Analysis of passive acoustic recordings made during a three month survey of cetaceans off the Northern Mariana Islands in the western North Pacific

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Passive acoustic monitoring using was used to complement a line-transect survey of marine mammals for a large (~580,000 km2) study site centered on the Northern Mariana Islands in the western North Pacific. A two-element towed hydrophone array was used to monitor and record during daylight hours. Sonobuoys were deployed opportunistically on sightings and areas of interest. Extremely poor sighting conditions hindered visual efforts but not the passive acoustics effort. Over 70 days of survey effort was completed from mid-January to April, 2007. Approximately 220 `unique acoustic detections' were made, of which 155 (70%) were preliminarily identified to 14 different species. The most frequent whale detected was the sperm whale (65), followed by minke whales (30) and humpback whales (12), respectively. The first recordings of calls from Sei whales in this region are characterized. Post-processing of minke and sperm whales recordings resulted in approximately 30 and over 70 localizations, respectively. We present the first acoustic-based estimates for minke whales abundance in this region. Numerous unidentified odontocete whistles were analyzed using ROCCA, a semi-automated whistle classification program, with promising results. We provide recommendations for additional analyses and improvements to methods of collecting and post-processing passive acoustic data on marine mammals.