Since 1991, NOAA's Pacific Marine Environmental Laboratory (PMEL) in Newport, Oregon and National Marine Mammal Laboratory Laboratory (NMML) in Seattle, Washington have collaborated since 1993 on a joint study to assess the potential of long-range acoustic monitoring of free-ranging populations of large cetaceans. NMML brings many years of experience in population stock assessment based on field observations, with supporting data on habitat, near-field acoustics and behavior. PMEL brings expertise in underwater acoustics and access to both the U.S. Navy's underwater SOund SUrveillance System ( SOSUS) and autonomous moored hydrophone recorders designed for long-term, deep-ocean deployment. This joint study has been largely funded through the Strategic Environmental Research and Development Program (SERDP), with additional support from the Office of Naval Research (ONR), NOAA's Environmental Services Data and Information Management (ESDIM) Program, NMML, and the NOAA VENTS program at PMEL.

PMEL has been monitoring and archiving sound recordings from the U.S. Navy' underwater (SOund SUrveillance System or "SOSUS") fixed hydrophone arrays since 1991. Although the research was directed to the study of underwater seismicity and volcanic activity, initial examination of the data indicated numerous marine mammal calls. PMEL/NMML joint acoustic research has concentrated on the low frequency calls of baleen whales, particularly the blue whale and fin whale. The most readily detected signals are produced by the blue whale, which produces sweeping calls over a several Hertz range (around 17 Hz) with durations of up to sixteen seconds. The call of the blue whale in the northeast Pacific is often sufficiently loud to be detected on more than one hydrophone array. This distinctive call allows the use of mathematical matched filters to detect the signal within the ocean's ambient noise and allow localization of calls to within a few kilometers from ranges of hundreds of kilometers. This powerful technique provides a remote means of acoustically surveying thousands of square kilometers of open ocean for the presence and movements of large whales.

In order to acoustically monitor areas of the world ocean not covered by existing fixed hydrophone arrays, PMEL has developed autonomous moored hydrophones (Fox and Matsumoto 1995) to record acoustic energy from both underwater seismic activity as well as that from whale calls. The hydrophones are designed to be moored in the oceanic sound channel (or SOFAR channel, for SOund Fixing And Ranging); the titanium case containing the data recorder can withstand pressure to at least 1000 m below sea level. These instruments are capable of recording frequencies from 1 - 20,000 Hz, and depending on the sampling rate, can record data for over a year before servicing is required. The hydrophones are designed to be deployed as an array of independent instruments whose geometry can be determined by the needs of the experimenter in order to localize acoustic sources of interest. Currently, 6 autonomous hydrophones are moored in the Eastern Tropical Pacific (ETP), the Gulf of Alaska and the mid-Atlantic Ocean. Hydrophone locations can be seen here

This powerful combination of SOSUS and autonomous moored hydrophone data has enabled PMEL researchers to record the low-frequency calls of blue and fin whales throughout the Pacific Ocean, and to identify regional differences in blue and fin whale vocalizations. Locating calling whales also enables PMEL to identify apparent seasonal shifts in whale distributions. Correlating these data with NMML current field observations and their extensive historical database of species distributions may help answer critical population and stock management questions.