Aquatic Mammals 2013, 39(1), 23-53, DOI 10.1578/AM.39.1.2013.23 A Review and Inventory of Fixed Autonomous Recorders for Passive Acoustic Monitoring of Marine Mammals Renata S. Sousa-Lima,1, 2, 5 Thomas F. Norris,3 Julie N. Oswald,3, 4 and Deborah P. Fernandes5 1Bioacoustics Research Program, Cornell Lab of Ornithology, 159 Sapsucker Woods Road, Ithaca, NY 14850, USA E-mail: [email protected] 2Programa de Pós-Graduação em Ecologia, Conservação e Manejo de Vida Silvestre, Instituto de Ciências Biológicas, Universidade Federal de Minas Gerais, Avenida Antônio Carlos 6627, Belo Horizonte, MG 31270, Brasil 3Bio-Waves, Inc., 144 W. D Street, Suite #205, Encinitas, CA 92024, USA 4Oceanwide Science Institute, PO Box 61692, Honolulu, HI 96839, USA 5Laboratório de Bioacústica e Programa de Pós Graduação em Psicobiologia, Departamento de Fisiologia, Centro de Biociências, Universidade Federal do Rio Grande do Norte, Caixa Postal 1511, Campus Universitário, Natal, RN 59078-970, Brasil Abstract sounds produced by marine mammals to more effectively study them. Fixed autonomous acoustic recording devices In 1880, Pierre and Jacques Curie (1880a, (autonomous recorders [ARs]) are defined as 1880b) discovered that when mechanical pressure any electronic recording system that acquires was exerted on a quartz crystal, an electric poten- and stores acoustic data internally (i.e., without a tial is produced. This finding enabled the devel- cable or radio link to transmit data to a receiving opment of the first device capable of listening to station), is deployed semi-permanently underwa- sounds underwater—passive acoustic monitoring ter (via a mooring, buoy, or attached to the sea (PAM), which was utilized during World War I. floor), and must be retrieved to access the data. Since then, the development of PAM technology More than 30 ARs were reviewed. They varied has made it possible for researchers to listen to, greatly in capabilities and costs, from small, record, store, and analyze marine mammal sounds. hand-deployable units for detecting dolphin and However, up until the turn of the century, limita- porpoise clicks in shallow water to larger units tions in PAM technologies and methods available, that can be deployed in deep water and can record as well as high costs, inhibited the development at high-frequency bandwidths for over a year, but and application of passive acoustics for marine must be deployed from a large vessel. The capa- mammal monitoring. In addition, the technical bilities and limitations of the systems reviewed expertise required to develop and apply these herein are discussed in terms of their effectiveness technologies was typically beyond that of most in monitoring and studying marine mammals. field biologists. The development of fixed autono- mous underwater sound recorders (ARs) in the Key Words: passive acoustic monitoring, fixed early 1990s greatly reduced the costs and exper- systems, marine mammals, acoustic monitoring, tise required to monitor marine mammal sounds mitigation, autonomous recorders for extended time periods. An AR is defined as any electronic recording device or system that Introduction acquires and stores acoustic data internally (i.e., without cable or radio links to a fixed platform or Marine mammals live most of their lives under the receiving station) on its own, without the need of ocean surface, out of view of humans. The diffi- a person to run it; is deployed semipermanently culties inherent in studying the effects of human underwater (i.e., usually via a mooring, buoy, or activities on these animals can be overcome only attached to the sea floor); and is archival (i.e., through the application of technology (Samuels must be retrieved after the deployment period to & Tyack, 1999). Most species of marine mam- access the data). mals are acoustic specialists that rely on sounds Today, ARs can be easily deployed on the for communication and navigational purposes. ocean bottom to record acoustic data for days, Scientists and engineers have developed passive weeks, or even months at a time. These archival acoustic-based technologies to detect and record ARs must then be retrieved to download data 24 Sousa-Lima et al. for post-processing and analysis. This approach in relation to oil and gas exploration and produc- allows ARs to be deployed and retrieved by tion (E&P) activities. field personnel with a relatively limited amount of training or expertise, which frees up valuable Historical Overview of the Development of time, resources, and funding. Autonomous Recorders ARs are most cost effective when used in During the late 1960s, a change in spatial scale extreme or remote locations where access is lim- occurred in marine geophysical research when ited or difficult—for example, polar regions, the scientists focused their studies on earthquakes in deep sea, and locations where travel distances are smaller areas of the sea floor. This shift required great or environmental conditions exist that