Passive acoustic methods for tracking the 3D movements of small cetaceans around marine structures

Abstract

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

Harbour porpoise (Phocoena phocoena) presence is reduced during tidal turbine operation

“…Noise varied by up to 20 dB between low and high flows and a 20 kHz tonal sound was present when the turbine was generating power (Gillespie et al, 2020). Whilst rotation of the blades was usually associated with power generation, there were brief periods of rotation without power generation and therefore the 20 kHz tone was not present (574 h; 5.1% of total study period).…”

“…The PAM system and its performance are described in Gillespie et al (2020). The system consists of three tetrahedral hydrophone clusters, one mounted on each leg of the TSS (Figure 2).…”

“…When triggered, the detector stored short (~1 ms) clips of unfiltered data. The 40 kHz lower frequency bound was selected as there was a strong band of noise present in the data at lower frequencies (Gillespie et al, 2020). While not optimal for the detection of dolphin clicks, which can have peak frequencies at lower frequencies (Soldevilla et al, 2008), the click detector is still triggered by higher‐frequency components of the clicks.…”

“…As described, the PAMGuard click detector was triggered by transient signals in the 40–150 kHz frequency band that increased >10 dB above background noise. Noise in the click detector band increased by up to 20 dB from slack tide to high flows (Gillespie et al, 2020); therefore, the absolute detection threshold was higher (and click detection less likely) during periods of high tidal flow and, if not accounted for, would confound perceived patterns in porpoise presence. Therefore, a single, high noise level and corresponding absolute detection threshold were selected and clicks with received levels below that detection threshold were discarded to provide a uniform probability of detection over time.…”

Collision risk modelling for tidal energy devices: A flexible simulation-based approach

“…<50 m) would improve our understanding of collision risk (Wilson et al, 2006). Developments to quantify and better understand underwater movement of animals in these tidally energetic environments is being addressed through three different technologies; active acoustic monitoring (Hastie et al, 2019;Lieber et al, 2014;Williamson et al, 2017), passive acoustic monitoring (Gillespie et al, 2020;Macaulay et al, 2017;Malinka et al, 2018) and through animalborne loggers (Gabaldon et al, 2019;McKnight et al, 2019). These methods provide information on how animals move in three-dimensional space, which will provide valuable empirical data, such as angle of approach, animal speed, dive profile, and avoidance and evasion behaviours, that can be used to better inform this simulation-based approach to collision risk models.…”