Measuring ADD Noise in TIdal Streams (MANTIS): Could Acoustic Deterrent Devices (ADDs) reduce risk of marine mammal collisions with tidal turbines?

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Expansion of the tidal-stream marine renewable energy (MRE) sector is desirable but poses potential risks to the surrounding marine environment. One potential risk involves marine mammals and other species colliding with rotating tidal turbine blades. Concern about potential collision-driven mortality or injury of these charismatic protected species has led to regulators (including project partners Marine Scotland [MS], Scottish Natural Heritage [SNH] and Natural Resources Wales [NRW]) adopting a precautionary approach when considering consent applications, resulting in a slowdown in MRE site development. If animals are to avoid collision they will need sufficient warning of turbines' presence to take timely action. With water flow and swimming speeds significantly restricting visual detection ranges, long-range detection (i.e. warning) will be through acoustic means. However there is uncertainty over whether animals can hear turbines over naturally occurring ambient noise in these energetic sites (from water flow, bed load transport and turbulence). As a result, developers and regulators (incl. MS) are considering whether to add extra noise sources to their turbines to ensure animals can perceive and avoid them. Other industries such as aquaculture, fishing and construction use Acoustic Deterrent Devices (ADDs) to scare seals and other animals away from potentially dangerous areas by producing loud, aversive sounds. For this reason ADDs are currently being considered as a mitigation method to reduce potential collision risk among tidal turbines. However, concerns exist regarding potential undue long-term exclusion of animals from large areas, especially as tidal-stream MRE turbines are intended to operate for decades rather than short periods. Furthermore it is not clear over what range ADDs will be audible given the already noisy environments targeted for turbine placement, and how this might vary over the tidal cycle. There is therefore an urgent need to assess the detectability (or not) of ADDs in these environments.The overall project objective is to clarify how the detectability of commercially available ADDs in real tidal-stream environments varies across collision-relevant spatial scales (at least tens to hundreds of metres, but potentially further dependent on ambient noise levels) and across the ebb-flood and spring-neap tidal cycles. A detailed analysis of existing acoustic datasets from tidal-stream environments will allow us to understand how ambient noise levels vary across tidal cycles across frequencies relevant to marine mammal hearing. This will allow development of improved range-dependent sound propagation models describing ADD signal transmission in tidal-stream environments. The transmission of different ADD signals will then be experimentally tested under varying flow conditions to assess ranges of acoustic detectability, validate propagation models and consider likely impacts on marine mammals. This will improve our understanding of whether or not ADDs could provide a suitable mitigation technique to reduce potential collision rates of marine mammals with tidal turbines. The impact of the expected outcomes of this proposal will be to allow the regulatory project partners (Marine Scotland, Scottish Natural Heritage and Natural Resources Wales) to develop long-term evidence-based policies concerning the use of ADDs as a practical and proportionate means to mitigate marine mammal collision risk potentially posed by tidal turbines. Such policies will contribute towards long-term environmental sustainability of the MRE sector and improve Scotland's and Wales' ability to fulfil their (inter-)national obligations in terms of conservation of marine mammals and other mobile marine species.