Results From A High-Resolution Acoustic Device For Monitoring Finless Porpoises In Coastal Precincts Off Japan

Finless porpoise populations in coastal precincts are adversely impacted by human activities that cause water pollution, lack of food and denial of migratory paths. Proper survey and management of their populations has therefore become necessary. Visual-based surveys are very tedious, labor intensive, have limited accuracy, and are strongly subject to visibility conditions. These cetaceans produce characteristic echolocation pulses that make them acoustically visible, night or day. Acoustic-based survey methods are expected to be indispensable for surveying finless porpoises that do not exhibit visually dramatic behavior like the oceanic dolphins and spend most of their time underwater. Passive observation of echolocation clicks on arrays of high frequency hydrophones can simultaneously localize several underwater sound sources as they move around. Since the click characteristics are known to be species-specific, individuals belonging to the same group need to be precisely discriminated on basis of their location and movement pattern alone. This paper reports the results from testing a compact and portable acoustic survey device designed for monitoring underwater behavior of groups finless porpoises. The acoustic sensor system consists of 3 hydrophones forming a main vertical linear array, together with two more hydrophones forming a small 3-element triangle array with the central hydrophone, in a plane perpendicular to the linear array axis. It is housed in a "bird-cage" structure that weighs 25 kg and is 3.6 m tall, 30 cm in diameter, and is deployed vertically with a buoy and a weight. The device has been tested in shallow waters of less than 10 meters depth off-Choshi port on the Pacific coast of Japan. Echolocation pulses from several porpoises were recorded. One feature has been the differences in strength of the click signal observed on the hydrophones in the vertical array that is indicative of a narrow beam pattern of the porpoise. The main challenge in localization has been to eliminate surface and bottom reflections that inevitably occur in shallow water. In addition, echolocation pulses of some dolphins in the vicinity were also simultaneously recorded. An attempt has been made to segregate the porpoise clicks from those of the dolphins, and then to localize the porpoises. The experiment demonstrates the capability of the device for individually tracking several vocalizing animals. This novel tracking system will be used to survey populations of finless porpoises in coastal precincts.