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Short-term disturbance by a commercial two-dimensional seismic survey does not lead to long-term displacement of harbour porpoises
Authors:Paul M. Thompson  Kate L. Brookes  Isla M. Graham  Tim R. Barton  Keith Needham  Gareth Bradbury  Nathan D. Merchant
Affiliation:1.Lighthouse Field Station, Institute of Biological and Environmental Sciences, University of Aberdeen, Cromarty IV 11 8YL, UK;2.Kongsberg Maritime Ltd, 11 The Briars, Waterberry Drive, Waterlooville, Hampshire PO7 7YH, UK;3.WWT Consulting, Slimbridge, Gloucestershire GL2 7BT, UK;4.Department of Physics, University of Bath, Bath, Somerset BA2 7AY, UK
Abstract:Assessments of the impact of offshore energy developments are constrained because it is not known whether fine-scale behavioural responses to noise lead to broader-scale displacement of protected small cetaceans. We used passive acoustic monitoring and digital aerial surveys to study changes in the occurrence of harbour porpoises across a 2000 km2 study area during a commercial two-dimensional seismic survey in the North Sea. Acoustic and visual data provided evidence of group responses to airgun noise from the 470 cu inch array over ranges of 5–10 km, at received peak-to-peak sound pressure levels of 165–172 dB re 1 µPa and sound exposure levels (SELs) of 145–151 dB re 1 µPa2 s−1. However, animals were typically detected again at affected sites within a few hours, and the level of response declined through the 10 day survey. Overall, acoustic detections decreased significantly during the survey period in the impact area compared with a control area, but this effect was small in relation to natural variation. These results demonstrate that prolonged seismic survey noise did not lead to broader-scale displacement into suboptimal or higher-risk habitats, and suggest that impact assessments should focus on sublethal effects resulting from changes in foraging performance of animals within affected sites.
Keywords:Phocoena phocoena   oil and gas exploration   acoustic disturbance   marine mammal conservation   marine spatial planning
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