are higher accuracy and more precise geophysical too harsh to conduct surveys from aboard research instrumentation and led to the development of vessels (Mellinger & Barlow, 2003; Munger et al., autonomous instruments called Ocean Bottom 2005; Sirovic et al., 2009). ARs are also useful Seismometers (OBSs) for monitoring underwater for detecting marine mammals in areas where earthquakes. OBSs were able to measure move- the occurrence of animals is infrequent, or where ments of the Earth’s crust (Loncarevic, 1977). An ship-based surveys have a very high cost per OBS is designed to rest on the ocean floor and detection (Mellinger & Barlow, 2003). The cost uses a sensor called a seismometer to take mea- savings in the use of ARs is achieved because of surements. The seismometer is comprised of a their autonomous nature—that is, their operation heavy mass suspended on a spring between two is independent of the presence of a human opera- magnets. Seismometers use the principle of iner- tor. The disadvantage is that these instruments tia: the resistance of an object to a change in its must be recovered to access the data; therefore, state of motion. When the earth’s crust shifts, the real-time monitoring is not possible. If archival seismometer and its magnets move concurrently, data are useful, such as for acoustic prospecting but the heavy mass momentarily remains in its efforts (i.e., during pilot studies), ARs should be original position. The relative movements of the considered as a cost-effective approach. In gen- mass through the magnetic field produce elec- eral, set-up and infrastructure costs are lower for trical currents that are then measured by instru- ARs than they are for other types of PAM systems mentation in the OBS (Dorman, 2001; Ocean (e.g., fixed-cabled hydrophones, towed-hydro- Instruments, n.d.). phone arrays, and real-time radio- or satellite- A typical OBS consists of a seismometer, a linked hydrophones; Mellinger et al., 2007a). In data logger, batteries to power the device, weight addition, ARs are more flexible in their configu- to sink it to the sea floor, a remotely activated ration, timing, and location of deployment, and (or timed) release mechanism, and flotation to they are less obtrusive to both animals and vessel buoy the instrument back to the surface (Dorman, traffic when compared to other types of PAM 2001; Ocean Instruments, n.d.). By 1975, the systems. Still, acoustic data bandwidth and col- OBS became an operational tool used by a dozen lection capabilities are usually higher for these or so research groups in at least seven countries other types of PAM systems than they are for ARs (Loncarevic, 1977). Since then, OBSs developed (Mellinger et al., 2007a; Van Parijs et al., 2009). by researchers from France, Japan, Australia, These trade-offs must be considered when decid- Germany, Russia, and the U.S. have been used ing which type of PAM system to use to reach a extensively in geophysical research efforts. particular goal. Small ground motions caused by earthquakes A critical view of state-of-the-industry AR tech- have relatively higher frequencies, so monitor- nology is provided herein, including both “tradi- ing them requires special short-period OBSs that tional” autonomous recording devices (i.e., those can record motions up to hundreds of times per designed specifically for recording geophysical second (Ocean Instruments, n.d.). These higher events, underwater noise, and marine animal frequency OBSs, originally intended to pick sounds) and “nontraditional” recording devices up the motion of the crust via the motion of the (e.g., electronic animal tags such as acoustic data- substrate upon which they rest, typically record loggers). A review of the history of AR develop- up to 100 Hz and are also capable of recording ment; their capabilities and constraints with respect low-frequency sounds produced by large baleen to different application requirements (monitoring whales (e.g., blue whale [Balaenoptera musculus] vs mitigation); specific environments in which they and fin whale [B. physalus])—sounds that con- can be used, and, perhaps most importantly, the tain frequencies below 100 Hz. McDonald et al. species to be monitored; and the biological ques- (1995) were the first to use OBS data to study tions that are to be addressed are presented herein. marine mammals: blue and fin whale calls were AR capabilities and constraints are discussed with detected and localized in deep waters during a respect to their use in monitoring marine mammals seismic study on the southern Juan de Fuca Ridge Fixed Autonomous Recorders 25 off the coast of Oregon. These data were recorded and engineers from these two areas of expertise, incidentally during a seismology experiment. and from two different institutions (SIO and BRP), A similar device called an Ocean Bottom exchanging technology and knowledge to help Hydrophone (OBH) is also used by geologists to create the initial design of that instrument (C.