Passive Acoustic Monitoring (PAM) systems are at the forefront of cetacean monitoring. PAM can be split into fixed, towed, or drifting PAM, and acoustic recording tags placed directly on the animal.
The order Cetacea is a specialised group of marine mammals commonly referred to as whales, dolphins, and porpoises. Cetaceans have evolved to inhabit marine and freshwater environments around the world (Pompa et al., 2011). As apex predators, they play a vital role in maintaining a stable and balanced ecosystem (Heithaus et al., 2008).
As offshore industries continue to expand, interest in research and conservation of cetaceans around industrial activities has increased. Offshore developers are faced with increasing pressure by Governments, Non-Governmental Organisations (NGOs) and the public worldwide to minimise impacts on the environment as part of growing concern over the ocean’s health. For example, recent increase in awareness of plastic pollution and marine litter has caused global change when autopsies of multiple stranding events of cetaceans revealed the fatalities were due to entanglement or starvation by prevention of normal feeding (Derraik, 2002; Poeta et al., 2017; Panti et al., 2019). Guidelines and legislation, such as those set out by the UK’s Joint Nature Conservation Committee (JNCC), Europe’s Marine Strategy Framework Directive (MSFD), New Zealand’s Department Of Conservation (DOC; see Ocean Science Consulting OSC - NZ also), USA’s Bureau of Safety & Environmental Enforcement (BSEE) and Bureau of Ocean Energy Management (BOEM) have obliged Governments to impose policies designed to minimise potential damage to the marine environment. Marine Mammal Observers (MMOs), Protected Species Observers (PSOs), and Passive Acoustic Monitoring (PAM) operators, in conjunction with established PAM systems (e.g. towed arrays, T-PODs, and C-PODs), are playing a vital role within oil & gas exploration and development, renewable energy installation, and harbour construction, military exercises, etc. (for a full list see sectors and services provided by OSC).
To minimise potential impacts of anthropogenic noise on marine mammals, protocols often stipulate that Environmental Impact Assessments (EIAs) must be carried out before, during, and after offshore industrial activities (for further details click here). Many EIAs require cetacean monitoring to estimate baseline abundance, distribution, and seasonal variation in cetacean populations around offshore operations (e.g. Todd and Todd, 2010; Todd et al., 2016).
Visual observations are an invaluable tool when monitoring cetaceans. Visual data collected using distance sampling can be analysed to produce population abundance and density estimates. Population-size estimates are often an essential component of environmental monitoring and EIAs. Regular visual monitoring throughout seasons and over many years using appropriate techniques can offer an effective long-term method for monitoring and management of cetacean populations. This has been demonstrated by many citizen science projects worldwide, which have proven effective and can be used to inform policy decisions (Harvey et al., 2018).
Observations can be made by MMOs offshore (vessels, offshore installations, planes) or onshore. For example, eight species of marine megafauna were observed over a 10-year period by MMOs around offshore installations in the North Sea (Todd et al., 2016), providing evidence that the animals are utilising these anthropogenic arenas. For large-scale surveys, visual observations can also be obtained by observers on planes or helicopters. This method is very useful when surveying large areas, and due to the speed at which observations can be carried out, the potential for bias caused by animal movement is reduced. Aerial surveys have been common for a number of years and produce robust reliable data (Williamson et al., 2016), as seen with the SCANS III project. Increasing advances in technology are allowing researchers to collect data using a fraction of the resources previously required and reducing disturbance; therefore, not subjecting the animal to any direct harm or interference. Unmanned Aerial Vehicles (UAVs) have been proven effective for population counts from large humpback whales (Megaptera novaeangliae) to small harbour porpoise, Phocoena phocoena (Aniceto et al., 2018).
Many cetacean monitoring methods are based on the principles of distance sampling, with line or point transects (Buckland et al., 2001; Buckland et al., 2004). Line transect sampling for cetaceans involve a vessel or aeroplane traveling along a pre-determined route, while observers record sightings. When confined to shore-based sampling, point transects can be used with theodolites to record accurate angle and distance. Data are then analysed statistically to produce an estimate of abundance and density of cetaceans within the area (e.g. Kaschner et al., 2012; Hammond et al., 2017).
Distance sampling can produce information at a basic scale, but more intricate details can be obtained by combining it with statistical models (e.g. generalized additive models) which can incorporate additional spatially referenced environmental data (such as depth, temperature or current speeds). For example, using Density Surface Modelling (DSM; Miller et al., 2013). DSM has been used to compare effectiveness of data collected using visual vs. video aerial survey data (Williamson et al., 2016), map the population of several species of cetacean in the entire North Sea (Hammond et al., 2013), and investigate efficacy of protected areas. For endangered fin whale (Balaenoptera physalus) in the Bay of Biscay (García-Barón et al., 2019), these methods have been used to estimate species abundance during a cold year, which decreased by 26.6% compared to warm years, suggesting the highest density to be in the south-eastern part of the bay (García-Barón et al., 2019). Results like this can be used to inform decisions regarding management and conservation of species.
