NOAA Technical Memorandum
Acoustic and visual detection of large whales in the eastern north Pacific Ocean
Two cruises were conducted in the eastern North Pacific Ocean to investigate the precision of large whale call locations as determined by the National Oceanic and Atmospheric Administration (NOAA) and Pacific Marine Environmental Laboratory's (PMEL) implementation of a localization and tracking routine using data from the U.S. Navy's SOund SUrveillance System (SOSUS). Attempts to confirm SOSUS-derived large whale call locations by comparisons to standard vessel-based visual survey data were largely unsuccessful due to spatial and temporal differences in sampling scale. However, the deployment of an autonomous array of six bottom-moored hydrophones provided a means to evaluate SOSUS precision using passive acoustic techniques. There was good correspondence between blue whale (Balaenoptera musculus) call tracks derived from data received at the autonomous hydrophone array and at SOSUS sites. The average difference between blue whale call locations from the two systems was 4.2 ± 8.0 km of latitude and 0.78 ± 1.07 km longitude. In addition, simultaneous reception of fin whale calls at the autonomous array and by SOSUS provided a confirmed detection distance of - 350 km for these short pulsed signals. SOSUS is an excellent tool for detecting fin whales (Balaenoptera physalus) and for detecting and locating blue whales over broad temporal and geographic scales. However, uncertainties regarding whale calling rates and full repertoires and variability in detection distances with seasonal oceanography limits SOSUS' capabilities to provide quantitative assessments of whale abundance or habitat selection. The strength of acoustic monitoring of pelagic waters for calls produced by large whales, either by SOSUS or autonomous arrays, lies in its capability to detect whales in habitats out of reach of conventional surveys over extended spatial and temporal scales. Subsequently, these detections can augment population estimates and habitat selection indices derived by conventional means.
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