Towards a numerical model to simulate the observed displacement of harbour porpoises Phocoena phocoena due to pile driving in Belgian waters

The impact of impulsive pile driving on marine mammals is a major environmental concern in offshore wind farm construction. Odontocetes, depending on emission and reception of sound for foraging, spatial orientation and social interactions, are likely to be impacted most. In Belgian and adjacent waters, specific concerns exist about the impact of underwater sound generated during pile driving on the harbour porpoise Phocoena phocoena, the most common cetacean in these waters. The results of visual aerial line-transect surveys performed before and during pile driving at the Thorntonbank (Belgian waters, North Sea) suggested a displacement of porpoises to a distance of around 20 km from the piling location. Such an apparent large-scale avoidance reaction is similar to the one observed in other countries' waters in the North Sea. Using survey data, we developed a simple numerical model that could reproduce the harbour porpoises' redistribution pattern during disturbance. The further development of our initial model can be useful to predict or simulate the redistribution of harbour porpoises in cases when field investigations are not possible or impractical, for pre-construction environmental impact assessments and for impact assessments in case of multiple piling operations ongoing in the same area.