Static Acoustic Monitoring Systems (SAMS) are surface- or subsurface-moored hydrophones used to detect vocalisations of marine mammals. C-PODs, and their predecessor T-PODs, are used commonly and are (with practice) reasonably easy to operate, and are thus often recommended options for SAMS. C-PODs were used recently as part of the international Static Acoustic Monitoring of the Baltic Harbour Porpoise (SAMBAH) project to gauge relative density and distribution of harbour porpoise populations (see here for an overview of the SAMBAH project). A variety of fixed PAM system types are used, each having their advantages for specific projects (click here for more details). This multifaceted approach has proven highly advantageous when environmental conditions are suboptimal (e.g. rain, fog or night) or inconspicuous animal behaviours render visual observations less effective. Recent research by Risch et al. (2019) investigated minke whale (Balaenoptera acutorostrata) acoustic presence in the North Sea and revealed a diel pattern in vocalisations, with detections peaking at night. The inclusion of PAM is a valuable means of limiting operational downtime.
Towed hydrophone arrays comprise a series of subsurface hydrophones deployed astern a vessel. Towed array systems receive and process real-time acoustic data. PAM operators decipher signals and attempt to identify marine mammal calls to species level through carful observation of call characteristics such as amplitude, frequency, duration and call contours etc. PAMGuard detection software is often used, whereby algorithms are employed to identify marine mammal species. Bittencourt et al. (2018) became the first to collect acoustic data using a wave glider (a service OSC also provides) with a towed PAM system in the southwestern Atlantic Ocean. Recordings of high and low frequency detections concluded the encounter of rough-toothed dolphins (Steno bredanensis) and the endangered bryde’s whale (Balaenoptera brydei), proving this is an effective tool for monitoring.
For both fixed and towed systems, it is possible to identify the origin of sounds to determine approximate location of cetaceans in relation to the array, tracking them through space; this is achieved by using triangulation methods (Li et al., 2014). Triangulation is the process by which the origin of sound emission can be verified through analysis of data from two or more stereo hydrophones using the Time Difference of Arrival (TDOA) of sound to each of the hydrophones (Li et al., 2014), followed by the production of a radar plot, showing rough loci (Li et al., 2014; Verfuss et al., 2018). Whilst not strictly necessary for EIAs, information on distance between sound source and array can be used to estimate detection range of marine mammals. Wiggins (2018) set up an array of passive acoustic monitoring recorders to track marine mammals, deployed May 2017 in the US Navy’s Virginia Capes Range Complex offshore of Cape Hatteras. The large-aperture tracking array was configured with three recorders at triangle vertices approximately 700 m apart. Two recorders were each outfitted with four hydrophones arranged in tetrahedron volumetric arrays with ~1 m sensor spacing, whereas the third recorder used one hydrophone. The two 4-hydrophone-small-aperture-arrays were used to track the focus species, beaked whales. TDOA analysis allowed the successful tracking of three individual beaked whales for a 30-minute period throughout the water column.
These systems can consist of, for example, a C-POD attached to Lagrangian drogue (floating device) and multiple floats and identifiers, deployed from a small vessel such as a Rigid Inflatable Boat (RIB). On Separate projects, both OSC and Wilson et al. (2013) have successfully recorded harbour porpoises (Phocoena phocoena) using these systems and were able to map tidally driven spatiotemporal variability in ambient noise levels which could influence porpoise detection. Drifting PAM can be more cost effective than traditional methods such as more logistically intensive methods, visual and acoustic boat-based surveys. Elusive beaked whales have also been also detected over abyssal planes (Griffiths and Barlow, 2016), proving their efficacy in deep strata. Drifters equipped with passive acoustic detectors should be considered as part of a comprehensive marine mammal monitoring project in these energetic environments in the context of marine renewable energy development and other industries.
Acoustic recording tags, such as the Digital Acoustic Recording Tag (DTAG), are attached directly to the cetacean, often using suction cups. DTAGs themselves comprise a hydrophone, orientation sensors (accelerometers), memory unit, software, infra-red data offload system, and rechargeable battery enclosed within a plastic skeleton and sealed by urethane sheeting. Unlike any other cetacean monitoring technique, once attached, DTAGs record continuously for up to 24 hours (sometimes longer) traveling with the host to a depth of over 1,900 m. The accelerometers are so sensitive that DTAGs can record individual fluke strokes. When used in conjunction with other cetacean monitoring techniques, DTAGs can bridge the gap between marine mammal underwater behaviour and surfacing contact with MMOs. A study by Tyack et al. (2006) used DTAGs to investigate the acoustic diving characteristics and foraging habits of beaked whales (Ziphiidae) in the Ligurian Sea (Mediterranean) and off the coast of the Canary Islands and were more recently used by Quick et al. (2018) to determine dialects in short‐finned pilot whales (Globicephala macrorhynchus) off North Carolina. Acoustic Tags (A-TAGs) are a different type of acoustic tag and have been used to determine abundance estimates of the Yangtze finless porpoises (Neophocaena phocaenoides asiaeorientalis). Another miniature acoustic recording tag is the Acousonde, developed by scientists at Greenridge Sciences Inc. It has two acoustic channels each of which has its own hydrophone. The tag is also outfitted with a 3D accelerometer, 3D compass, depth transducer, and a temperature monitor. In addition to being used in behavioural studies on marine wildlife, the Acousounde can be used as an autonomous recorder suspended from a cable, placed on the seafloor, or housed in a robotic or remotely operated vehicle.