Acoustic Mitigation Devices (AMDs), also known as Acoustic Deterrent Devices (ADDs), Acoustic Harassment Devices (AHDs), ‘pingers’, ‘scarers’ or ‘scrammers’ are designed to create a noise aversive enough to cause an animal to move away from a specific site, with the intention of preventing it from being present during anthropogenic activities (Schakner & Blumstein 2013). Acoustic Mitigation Devices have been used commonly in aquaculture and the fishing industry, in an attempt to prevent by-catch, and to minimise conflicts with humans (Götz. & Janik 2013). More recently, AMDs have been implemented for mitigation during high-intensity activities offshore, such as pile driving during wind farm construction. Acoustic Mitigation Devices, along with real time detection of marine mammals by dedicated Marine Mammal Observers (MMOs) (www.marinemammalobserver.com) and Passive Acoustic Monitoring (PAM) Operators (www.passiveacousticmonitoring.com), are becoming more common, especially as the renewable energy industry expands, and increasing numbers of offshore wind farms are constructed.
ACOUSTIC MITIGATION DEVICE DESIGN
Many factors need to be taken into consideration when designing an Acoustic Mitigation Device. For example, it is important that sounds produced are aversive enough to deter target species, whilst minimising potential of causing hearing damage, or impacts on non-target species.
Source levels vary considerably. Generally, although the distinction is less defined now than it used to be, sounds emitted by Acoustic Deterrent Devices have a lower source level than those produced by Acoustic Harassment Devices, the appellation of which is being discouraged, but is still used occasionally (Tixier et al. 2015).
Hearing ranges of marine mammal species vary substantially (see Chapter One of the Marine Mammal Observer and Passive Acoustic Monitoring Handbook for a summary); therefore to be effective, frequencies of Acoustic Mitigation Devices also differ depending on target species. Those designed to deter harbour porpoises (Phocoena phocoena) from industrial or commercial fishing sites emit sounds that are, for example, higher frequency than those intended to keep seals from aquaculture facilities.
Devices also need to be designed with habituation (http://en.wikipedia.org) in mind. If the same sounds are played repeatedly over a prolonged period, it is likely marine mammals will adjust to the noise, and reactions will reduce over time. Therefore, to be effective in the long-term, sounds should be variable and irregular.
For a review of the acoustic characteristics of some devices used commonly see Kastelein et al., 2010.
AMDs AND MARINE MAMMAL MITIGATION
The use of Acoustic Mitigation Devices during pile driving for wind farm construction is a mandatory mitigation technique in some countries such as the Netherlands or Germany, or use may be determined by on project specifications; for a review of international worldwide regulations, see ICES (2010) and Erbe (2013). When forming part of an additional mitigation procedure, a licence should be obtained, and their use justified (www.gov.uk).
For pile driving operations, Acoustic Mitigation Devices are often used in addition to a pre-piling search of at least 30 minutes by Marine Mammal Observers (www.marinemammalobservers.co.uk) and/or Passive Acoustic Monitoring (PAM) Operators (www.pamoperator.co.uk), and a soft-start of gradual increasing power on the pile driver of no less than 20 minutes, depending on local (country-specific guidelines) procedures recommended by governmental committees.
Ocean Science Consulting Ltd (OSC) (www.osc.co.uk) supplies high performance pingers with an output that meets regulation (EC) 812/2004. Devices can be spaced as far apart as 200 m, but OSC can calculate the optimal number of pingers required to satisfy specific mitigation requirements stated in licences/permits or by government authorities. Pingers spaced more closely ensonify an area more effectively.
OSC’s devices operate continuously (i.e. no need to activate in the field), typically for around 2 years’ on a single C-cell battery, which can be changed easily. pingers are small, light, brightly coloured, and have a durable carrier with attachment points on either end, so they are easy for field personnel to deploy. In field studies, these pingers have proved >80% effective at excluding harbour porpoises, the species that authorities are often concerned about, but these pingers also exclude dolphins. The German BfN authority’s standards are arguably the strictest in Europe, so OSC tends to use their mitigation requirements as a benchmark, and these pingers are compliant.
Dr Victoria Todd (www.victoria-todd.com), OSC’s Chief Scientist, is probably the only individual in the world who has managed to coax several major Acoustic Mitigation Device manufacturers onto the same field trial, so we know which technologies work; consequently, OSC can supply what is arguably the best system available, which is easy to install and operate.
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