Auditory studies on harbour porpoises in relation to offshore wind turbines

The effects of offshore wind turbines on harbour porpoises (Phocoena phocoena) can be studied from different perspectives. Line transect surveys and static or towed acoustic monitoring are valuable tools to describe the status or trend in distribution and abundance of these animals within certain areas and telemetry studies provide insight in the behaviour and habitat use of individual animals. These methods are indispensable in many respects but they are descriptive by nature and can not explain or predict why extend the observed effects occur. In this sense they are complimentary to studies on the cause-effect relationship of the presence of or emissions from offshore wind turbines (OWT’s) and their direct effect on individual animals. Electromagnetic and visual inputs from OWT’s are likely to be negligible in this context, either because of their low strength of emissions or comparatively low sensitivity of harbour porpoises to such stimuli. In contrast the OWT-related acoustic emissions can repeatedly reach extreme intensities. There is a direct and highly relevant link between acoustic emissions and harbour porpoises as these animals have a very acute hearing and rely vitally on this sense. The understanding of noiseinduced effects and data on the tolerance of the animals hearing system to such sounds is critical for the assessment of the overall effect of OWT’s on harbour porpoises.