Between 1999 and 2006 harbour porpoises were studied by ship surveys to evaluate their use of Horns Reef, and the potential effect of construction and operation. At both Horns Rev and Nysted Offshore Wind Farms the potential effect of construction and operation was also investigated by means of acoustic monitoring from 2001 to 2005. Only a slight decrease in porpoise abundance was found at Horns Reef during construction and no effect during operation of the wind farm was observed. A clear decrease in the echolocation activity of porpoises was found at Nysted during construction and operation of the wind farm. This effect still persisted after two years of operation, however with indications of a slow, gradual recovery. At both wind farms a substantial but short lived effect of pile driving was observed with larger responses at Nysted, where silent periods after pile driving were several days compared to hours at Horns Reef.
The stronger response Horns Rev compared to Nysted may be speculated to be caused by a higher motivation/competition to find food at Horns Rev regardless of the presence of a wind farm. Another explanation could be that the more turbulent and noisy environment at Horns Rev makes the turbines and the noise less detectable to the porpoises.