The use of passively drifting acoustic recorders for bioacoustic sensing

Abstract

Passively drifting recording systems offer several advantages over autonomous underwater or surface vessels for mobile bioacoustic sensing in the sea. Because they lack of any propulsion, self noise is minimized. Also, vertical hydrophone arrays are easy to implement, which is useful in estimating the distance to specific sound sources. We have developed an inexpensive (<$5000) Drifting Acoustic Spar Buoy Recorder (DASBR) that features up to 1 TB of stereo recording capacity and a bandwidth of 10 Hz–96 kHz. Given their low cost, many more recorders can be deployed to achieve greater coverage. The audio and GPS recording system floats at the surface, and the two hydrophones (at 100 m) are de-coupled from wave action by a dampner disk and an elastic cord. During a test deployment in the Catalina Basin (Nov 2013) we collected approximately 1200 hours of recordings using 5 DASBRs recording at 192 kHz sampling rate. Each recorder was recovered (using GPS and VHF locators) and re-deployed 3–4 times. Dolphin whistles and echo-location clicks were detectable approximately half of the total recording time. Cuvier’s beaked whales were also detected on three occasions. Cetacean density estimation and ocean noise measurements are just two of many potential uses for free-drifting recorders.