Evaluating effects of wind farm construction on porpoises – a model-based approach

To combat impacts of climate change, offshore windfarm construction is accelerating in many parts of Europe, raising concerns about construction-associated noise impacts on acoustically sensitive species such as the harbour porpoise (Phocoena phocoena). This work investigated potential impacts from the construction of ScottishPower Renewables’ East Anglia ONE (EA1) offshore windfarm, located off southeast England, on the North Sea harbour porpoise Management Unit (MU) using iPCoD and DEPONS population impact modelling frameworks. Both models were parameterised using project-specific input values derived from sound propagation modelling (calibrated by means of empirical broadband recordings) and passive acoustic monitoring of porpoises (C-PODs) collected at the EA1 development site before, during, and after construction. To evaluate model sensitivity to parameter uncertainty, multiple iPCoD and DEPONS scenarios were considered, using different values across several parameters, and modelling over different spatial scales.
Analysis of porpoise detection data in the presence and absence of construction activity revealed clear negative local effects of pin-piling, with an overall decrease in porpoise detections out to 14.0 km from the piling locations. At this distance, the predicted frequency-weighted (for high-frequency cetaceans) received level was 103.0 dB re 1 μPa2s SEL.
Results of the simulations undertaken here revealed that neither iPCoD nor DEPONS model predicted long-term population-level impacts in response to pin-piling activities at EA1. There was no indication that population size would decline, and no differences were apparent between simulated unimpacted reference populations and impacted populations when subjected to multiple scenarios representing both uncertainty in parameter input values and uncertainty in model sensitivity to these input values.
Based on these outcomes, and ignoring all other pressures that may affect population dynamics (e.g. bycatch), it appears likely that a population-level impact of a single windfarm development cannot be demonstrated with either of these modelling approaches. This is largely due to the discrepancy in scale between localised and relatively short-term impact of an individual development and the large and wide-ranging North Sea harbour porpoise MU.
Offshore construction activity co-occurs with other pressures, and cumulative impact assessments are recommended to obtain a more realistic understanding of population-level impacts. There is a risk that ignoring other co-occurring pressures may result in an incomplete picture of simulated long-term population trajectories that should be considered carefully if these models are to be used more widely in environmental impact assessments.