predicting the proportion of seabirds moving at RSZ heights can be achieved by quantifying: (1) the likelihood that significant numbers of individuals of various species, of those present, will reach RSZ heights across the full spectrum of wind speeds, (2) species-specific density in 2D space, and (3) the windscape. To address these goals, we describe a novel 3D Seabird Collision Vulnerability Framework (3D Framework) that integrates historical at-sea observations with the offshore windscape to predict the densities of 44 species with sufficient sample sizes to support a 3D assessment of their expected distribution below versus within RSZ heights. The prediction region encompasses all offshore waters capable of supporting current OSW mooring technologies, which, in CA and southern Oregon, includes waters overlying the continental shelf and upper continental slope. This 3D Framework can be modified to incorporate new data and new locations as the OSW industry expands. This effort supports the broader goal of identifying sites within the CCS that optimize power generation while minimizing interactions with seabird species whose flight behavior makes them vulnerable to collision.