The greatest impact on marine mammals from offshore wind is widely considered to occur primarily during the construction phase when turbine foundations are being driven into the seabed. Marine mammals rely heavily on sound to navigate, feed and conduct social interactions, and are sensitive to increased underwater noise. Potential effects (of piling noise on marine mammals) include lethal effects and physical injury, auditory injury and behavioural displacement. Standard mitigation should be effective in negating or significantly reducing the potential for lethal effects and physical injury, but even with mitigation, PTS onset and behavioural displacement may occur. Because the majority of the currently proposed offshore wind farms consist of several hundred rather than tens of turbines, pile driving may be carried out over a period of years rather than months, and therefore the potential for longer term effects on marine mammals may exist. The consideration of long term impacts is important for both EIA and HRA, where an assessment against a designated site’s conservation objectives ‘in the long term’ is required. We used data from a proposed UK offshore wind farm – Inch Cape (Scottish east coast) – to investigate the potential for long term effects of noise from piling on bottlenose dolphins and harbour seals (SACs have been designated locally for both species). Noise impact contours were modelled by Subacoustech Environmental Ltd using their INSPIRE model. The harbour seal density surface was produced by SMRU using their telemetry and haul out count data while the bottlenose dolphin density surface was inferred using the most appropriate information available. The numbers of animals predicted to be affected by PTS onset and behavioural displacement were then estimated using a dose-response approach. Because the consequences of PTS onset and behavioural displacement were unknown, assumptions had to be made (about how exposure to piling noise might influence demographic parameters) in order to model potential impacts on population dynamics. We assumed that the mortality risk of PTS onset was likely to be similar to that of old age (and ‘harvested’ 25% of the animals estimated to be exposed to SELs sufficient to induce PTS onset in each year in the PVA). Behavioural displacement was assumed to result in breeding failure due to a reduction in condition of breeding females (and a reduction in reproduction proportional to the percentage of the population that had the potential to be displaced in each year was modelled in the PVA). All assumptions were conservative and the magnitude of impact considered was always worst case. PVA modelling indicates that worst case scenarios could lead to short term impacts upon population size, but not to longer term impacts on population viability. Application of PVA to inform EIAs and HRA has proved useful and may be further enhanced through development of clear guidance to aid interpretation of the modelled outputs by the SNCBs.
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