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Cetacean Assessment & Ecology Program

Finding the Needle in the Haystack: Using Sonobuoys to Locate a Critically Endangered Species

The National Marine Mammal Laboratory's (NMML) Cetacean Assessment and Ecology Program (CAEP) has been conducting large-scale marine mammal surveys for many years, and the inclusion of passive acoustics has become an increasingly vital component. Since 2007, CAEP acousticians have been deploying both long-term recorders and short-term instruments called sonobuoys to acoustically detect and monitor whale populations. Designed for military purposes, sonobuoys are free-floating, expendable, short-term hydrophones that transmit signals in real time via VHF radio waves to a receiver on a vessel (or aircraft). Acoustic detection ranges are highly dependent on water propagation conditions, but typically average 10-15 nautical miles (nmi), allowing for greater coverage than visual surveys alone.

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Figure 1. North Pacific right whale (NMML #27, "Blip"), NMML Permit #172-1719. Photo by Amy Kennedy.

Because they contain batteries, sonobuoys have a limited shelf life. The military is often unable to use all of their sonobuoys before the expiration date. Because their operations have no room for equipment failure, expired sonobuoys are sent to surplus, where many are donated to marine mammal scientists for passive acoustic research. Sonobuoys come in two main types: omnidirectional sonobuoys can record up to 100 kHz, a frequency range that includes most marine mammal vocalizations. DiFAR (Directional Frequency Analysis and Recording) sonobuoys can only record up to 2.5 kHz, which is still sufficient for most vocalizations, but they transmit directional bearing information in addition to the acoustic signals.

By deploying two or more sonobuoys separated by a few miles, we can obtain a cross-fix on a calling whale and localize on the whale's position in real time. Usually this is done to verify that the calling whale is the same individual spotted by the observers and to conduct focal follows to correlate acoustic behavior with surface and dive patterns. On occasion, however, this directional information becomes much more important.

North Pacific right whales (Eubalaena japonica) are arguably one of the most endangered marine mammals in the world (Fig. 1). Current estimates put the size of this population at fewer than 40 individuals, which makes them difficult to locate in even the best conditions. In thick fog and high sea states (common in the southeast Bering Sea), it becomes nearly impossible to visually spot a right whale. However, the use of sonobuoys and the ability to localize on a calling whale dramatically increase the odds of finding these extremely rare animals.

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