A temporary displacement of porpoises was observed by pile driving for offshore wind turbines, but it is not known what effects this has on a population level. Given the measured and modelled noise levels, the assessment of the disturbance distance for porpoises during pile driving and their seasonal density (see Descriptor 1), a number of measures were taken to limit exposure and consequences for porpoises: 1) If the underwater noise level (Lz-p ) during pile driving is higher than 185 dB re 1μPa at 750 m from the source, noise mitigation measures must be applied, such as bubble curtains or an alternative technique that produces less noise. 2) Piling may not take place between January 1 and April 30, the period with the highest density of porpoises in Belgian waters (this measure took effect from 2014). 3) To prevent exposure, preventive measures such as acoustic deterrents and a procedure for gradually increasing the energy of the hammer must be applied. 4) Piling cannot be started or continued if marine mammals are observed in the vicinity of the sanctuary (200 - 500 m, depending on visibility and the type of marine mammals).

Due to the currently limited data set, no trend in environmental noise levels can be determined.