Responses of bottlenose dolphins and harbour porpoises to impact and vibration piling noise during harbour construction

The development of risk assessments for the exposure of pro tected population s to noise fromcoastal construction is constrained by uncertainty over th e nature and extent of marine mammalresponses to man-made noise. Stakeholder concern often focuses on the potential for local displa cementcaus ed by impact piling, whe re piles are hammered into the seabed. To mitigate this threat, use of vibra-tion piling, where pil es are shaken int o place with a vibratory hammer, is often encouraged due to pre-sumed impact reduction. However, da ta on comparative responses of cetaceans to these different noisesources are lacking. We stu died the responses of bottlenose dolphins an d harbor porpoises to both impa ctand vibration pile driving noise during harbor constructi on works in northeast Scotland, using passiveacoustic monitoring devices to record cetacean activity and noise recorders to measure and predictreceived noise levels. Local abundance and patterns of occurrence of bottlenose dolphins were also com-pared with a five-year baselin e. The median peak-to-peak source level estim ated for impact piling was240 dB re 1 lPa (single-p ulse sound exposur e level [SEL] 198 dB re 1 lPa2s), and the r.m.s. source levelfor vibration piling was 192 dB re 1 lPa. Predicted received broadband SEL values 812 m from the pilingsite were markedly lower due to high propagation loss: 133.4 dB re 1 lPa2s (impac t) and 128.9 dB re1 lPa2s (vibration). Bottlenose dolphins and harbor porpoises were not excluded from sites in th e vicin-ity of impact piling or vibration piling; nevertheles s, some small effects were detected. Bot tlenose dol-phins spent a reduced period of time in the vicinity of construction works during both impact andvibration piling. The prob ability of occurrence of both cetacean species was also slightly less during peri-ods of vibration piling. This work provides developers and managers with the first evidence of the com-parative effects of vibration and impact piling on small cetaceans, enabling more informed riskassessments, policy frameworks, and mi tigation plans. In particular, our results emphasize the need forbetter understanding of noise levels and behavioral responses to vibration piling before recommendingits use to mitigate impact piling.