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Cetacean Assessment & Ecology Program

Passive Acoustic Monitoring

acoustic recorders
Figure 1.  Autonomous passive acoustic recorders.  From left to right: EAR, Haruphone, and AURAL. (Not to scale.)

In 2007, NMML's Cetacean Assessment and Ecology Program (CAEP) began ramping up its use of passive acoustics to detect and monitor marine mammals, in collaboration with Sue Moore (NOAA Office of Science and Technology (S&T)–Pacific Marine Environmental Laboratory (PMEL)), Kate Stafford (Applied Physics Laboratory–University of Washington (APL–UW)), and Dave Mellinger (Cooperative Institute for Marine Resources Studies (CIMRS), Oregon State University, and PMEL).

Currently, there are acoustic components in the PRIEST (Pacific Right Whale Evaluation Study) and BOWFEST (Bowhead Whale Feeding Ecology Study) projects, funded by the Minerals Management Service, as well as in the Passive Acoustic Monitoring in the High Arctic project, which is funded by the S&T and is part of NOAA's contribution to the International Polar Year.

Acoustics Arsenal

In all projects, bottom-moored passive acoustic recorders are being used to collect long-term recordings. Several types of recorders have been deployed (Fig. 1 above): EARs (ecological acoustic recorder in collaboration with Drs. Marc Lammers and Whitlow Au, Hawaii Institute of Marine Biology, University of Hawaii); Haruphones (Haru Matsumoto, CIMRS, Newport, Oregon); and AURALs (autonomous underwater recorder for acoustic listening; Multi-Électronique, Inc., Rimouski, Quebec, Canada).

These recorders all have the same basic components: an underwater sensor (hydrophone) and a pressure housing containing the recording circuitry, batteries, and hard drives. They are all programmable to record on a user-chosen duty cycle and frequency range. The Haruphone and AURAL are substantially larger than the EAR unit (~6 ft vs. ~2 ft) and do not have a built in autodetector. The AURAL is the only recorder that can be purchased instead of rented, which is very cost-effective over the long term for these projects.

Sonobuoys were deployed only during the PRIEST project. These are free-floating expendable underwater listening devices created for military applications. Simple in concept, the underwater sensor picks up acoustic signals and sends them to a surface float where they are transmitted back to the ship (or aircraft) via VHF radio waves. Sonobuoys are very compatible with visual surveys in that they can be deployed and monitored while the vessel is under way.

Monitoring distances (from the ship to the sonobuoy) are based on line-of-sight and battery strength of the sonobuoy but typically fall within the 10-15 mi range with the receiving antenna installed high on the vessel. Acoustic detection distances (from whale to sonobuoy) are highly dependent on underwater propagation conditions and so can vary widely among research areas.

Sonobuoys come in two main types: omni-directional sonobuoys, which can record frequencies up to 20 kHz and transmit just the acoustic signal; and DiFAR (directional fixing and ranging) sonobuoys, which have a limited frequency bandwidth (<2.5 kHz), but which transmit both the acoustic signal and bearing information of the sound source from the buoy.

Cross-bearings calculated from two or more sonobuoys allow for real-time whale localizations. Sonobuoys have a limited shelf life, and the military sends them to surplus after they reach their expiration date.

Because new sonobuoys are budget-prohibitive and because our research can tolerate a much higher failure rate than the U.S. Navy's, most marine mammal scientists rely on donations of expired sonobuoys from the military.


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