1. Offshore windfarms require construction procedures that minimise impacts on protected marine mammals. Uncertainty over the efficacy of existing guidelines for mitigating near-field injury when pile-driving recently resulted in the development of alternative measures, which integrated the routine deployment of acoustic deterrent devices (ADD) into engineering installation procedures without prior monitoring by Marine Mammal Observers. 2. We conducted research around the installation of jacket foundations at the UK’s first deep-water offshore windfarm to address data gaps identified by regulators when consenting this new approach. Specifically, we aimed to a) measure the relationship between noise levels and hammer energy to inform assessments of near-field injury zones, b) assess the efficacy of ADDs to disperse harbour porpoises from these zones. 3. Distance from source had the biggest influence on received noise levels but, unexpectedly, received levels at any given distance were highest at low hammer energies. Modelling highlighted that this was because noise from pin pile installations was dominated by the strong negative relationship with pile penetration depth with only a weak positive relationship with hammer energy. 4. Acoustic detections of porpoises along a gradient of ADD exposure decreased in the 3-hours following a 15-minute ADD playback, with a 50% probability of response within 21.7 km. The minimum time to the first porpoise detection after playbacks was > 2 hours for sites within 1 km of the playback. 5. Our data suggest that the current regulatory focus on maximum hammer energies needs review, and future assessments of noise exposure should also consider foundation type. Despite higher piling noise levels than predicted, responses to ADD playback suggest mitigation was sufficiently conservative. Conversely, strong responses of porpoises to ADDs resulted in far-field disturbance beyond that required to mitigate injury. We recommend that risks to marine mammals can be further minimised by: 1) optimising ADD source signals and/or deployment schedules to minimise broad-scale disturbance; 2) minimising initial hammer energies when received noise levels were highest; 3) extending the initial phase of soft start with minimum hammer energies and low blow rates.Minhyuk Seo

Data consist of the following 16 files/file packages. A full description of data identifiers, R code and the data files required to repeat each analysis is provided in the text file: Thompson_BOWL_cMMMP_Data_Packages_Readme.txt Raw data for analyses of echolocation detections are available at: https://doi.org/10.5061/dryad.5qg30sd 1_Thompson_BOWL_cMMMP_noise_wav_files_data_2020-10-12.zip 2_Thompson_BOWL_cMMMP_piling_pulse_data_2020-10-12.txt 3_Thompson _BOWL_cMMMP_R_code_to_analyse_piling_noise_variation_2020-10-12.R 4_Thompson_BOWL_cMMMP_piling_times_data_2020-10-12.txt 5_Thompson_BOWL_cMMMP_ADD_wav_files_data_2020-10-12.zip 6_Thompson_BOWL_cMMMP_R_code_to_prepare_porpoise_response_data-2020-10-12.R 7_Thompson_BOWL_cMMMP_ADD_experiment_data_2020-10-12 8_Thompson_BOWL_cMMMP_Distances_between_ADD_tests_&_CPODs_data_2020-10-12.txt 9_Thompson_BOWL_cMMMP_CPOD_porpoise_ClickCounts_data_2020-10-12 10_Thompson_BOWL_cMMMP_R_code_to_analyse_porpoise_responses_to_ADD_2020-10-12.R 11_Thompson_BOWL_cMMMP_Porpoise_responses_to_ADD_experiments_data_2020-10-12.txt 12_Thompson_BOWL_cMMMP_R_code_to_prepare_porpoise_return_times_data_2020-10-12.R 13_Thompson_BOWL_cMMMP_CPOD_porpoise_ClickDetails_data_2020-10-12.zip 14_Thompson_BOWL_cMMMP_expt_return_times_CPOD_data_2020-10-12.txt 15_Thompson_BOWL_cMMMP_baseline_return_times_CPOD_data_2020-10-12.txt 16_Thompson_BOWL_cMMMP_R_code_to_analyse_porpoise_return_times_after_ADD_playbacks_2020-10-12.R

Piling noise and Acoustic Deterrent device signals were measured using calibrated broadband noise recorders (Soundtrap ST300HF, Ocean Instruments) moored 2 m above the seabed. An array of moored echolocation detectors (V.0 and V.1 CPODs (www.chelonia.co.uk)) was used to assess variation in harbour porpoise detections in relation to experimental ADD exposure. Data on the timing of piling operations, pile-hammer energies used, and pile penetration depths were provided by the wind farm developer (Beatrice Offshore Wind Ltd).