Odontocete (toothed whales, dolphins, and porpoises) detection is an increasingly important field, as regulatory pressure mounts on offshore industries (e.g. oil & gas exploration and production firms, military sonar operations, wind farm construction, etc.) to minimise effects of anthropogenic noise on marine mammals. The purpose of detecting marine mammals during industrial operations is most often employed for real-time mitigation (www.marinemammalmitigation.co.uk). Dolphin detection is also used for research purposes, such as establishing species distribution and population numbers, and may occur in conjunction with industrial activities. There is, therefore, overlap in the methods used for dolphin detection during mitigation and research.
Dolphin detection methods are developing continuously. Examples of two dolphin detection methods used currently include visual detections by Marine Mammal Observers (MMOs; www.marinemammalobservers.com), and acoustic detections using Passive Acoustic Monitoring (PAM; www.passiveacousticmonitoring.co.uk). Visual and acoustic detection methods are complementary, and are often best used together.
Visual detection is the most common and basic method of detecting dolphins and other marine species. Marine Mammal Observers use specific cues (e.g. feeding birds, splashes, etc.) to assist in the visual detection of dolphins. Certain dolphin behaviours make them easy to detect visually (e.g. bow riding, lob tailing, continuous breaching, and backflips). Once detected, MMOs use distance-measuring techniques (e.g. reticle binoculars) to record the distance to the animal accurately. Accurate range estimation (www.osc.co.uk) is important for both mitigation (e.g. determines if operations need to be delayed or ceased) and research (e.g. distance sampling; http://en.wikipedia.org).
Species identification is also an important aspect of mitigation and research. When a marine mammal is detected, experienced MMOs may be able to determine the type and species of mammal seen based on a variety of characteristics, such as: size, colour, and shape. For example, the hour-glass markings on the side of common dolphin (Delphinus delphis; http://en.wikipedia.org) distinguishes them from other dolphin species such as Atlantic white-sided dolphin (Lagenorhynchus acutus; http://en.wikipedia.org), which can be identified by their characteristic white patch below the dorsal fin, and a second patch, usually yellow or tan in colour, behind the first, on the tail stock.
Visually, the majority of dolphin species tend to differ from other marine mammals by having enlarged dorsal fins relative to body size, and sleeker, less bulky bodies than most whales.
For further information on MMOs, their roles, and research conducted using visual detection, see www.marinemammalobservers.com
Passive Acoustic Monitoring systems come in various guises (www.pamsystem.co.uk), and can be configured to detect specific frequencies, thereby enabling the user to monitor for a particular species’ acoustic signals. For example, bottlenose dolphin (Tursiops truncatus) may socialise using sounds around 300 Hz to 60 kHz; whereas, they use high frequency clicks (known as echolocation) of up to around 150 kHz for navigation and hunting (Cranford and Amundin, 2004). If the goal is to determine the presence of bottlenose dolphin, a PAM system needs to cover both ranges.
Advanced detection systems developed specifically for marine mammal detection may be able to identify and log frequency, duration, bandwidth, and power of each vocalisation, which, in turn, may assist in species identification.
Acoustic detection by Harris (2012), focused on invariant and distinctive features of marine mammal vocal elements, to identify the presence of dolphin, and other marine mammal species. This study compared data gathered from a High-frequency Acoustic Recording Package (HARP) (Wiggins and Hildebrand, 2007), designed specifically for detecting marine mammals, to data gathered from three hydrophones set-up in the Southern California Offshore Range (SCORE) system, which was not developed specifically for detecting marine mammals. Results showed that both systems were able to detect dolphin or whale presence but some data gathered by the non-specialised SCORE system appeared to be ‘modified’, in that the hydrophones appeared randomly more sensitive at different frequencies, leading to over or under emphasis of some acoustic signals. Furthermore, the number of sperm whale (Physeter macrocephalus) clicks detected by the SCORE system seemed greatly reduced, when compared to the HARP data; however, it was later established that the SCORE system had in fact detected sperm whale clicks, but had disregarded them as non-marine mammal noise. It was concluded that, although the SCORE system could be used to alert the user to animal presence, it could not reliably identify or distinguish between species. It is believed that, with adjustment to both hardware configuration and detection parameters, the SCORE system could offer more reliable results; however, it was deemed unlikely to become accurate enough for use as a specialised marine mammal detection system. These findings clearly emphasise the importance of using a dedicated PAM system over a more generalised setup.
Towed vs static acoustic detection
In broad terms, PAM systems can be static (i.e. stationary), or towed (www.towedarray.co.uk). For more information on T-PODs and C-PODs, a type of Static Acoustic Monitoring System (SAMS; www.staticacousticmonitoringsystems.co.uk), please see www.echolocationclickdetectors.co.uk
Dolphin detectors used with SAMS are capable of collecting data easily over large temporal ranges, whereas towed systems are better suited for detecting dolphins over large spatial ranges. Towed arrays can often be subjected to more noise pollution than static arrays, due to propeller/thruster resonance and increased water turbulence as the vessel moves. Simard (2009) found that, in ideal conditions (i.e. calm weather and reduced anthropogenic noise with little unsolicited disturbance), acoustic detection ranges regularly exceeded the minimum 500 m safety zone required during seismic survey mitigation; however, when ambient vessel noises increased, this was not always the case. During periods of increased disturbance, detection range dropped consistently below 300 m for both low and high frequency sounds. In order to combat this, some acoustic detector systems can be configured specifically to disregard excess noise, or in the case of marine mammal noise-processing software (such as PAMGuard, www.pamguard.org/), employ seismic vetos that exclude airgun noise.
Advantages and limitations of marine mammal detection methods
Visual detections of marine mammals require animals to surface, and (with the exception of inefficient infrared techniques) are often limited to daylight hours and good weather conditions. Acoustic detections are less influenced by adverse weather conditions, but require marine mammals to be vocalising. By using both visual and acoustic methods simultaneously, the likelihood of detection and species identification is increased.
Visual detection can be difficult for deeper-diving species such as beaked whales, that may appear only very briefly on the surface, or reveal limited body mass when breathing. Passive Acoustic Monitoring is useful for detecting these species, as they echolocate during their long, deep dives. Shallower-diving species, such as many dolphins, may be detected visually up to several miles away, as they are often very lively at the surface, whereas their vocalisations have much shorter detection ranges. In deeper water, lower frequencies travel further than high frequencies, which dissipate very quickly; this allows the great whales to communicate over vast distances using incredibly low frequency sounds, while porpoises communicate in relatively close-quarters using their ultra-high frequency vocalisations. In general, acoustic detection of medium-sized odontocetes (e.g. dolphins), appears to have a relatively consistent detection range-limit of around 300 m (Parvin et al., 2007).
The ability to detect marine mammals at night has revealed some interesting findings. Using specialised recorders situated at depths between 400-800 m, Au et al. (2013) found that, across five locations chosen around Hawaii, 70–84% of echolocation clicks were detected at night. Todd et al. (2009) also found a similar 24-hur (diel) pattern in harbour porpoise (Phocoena phocoena) clicks around offshore oil & gas installations in the North Sea.