Using multi-sensor drifter platforms to observe mid-water bioacoustic activity

Conducting unobtrusive observations of physical, acoustic, and biological properties of the mesopelagic zone remains challenging, despite the proliferation of mobile autonomous platforms. Presented here is the Optical Passive Acoustic Adaptive Drifter System (OPAADS), a 2-m tall Lagrangian drifter platform that incorporates a CTD, stereo video cameras, vertical and tetrahedral hydrophone arrays, and acoustic vector sensors. The OPAADS uses a commercial buoyancy engine to dynamically adjust its target isobar via either a preprogrammed dive sequence or real-time commands relayed by an ultra-short baseline underwater tracking and communication system. The platform can descend to depths to 1 km for 3- to 5-day missions. Since 2022 we have conducted 29 deployments of three units off the New England Seamounts, Southern California, and Kona-Kohala Hawaii, all in water depths greater than 700 m. Many deployments find bioacoustic activity a dominant component of the ambient sound, from clusters of sperm whale clicks off seamounts, to fish chorusing off Hawaii, to distinctive 0.5–2 and 3–5 kHz diffuse noise bands off San Diego and Hawaii often associated with dolphin echolocation but whose origin remains uncertain. We also discuss the directionality and localization of these sources. Work sponsored by ONR TFO.