We are happy to announce the release of a new report exploring how noise reduction methods might change the risk of impact associated with pile driving on the North Sea harbour porpoise population.

It is well established that the noise produced by pile driving during the construction of offshore wind farms has the potential to impacts marine mammals. There are several different methods that can be used to try to reduce this impact.  Some of them attempt to limit the noise emitted into the water, for example: air bubble curtains around the piling site can be used to scatter and absorb underwater noise (to reduce the level of noise animals hear). Bubble curtains have been developed and tested in shallow water in Germany where studies have shown that they are successful in reducing the overall levels of sound harbour porpoises are exposed to and therefore can reduce the numbers of animals affected.

The purpose of this study was to explore how useful noise reduction could be in reducing impacts on the harbour porpoise population. To do this we constructed hypothetical situations where the North Sea harbour porpoise population was exposed to piling noise during offshore wind farm construction both with and without noise reduction methods. We then used iPCOD to understand if population level impacts can be reduced by using noise reduction. We approached this by creating a “baseline” model which used information provided in Environmental Statements such as the predicted number of porpoise experiencing hearing damage and behavioural disturbance from each pile driving event across 39 wind farms (spanning 25 years) in the UK North Sea. We then created different hypothetical scenarios where we assumed that noise reduction methods were used at all or only selected the wind farms in order to reduce the impact area and so reduce the number of porpoises disturbed.

Our results showed that using noise reduction measures has the potential to reduce the risk of a population decline caused by cumulative impacts of wind farm construction. The results showed that similar levels of risk reduction can be achieved by large scale reductions of the impact area at a limited number of construction sites, or a smaller reduction in the impact area at a large number of sites. However, it is important to recognise that this report used hypothetical situations to develop a baseline model that was based purely on data provided in Environmental Statements and that every planned windfarm is built over 30 years. And as a result, the amount of potential risk reduction that noise reduction can provide is closely linked to the forecast of risk from the baseline scenario.

So it is important to consider the uncertainties and limitations in this kind of approach – it does not allow for forecasting absolute numbers. It was not the aim of this study to forecast absolute values of a risk of population decline, but rather to investigate if and how the use of noise reduction measures can influence the forecasted risk of a decline.

There are uncertainties in the density estimates of porpoise at development sites, and developers often report only worst-case scenario impacts which are likely to be larger than the actual impacts. There are also differences in the noise impact threshold used in Environmental Statements and so they are not directly comparable. We modelled all 39 offshore wind farms in the UK North Sea as an example – these included operational, consented and not yet consented developments – it is unclear whether each of these wind farms will be consented and built in reality but this served as a useful example dataset for the purposes of this noise reduction project. It is also unclear how noise reduction systems will perform at the deeper water sites planned for offshore wind farm development.

SMRU Consulting’s full report can be viewed here for more information on the methods, results, assumptions and limitations.

A summary of our work was published by WWF-UK here. Please note that the views and opinions expressed in the WWF-UK summary document are those of WWF-UK and do not necessarily reflect the views of SMRU Consulting.

SMRU Consulting are currently working with JNCC and Natural England (with up-to-date planning input from developers) to detail a robust cumulative impact assessment for offshore wind farms off the east coast of England. The goal of that project is to generate a realistic baseline and generate an assessment of the potential for impact of offshore wind farm on the North Sea harbour porpoise population.