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

Abstract

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.

Applying Two Active Acoustic Technologies to Document Presence of Large Marine Animal Targets at a Marine Renewable Energy Site

“…Three-dimensional distribution of animals in the water column is a data gap and a need for regulators. These data also provide information on depth usage by marine animals, which is missing from surface-based observations and often from passive vocalization methods, though some studies do conduct 3D localization [23]. Depth use of monitored organisms will be important in areas where MRE devices are deployed [9,11,37,[40][41][42].…”

“…This method has limitations as well, e.g., not all mammals vocalize all the time and vocalizations can be difficult to link to species or individuals [22]. Hydrophone data also have the additional complication of background noise in energetic environments, such as channels with high-velocity tidal currents, which impedes marine mammal detection [23]. Preliminary empirical data are needed to better understand large animal interactions with MRE devices but collecting such data will require new survey methods.…”

Real-Time Continuous Acoustic Monitoring of Marine Mammals in the Mediterranean Sea

“…As another example of near-real-time monitoring, there is the system implemented between 2017 and 2019 in Scotland in the Gills Bay Port, where tidal energy generators turbines were active [11]. Twelve hydrophones arranged in three small tetrahedral clusters were deployed on an operational commercial-scale tidal turbine.…”

Marine Mammal HiCUP: A High Current Underwater Platform for the Long-Term Monitoring of Fine-Scale Marine Mammal Behavior Around Tidal Turbines

“…Underwater cameras have been used to some extent, but have limited fields of view, low detection ranges, and can only operate effectively in daylight. Potential alternative tools which have the capacity to detect and track marine mammals include active and passive acoustic systems (Williamson et al, 2015;Malinka et al, 2018;Hastie et al, 2019a;Gillespie et al, 2020;Polagye et al, 2020).…”