Passive Acoustic Monitoring (PAM) of marine mammals using Static Acoustic Monitoring Systems (SAMS) has increased over the last decade due to advances in technology. Devices are either autonomous (i.e. collect and record data internally) or transmit data via cable or radio link.
Autonomous recording devices vary from small units, which can be deployed from the sides of boats/installations by hand e.g. the T-POD www.t-pod.co.uk and C-POD www.cpodclickdetector.com), used for detecting dolphin and porpoise clicks, to larger deep water systems that can record at low and high frequency bandwidths (e.g. Acoustic Recording Packages (ARPs) and High-frequency ARPs (HARPs)) (Sousa-Lima et al. 2013). Scripps Institution of Oceanography developed ARPs (http://www.whoi.edu) for long term marine mammal monitoring. Large hard disk storage capacity makes ARPs capable of 400 day deployment durations. Autonomous Recording Units (ARUs; http://www.dosits.org) using ‘pop-ups’ have also been deployed successfully on the ocean bottom to collect marine mammal data. These units were developed by Cornell University’s Bioacoustics Research Program. Units are designed to be anchored temporarily to the ocean floor with sand bags and any data collected retrieved by an acoustic release trigger from a shipboard transponder, which allows surface retrieval (Hatch et al. 2008). For a comprehensive overview of autonomous acoustic monitoring systems see http://www.bio-waves.com
Cabled bottom-mounted hydrophone arrays consist of arrays connected by underwater cables to facilities ashore, e.g. the United States Navy Sound Surveillance System (SOSUS; http://www.dosits.org). The US Navy SOSUS (http://en.wikipedia.org), deployed in the North Atlantic and North Pacific Oceans, was developed originally by the navy to track submarines; however, in the early 1990s scientists were allowed to use the system to track and study whale vocalisations. The US Navy also has the Atlantic Undersea Test and Evaluation Centre (AUTEC; http://en.wikipedia.org), which consists of 82 bottom-mounted hydrophones deployed at depths of 2,000 m. The AUTEC range is based in a deep canyon known as the Tongue Of The Ocean (TOTO; http://en.wikipedia.org) in the Bahamas. Bottom mounted hydrophone arrays, especially at AUTEC, are used commonly to study beaked whales (e.g. DiMarzio et al. 2008; Ward et al. 2008; Wiggins et al. 2012; Baumann-Pickering et al. 2013), as they are deep-diving foragers and spend little time on the surface (Tyack et al. 2006). Beaked whales (http://en.wikipedia.org) are observed infrequently, and are one of the most mysterious and least known groups of marine mammals.
Ocean Bottom Seismometers (OBS), used originally to monitor underwater earthquakes, but are also capable of recording low frequency sounds produced by large baleen whales (e.g. blue (Balaenoptera musculus) and fin whales (Balaenoptera physalus)).
Over the last few decades, our oceans have become dominated by low frequency noise from shipping, military, research activities, and oil and gas exploration. This increase in anthropogenic noise has led to concern over the level of impact on marine mammals, in particular baleen (mysticetes) whales, as they are acoustically sensitive to and utilise low frequency sound for communication. Bottom-mounted hydrophones have not only increased our knowledge of marine mammal acoustics, but have also provided scientists with valuable information on species presence, locating and tracking individuals, and determining patterns of seasonal distribution and relative abundance. In order to manage and minimise impacts, accurate information on the distribution and behaviour of cetaceans is essential.
INCREASE OUR KNOWLEDGE OF MARINE MAMMALS
The SOSUS has been used successfully to study marine mammals, including blue and fin whales (Stafford et al. 2009). In a study carried out in the North Pacific, a seasonal difference in calling rates was observed with the majority of call detections occurring in autumn and fewest in spring (Stafford et al. 2009). Fin whales were recorded in all months of the year, with a peak in detections occurring from December to March (Stafford et al. 2009).
Watkins et al. (2000) recorded whale calls successfully with US Navy bottom mounted arrays in the North Pacific. Results from this study highlighted blue, fin, and humpback (Megaptera novaeangliae) whale distribution and seasonality in the North Pacific. Blue whales were found to be distributed widely in the North Pacific, with a peak in whale calls during autumn months. Fin whale calling peaked in midwinter, in the offshore waters of the North Pacific; while humpback whale songs where recorded from December to May. Both call sequences recorded for fin and humpback whales are known to represent male breeding displays (Watkins et al. 2000).
McDonald et al. (1995) were the first to study marine mammal sounds recorded from OBSs. During this study blue and fin whale calls were detected and localised in deep waters on the southern Juan de Fuca Ridge (http://en.wikipedia.org) off the coast of Oregon, USA.
USING FIXED AUTONOMOUS MONITORING SYSTEMS DURING OFFSHORE INDUSTRIAL ACTIVITIES
Bottom mounted arrays have also been used to assess anthropogenic impacts on marine mammal vocal behaviour. In a study carried out in the St Lawrence Estuary, Canada, the calling behaviour of blue whales relative to seismic operations was recorded using five bottom-mounted Marine Acoustic Recording Units (MARU). During the 11 day survey, acoustic recordings of blue whale calls were significantly higher on days with seismic operations compared to days with no seismic operations (Di Iorio & Clark 2010). The results are suggestive that whales increased their call rates as a response to the increased ambient noise from seismic survey operations. For a review of the effects of seismic surveys on marine mammals please see www.marinemammalseismic.co.uk and www.marinemammalseismic.com. A review looking at harbour porpoises (Phocoena phocoena) specifically, can be found at www.porpoisedetectors.com
A study carried out in the Bahamas measured behavioural responses of Blainville’s beaked whales to vessel noise. Results highlighted that duration of foraging sessions was not affected significantly by exposure. Changes in spatial pattern of vocalisations were observed, with changes in the hydrophone which the group was detected most frequently. The number of detections was significantly less the closer the whales were to the sound source. Results therefore suggest that shipping noise caused a significant change in beaked whale behaviour up to at least 5.2 km away from the vessel (Pirotta et al. 2012).
Using T-PODs, Ocean Science Consulting (OSC; www.osc.co.uk) is continuing their research on the presence and behaviour of harbour porpoises around offshore drilling rigs and production platforms. There is already evidence by Todd et al. (2009) that harbour porpoise feed around offshore installations, and that there is a pronounced dial pattern in echolocation activity (i.e. a greater number of porpoise encounters at night than during the day). Check-out OSC’s news page for the latest (www.osc.co.uk). You can also follow OSC on Facebook (https://www.facebook.com/OSCLtd) and Twitter (https://twitter.com/OSC_Blogger).
Results from research on the AUTEC range have highlighted that Blainville’s beaked whales dive as a group, and vocalise at depth (e.g. DiMarzio et al. 2008). A six day study was carried out at the AUTEC range using bottom mounted hydrophones and DTAGs (www.whoi.edu) to measure the estimated density of beaked whales. Results estimated Blainville’s beaked whale density at 25.3 or 22.5 animals per 1000 km² (Marques et al. 2009).