A wide range of anthropogenic structures exist in the marine environment with the extent of these set to increase as the global offshore renewable energy industry grows. Many of these pose acute risks to marine wildlife; for example, tidal energy generators have the potential to injure or kill seals and small cetaceans through collisions with moving turbine parts. Information on fine scale behaviour of animals close to operational turbines is required to understand the likely impact of these new technologies. There are inherent challenges associated with measuring the underwater movements of marine animals which have, so far, limited data collection. Here, we describe the development and application of a system for monitoring the three-dimensional movements of cetaceans in the immediate vicinity of a subsea structure. The system comprises twelve hydrophones and software for the detection and localisation of vocal marine mammals. We present data demonstrating the systems practical performance during a deployment on an operational tidal turbine between October 2017 and October 2019. Three-dimensional locations of cetaceans were derived from the passive acoustic data using time of arrival differences on each hydrophone. Localisation accuracy was assessed with an artificial sound source at known locations and a refined method of error estimation is presented. Calibration trials show that the system can accurately localise sounds to 2m accuracy within 20m of the turbine but that localisations become highly inaccurate at distances greater than 35m. The system is currently being used to provide data on rates of encounters between cetaceans and the turbine and to provide high resolution tracking data for animals close to the turbine. These data can be used to inform stakeholders and regulators on the likely impact of tidal turbines on cetaceans.

13A wide range of anthropogenic structures exist in the marine environment with the extent of 14 these set to increase as the global offshore renewable energy industry grows. Many of these 15 pose acute risks to marine wildlife; for example, tidal energy generators have the potential to 16 injure or kill seals and small cetaceans through collisions with moving turbine parts. Information 17 on fine scale behaviour of animals close to operational turbines is required to understand the 18 likely impact of these new technologies. There are inherent challenges associated with 19 measuring the underwater movements of marine animals which have, so far, limited data 20 collection. Here, we describe the development and application of a system for monitoring the 21 three-dimensional movements of cetaceans in the immediate vicinity of a subsea structure. The 22 system comprises twelve hydrophones and software for the detection and localisation of vocal 23 marine mammals. We present data demonstrating the systems practical performance during a 24 deployment on an operational tidal turbine between October 2017 and October 2019. Three-25 dimensional locations of cetaceans were derived from the passive acoustic data using time of 26 arrival differences on each hydrophone. Localisation accuracy was assessed with an artificial 27 sound source at known locations and a refined method of error estimation is presented. 28Calibration trials show that the system can accurately localise sounds to 2m accuracy within 20m 29 of the turbine but that localisations become highly inaccurate at distances greater than 35m. The 30 system is currently being used to provide data on rates of encounters between cetaceans and 31 the turbine and to provide high resolution tracking data for animals close to the turbine. These 32 data can be used to inform stakeholders and regulators on the likely impact of tidal turbines on 33 cetaceans. 34

Tidal energy generators have the potential to injure or kill marine animals, including small cetaceans, through collisions with moving turbine parts. Information on the fine scale behaviour of animals close to operational turbines is required to inform regulators of the likely impact of these new technologies. Harbour porpoise movements were monitored in three dimensions around a tidal turbine for 451 days between October 2017 and April 2019 with a 12‐channel hydrophone array. Echolocation clicks from 344 porpoise events were localized close to the turbine. The data show that porpoises effectively avoid the turbine rotors, with only a single animal clearly passing through the rotor swept area while the rotors were stationary, and none passing through while rotating. The results indicate that the risk of collisions between the tidal turbine and porpoises is low; this has important implications for the potential effects and the sustainable development of the tidal energy industry.

Uptake of tidal turbine technology to generate renewable energy has been partly limited by poor understanding of ecological impacts, including the potential for collisions between cetaceans and rotating turbine blades. To address this concern, it is necessary to identify whether cetaceans behaviourally respond to operating turbines. A turbine in Scotland was instrumented with hydrophones to detect cetacean vocalizations. A generalized additive model was used to investigate temporal variability in harbour porpoise presence close to the turbine. As there were incidentally periods when the turbine was not operating, it was possible to determine the effect of blade rotation, whilst accounting for the potentially confounding effect of tidal flow. Harbour porpoise presence varied intra‐annually, diurnally and with tidal state. Peak presence occurred during winter (September–February), at night and at high flow speeds on the flood tide. Porpoises exhibited significant avoidance of the tidal turbine when it was operating; avoidance increased with flow speed, whereby mean porpoise presence was reduced by up to 78% (95% CIs, 51%, 91%) on the flood tide and up to 64% (95% CI, 3%, 91%) on the ebb tide. The temporal variability in encounter rate in the present study highlights that collision risk assessments assuming static densities probably fail to capture the temporal variability of collision risk. Future studies should conduct long‐term baseline monitoring to derive encounter rates at larger spatio‐temporal scales and as a reference from which to measure change in habitat use. It is also critical that the generality of the avoidance rates presented here is assessed for other sites, turbine types, array sizes and cetacean species. As the tidal industry expands, it will be important to reconcile the benefits of avoidance responses from a collision risk perspective with potential chronic effects of displacement from, or barriers between, important habitats.