The use of passive acoustic monitoring (PAM) has increased recently with the availability of inexpensive and capable devices to collect acoustic data to observe and monitor marine animals. One type of passive acoustic sensor is thearchival marine acoustic recording unit (ARU). Here, archival means that the data are collected and stored onboard the unit for analysis after the instrument is recovered.

      Many different archival ARU systems have been developed. They can be used in many different marine areas and vary widely in their size and length of deployment. However, archival ARU sensors may be deployed for up to a year, have a self-contained power supply, store the data internally, and must be retrieved in order to download the acoustic information gathered. Several archival ARUs may be deployed together to increase the accuracy of data collected and make it possible to detect, localize, and track vocalizing fish and marine mammals.

     Scripps Institution of Oceanography has developed acoustic recording packages (ARPs) to monitor marine mammals and study ambient ocean noise over long periods. The instrument package includes an arrangement of floats, data recording electronics, batteries, and ballast and release systems. The package is moored to the ocean bottom with the components distributed vertically along a line buoyed up from the seafloor. The hydrophone sensor is tethered approximately 10 m above the seafloor package.

    ARPs record sounds up to 1 kHz to detect the low-frequency vocalizations made by calling baleen whales. ARPs have been deployed to record baleen whale sounds in the Bering Sea, off the coast of southern California, near the West Antarctic Peninsula, and near Hawaii. ARPs deployed in the southeastern Bering Sea recorded North Pacific right whale calls as early as May and as late as December. These data indicate that right whales occur intermittently in the southeastern Bering Sea and remain later in the year than previously known. Such findings underscore the usefulness of acoustic monitoring during times of the year when visual surveys are typically not conducted.

   High frequency acoustic recording packages (HARPs) record sounds from 10 Hz up to 100 kHz to study toothed whales. HARPs have been successfully used to research and monitor many cetacean species including, Risso’s andPacific white-sided dolphins, killer whales, and beaked whales.

                             Diagram of a high frequency acoustic recording package (HARP). Image courtesy of Sean Wiggins/Scripps Whale Acoustic Lab (http://cetus.ucsd.edu/technologies_AutonomousRecorders.html).

     The National Oceanic and Atmospheric Association’s (NOAA’s) Pacific Marine Environmental Laboratory (PMEL) has developed moored systems that place hydrophones in the deep sound channel (SOFAR channel). Hydrophones in the deep sound channel can detect signals from very distant sources. Each acoustic package consists of an anchor,acoustic release, hydrophone, data logging system with one to six hard disk drives, and flotation.

       The moored hydrophone recorders are capable of recording frequencies from 1-20,000 Hz, and depending on the sampling rate, can record data for over a year. These hydrophones have been deployed in the eastern equatorial Pacific, northern North Atlantic, central North Atlantic, Marianas, and the Gulf of Alaska. In the offshore waters of Alaska, the recorders have yielded unprecedented information on the seasonal occurrence and calling behavior of blue, fin, humpback,sperm, North Pacific right, bowhead, and gray whales.

 Diagram of the PMEL moored hydrophone system. Note that the autonomous hydrophone floats a depth equivalent with the SOFAR channel (shown in light blue above). Image credit: NOAA PMEL Vents Program.

      Cornell University has developed bottom-mounted ARUs known as Marine Acoustic Recording Units (MARU’s) or“pop-up” buoys. These systems differ from other archival AUR systems in that they are smaller and more portable. Pop-ups are packaged in 17-inch diameter glass spheres that are in plastic “hardhats” for protection. The buoys can be deployed up to depths of approximately 6,000m and float just above the sea floor, recording acoustic data for up to 90 days. In addition to monitoring other cetacean species and measuring ocean noise, pop-up buoys are being used to study critically endangered right wales in Cape Cod Bay, the Great South Channel, Bay of Fundy, Canada, as well as in waters off New York, New Jersey, North and South Carolina, and in right whale calving grounds off Georgia and Florida. For more information on how pop-up buoys are being used to monitor marine mammals, please see How is Sound Used to Study Marine Mammal Distribution?

      The Ecological Acoustic Recorder (EAR) is another acoustic monitoring unit, however, instead of being used specifically for marine mammal studies, this system is used to monitor the ambient sound field of coral reefs and other tropical systems. It records sounds at frequencies up to 30kHz. EARs have proven to be an effective tool for documenting the temporal patterns of a wide range of acoustic signal types on coral reefs, and can detect and monitor the occurrence of anthropogenically-produced sounds, such as vessel engine noise.

Photo of a deep-water EAR suspended in the water showing the hydrophone(1), aluminum housing(2), syntactic foam collar(3), and two acoustic releases(4). Image credit: NOAA

        A deep water EAR deployed off of Oahu, Hawaii. Image credit: NOAA.

       Some ARU systems process the data on board in addition to storing it for later analysis. Porpoise detectors, PODs, are fully automated, passive acoustic monitoring systems that detect toothed whales by recognizing their echolocation clicks. Onboard software classifies the click trains based on their properties. Data are automatically filtered to remove sounds from noise and boat sonar. T-PODs have been used to monitor harbor porpoise activity and behavior. They have also been useful in assessing the reaction of cetaceans to wind turbine construction. The digital C-POD has recently replaced the TPOD- it detects broader-band clicks and collects a much wider range of data to advance species identification. A deep-water version of the POD - the DeepC-POD - can operate in depths down to 3000 m, sufficient to cover the whole depth range of deep-diving species such as beaked whales.

        Other ARU systems have a directional component to them. An array of Directional Autonomous Seafloor Acoustic Recorders (DASARs) are being used in the Beaufort Sea to analyze locations of bowhead whales and assess the relationship between sounds associated with an offshore oil facility or seismic exploration and whale call distributions. DASARs are fashioned after Directional Frequency Analysis and Recording (DIFAR) sonobuoys, which have been used previously to obtain bearings to calling bowhead whales and other cetacean species. However, unlike sonobouys, DASARs are designed to sit on the ocean floor and record acoustic data for up to 116 days. This allows long-term deployment in areas where floating ice, surface activities, or other factors might prevent the use of sonobouys. Each DASAR includes an omnidirectional hydrophone to record sound pressure levels (of, for example, whale calls) and two particle motion sensors that determine the direction to the sound source. Sounds up to 1000 Hz can be recorded by DASARS, a bandwith that is appropriate for bowhead calls.

  A DASAR being deployed in the Beaufort Sea (on a very calm day). Photo courtesy of Shell Exploration and Production Company and Greeneridge Sciences, Inc.