Monitoring cetaceans in the North Pacific: analysis of retrospective SOSUS data and acoustic detection on the Northern Edge Range

Abstract

To comply with environmental laws, the U.S. Navy has increasingly relied on acoustic detection and tracking for marine mammal monitoring/mitigation. In the North Pacific, much of what is known about large whale seasonal occurrence comes from Sound Surveillance System (SOSUS) arrays. SOSUS provides broad basin-scale assessments of large whale seasonal occurrence, but cannot provide detail at the regional level, in particular here for the Navy's Northern Edge (Training) Range in the northern Gulf of Alaska. Here, data from long-term acoustic observations of basin-wide data, and from a short-term, nearshore acoustic deployment in the NER, are presented. For the long-term observations, Navy-analyst derived detections of blue and fin whales were compared with spectral data to determine if there is a reliable way to separate the two species in the spectral data. Although the degree to which detections matched the spectra varied with frequency range, generally blue and fin whale detections matched the spectra. However, when only blue, and no fin, whales (or vice versa) were detected by analysts, there were no discernable differences in the spectral levels of different frequency ranges. This suggests that, absent other confirmation of the presence of blue and fin whale vocalizations, these species cannot be reliably discerned from spectra data alone in regions and seasons where the two overlap.