Offshore industrial activities such as seismic surveys, drilling, dredging and pile driving during construction can introduce loud, impulsive sounds into the underwater environment, which might cause auditory damage to marine mammals (Hastie et al., 2019). Under UK legislation (Conservation of Habitats and Species Regulations 2017, Conservation (Natural Habitats, &c) Regulations (Northern Ireland) 1995 (as amended), Conservation (Natural Habitats, &c) Regulations 1994 (as amended in Scotland) and Conservation of Offshore Marine Habitats and Species Regulations 2017), it is an offense to significantly injure or disturb marine mammals; therefore, mitigating underwater noise impacts during industrial operations is required. Marine Mammal Observers (MMOs), and in certain circumstances Passive Acoustic Monitoring (PAM) operators, monitor the area for presence of marine mammals, and advice on mitigation procedures according to the Marine Mammal Mitigation Plan (MMMP) when marine mammals are detected. Noise from some industrial activities such as impact piling has potential to affect marine mammals over a greater distance than is possible to monitor visually or acoustically (Bailey et al., 2010; Hastie et al., 2015; Leunissen and Dawson, 2018). In such instances, additional mitigation measures such as Acoustic Deterrent Devices (ADDs) may be recommended, as suggested in the JNCC piling protocol, to exclude marine mammals from the area.
What are acoustic deterrent devices?
Early ADDs or pingers were devices developed originally to deter small cetaceans, such as harbour porpoise (Phocoena phocoena), from approaching fishing gear, thus reducing entanglements (Northridge et al., 2006). Early acoustic alarms were developed to discourage seals from approaching (and damaging) commercially important fish stocks (Mate and Harvey, 1986). These emitted sounds that were louder than those produced by ADDs and came to be known as Acoustic Harassment Devices (AHDs) or seal scarers, (Johnston and Woodley, 1998). Both devices are relatively simple and use a transducer to convert electrical signals to sound signals, which are then emitted into the underwater environment. More recently, there has been some overlap between the sound levels that these devices produce and the marine mammals being targeted; consequently, both devices are now commonly referred to as ADDs (Schakner and Blumstein, 2013). Additionally, when deployed to deter marine mammals from industrial sites, the devices may be referred to as Acoustic Mitigation Devices (AMDs).
Can acoustic deterrent devices deter marine mammals from offshore industrial sites?
Hearing is a marine mammal’s most acute sense, and is used to navigate, communicate, and forage in the underwater environment (Ketten, 2002). Consequently, it is likely that animals react to any loud sounds within their hearing range, including those produced by ADDs.
The harbour porpoise is probably the species of cetacean most affected by offshore industrial operations due to its common occurrence and wide distribution throughout temperate coastal regions of the northern hemisphere (Bjørge and Tolley, 2018). The harbour porpoise is able to hear sounds over a wide frequency range of 0.3–160 kHz (Southall et al., 2019), and being exposed to industrial noise can likely cause permanent or temporary hearing loss in the lower part of this range (Kastelein et al., 2015). This could reduce ability to passively detect species that could pose a threat, such as killer whales, (Orcinus orca, Cosentino, 2015), bottlenose dolphins, (Tursiops truncatus, Ross and Wilson, 1996), and grey seals, (Halichoerus grypus, Leopold et al., 2015). Multiple studies have found that ADDs are able to deter harbour porpoises (Johnston, 2002; Olesiuk et al., 2002; Brandt et al., 2013a; Dawson et al., 2013; Kastelein et al., 2017; Kindt-Larsen et al., 2019). One study recorded significant deterrent effects up to 7.5 km from a Lofitech seal scarer ADD (Brandt et al., 2013b).
There is evidence that other marine mammal species also avoid acoustic deterrents. Captive common seals (Phoca vitulina) responded to ADDs, including seal scarers, by keeping their heads above the water and hauling out more frequently (Kastelein et al., 2010), and seal numbers decreasing during activation of low-frequency devices around a fish farm on the west coast of Scotland (Götz and Janik, 2015). In the Broughton Archipelago in Canada, presence of killer whales decreased when ADDs were deployed around salmon (Salmo salar) farms over a five-year period (Morton and Symonds, 2002). In the same area, numbers of Pacific white-sided dolphins (Lagenorhynchus obliquidens) also decreased, which was likely linked to the introduction of ADDs (Morton, 2000). Additionally, deploying acoustic deterrent pingers significantly reduced bycatch of Franciscana river dolphins (Pontoporia blainvillei, Bordino et al., 2002) and short-beaked common dolphins, (Delphinus delphis, Barlow and Cameron, 2003), whilst completely eliminating beaked whale bycatch (Carretta et al., 2008). Bottlenose dolphins were also less likely to interact with nets equipped with ADDs (Waples et al., 2013). More recently, minke whales (Balaenoptera acutorostrata) off the coast of Iceland reacted to ADDs at a distance of a 1,000 m, by increasing their speed and moving away from the device (McGarry et al., 2017).
Although there is evidence that ADDs can encourage marine mammals to move away from some areas, the opposite might also occur, since seal scarers are often deployed around fish farms. Seals may thus begin to associate these sounds with increased food supplies, viewing these ADDs as dinner bells (Bordino et al., 2002). In addition, several studies focusing on common seals have found that some individuals might not be affected at all by ADDs (Jacobs and Terhune, 2002; Mikkelsen et al., 2017; Basran et al., 2019). ADDs are therefore often considered a short-term approach of deterring seals from aquaculture sites (Nash et al., 2000). Although, seals will not necessarily have the same motivation to approach seal scarers at offshore industrial sites as they do at fish farms, the devices could still attract seals if associations with food have already been created. Even if seals are not attracted initially, they may become habituated to ADD noise and not move away. Coram et al. (2014), suggested existing aquaculture ADDs are probably unsuitable, when mitigating the effects of industrial noise (e.g. pile driving) on seals.
Limiting auditory damage to marine mammals
Overall, ADDs seem capable of deterring some species of marine mammals; however, further research into why some harbour porpoises have been recorded close to active seal scarers (Brandt et al., 2013b) is required, to explain the reason why current acoustic deterrents do not have similar effects on every individual. Combining acoustic deterrent deployments with more traditional Marine Mammal Observations (MMO) and Passive Acoustic Monitoring (PAM) practices is likely to lower the risk of commencing industrial operations when marine mammals may be close to the sound source. It might also be beneficial to consider dampening the noise around the source, e.g. using air bubble curtains or hydro sound dampers (Dähne et al., 2017), to reduce risk of auditory damage to marine mammals at considerable distances from the industrial site (Hastie et al., 2015).
Ocean Science Consulting Limited (OSC) has extensive experience deploying ADDs during industrial operations and can provide further advice when mitigating effects of industrial noise on marine mammals. OSC is also able to provide experienced personnel, including MMOs and PAM operators for offshore projects.
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