Abstract
Cetacean distribution and abundance data are traditionally collected by large vessels and aircraft conducting surveys in offshore areas. These surveys provide important data, but due to the expense and difficulty in collecting data during bad weather or during times of low visibility, these surveys are generally conducted intermittently during the summer and fall seasons. As such, these data suffer from spatial and temporal gaps, especially for cryptic species. Since sound is the primary sensory modality of marine mammals, passive acoustic monitoring (PAM) is an efficient approach to monitoring marine mammals while allowing simultaneous characterization of the overall soundscape.
There are a variety of PAM platforms that vary in strengths and limitations: towing hydrophones behind a ship provides good geographic resolution, while seafloor hydrophones allow for good temporal resolution. Passive acoustic drifting recorders can record for weeks or months (depending on recording characteristics and local currents) and their low cost allows for deployment of multiple instruments, which increases spatial coverage and provides a model for intermediate geographic and temporal resolution. Furthermore, the hydrophones for drifting recorders can be positioned near animals in the water column (and away from surface noise), which allows them to collect high-quality data without affecting animal behavior. Drifting recorders have been increasingly deployed during large scale shipboard surveys to augment visual line-transect surveys for cryptic and deep-diving species (Keating et al. 2018; Simonis 2020), and methods have been developed to estimate density and abundance of goosebeaked whales (Ziphius cavirostris) (Barlow et al. 2021). As drifting recorders are not tethered to the seafloor or to a ship, they have shown potential as an alternative PAM platform for the Wind Energy Areas (WEAs) identified in the deep waters offshore the U.S. West Coast.
The goal of the Adrift in the California Current Project (“Adrift”) was to use passive acoustic drifting recorders deployed offshore the U.S. West Coast to assess the distribution of marine mammals and to characterize the marine soundscape. This three-year study was initiated in the Northern California region in 2020, was extended to Central California in 2021, and an additional pilot study off Oregon was initiated in 2022. A concerted effort was made to develop a streamlined open-source workflow for passive acoustic analysis that would promote reproducible research, with all methods, data, and metadata being publicly accessible. This report outlines methods, results, and recommendations for future research.