The development and use of towed hydrophone arrays to inform Southern Resident killer whale Critical Habitat in outer coastal waters

Southern Resident killer whales (SRKW) were listed as endangered in 2005, requiring designation of Critical Habitat. At that time little was known about SRKW distribution in winter/spring along the outer coast. From 2004 to 2016, we conducted nine winter/spring surveys aboard NOAA ocean-class vessels to locate and follow SRKW pods using visual sightings, satellite telemetry, and passive acoustic monitoring using towed hydrophone arrays. We present information on the evolution of the towed hydrophone array system, and insights gained on the movements and acoustic behavior of this population. Beginning with a linear two-element hydrophone array system, operations evolved to include a secondary, five-element array towed in parallel, to resolve left-right bearing ambiguity to vocalizing animals. Acoustic localization allowed tracking of vocalizing whales when visual surveys were not possible (e.g., nighttime/inclement weather). When visual and/or satellite tag position data indicated whales were within acoustic range, vocalizations were detected only 45% of the time, a result that has implications for other PAM efforts. These surveys helped fill important gaps in our understanding of SRKW occurrence to inform Critical Habitat in the outer coastal portion of their range. Despite limited monitoring, we established the importance of this key portion of their range in winter/spring.