| Summary: | Harbour porpoises (Phocoena phocoena) face a wide variety of stressors with potentially harmful effects. In this study the impact of pile driving during construction of the offshore wind farm DanTysk was analyzed using behavioural information in click activity recorded by C-PODs (Chelonia Ltd.). Harbour porpoise echolocation activity, landmark orientation and foraging were analyzed in 14 of 80 pile drivings. For each driving swim speeds and distances travelled were compared for the periods before (24 h), during and after pile driving (24 h). This was done based on a visual screening for feeding events and landmark orientation. The results show a distinct decrease in echolocation activity within 12 km radius during the construction of DanTysk. In addition results show that within a distance of 6 km harbour porpoises swam faster, traveled further and showed an increase in foraging activity in the 24 h after pile driving in comparison to before pile driving. Porpoises were recorded in close vicinity already below 20 minutes after cease of pile driving, indicating that not all porpoises evacuated the area of concern, but remained silent. This study is therefore a first indication that behavioural responses of harbour porpoises to pile driving are more complex than previously assumed. A moderately increased travel velocity after pile-driving may be an indication, that porpoises return to the pile driving site. Increased foraging activity on the other hand can be interpreted as a sign of reduced energy intake during pile driving and therefore increased food demands after cease of noisy activities.
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