Assessing the Performance of Passive Acoustic Monitoring Technologies for Porpoise Detection in a High Flow Tidal Energy Test Site
Current interests in harnessing tidal energy from Minas Passage, a high flow (up to 6 m/s) channel in the upper Bay of Fundy, require examination of potential effects of tidal turbines on the environment, including impacts on commonly occurring harbour porpoise, Phocoena phocoena (Linnaeus, 1758). To collect baseline data on porpoise presence in the FORCE turbine test site in Minas Passage, two Passive Acoustic Monitoring (PAM) technologies were used: the C-POD (porpoise click logger, Chelonia Ltd) and the icListenHF (digital hydrophone, Ocean Sonics). Prior multi-year PAM studies in the Minas Passage involved C-POD hydrophones housed within streamlined SUB buoys suspended 3m above the seafloor. During high flows, these SUB buoys experienced high variability in tilt. In addition, high-flow induced noise in the Minas Passage exceeded the C-POD’s maximum recordable clicks per minute and thus under-detected harbour porpoise click trains, an effect referred to as lost time. To increase detection efficiency, changes to mooring design and the use of acoustic shrouds were tested. The housing of hydrophones (two icListenHFs and two C-PODs) in a stationary bottom moored platform (i.e. minimal instrument tilt) was shown to reduce the detection of non-target noise and thus reduce time lost. To minimize pseudonoise, various shroud setups were tested before deployment and the best performing shroud (20 ppi, 1.27cm thick acoustic foam) selected for field deployment. The shroud did not significantly reduce pseudonoise within the bandwidth of harbour porpoise clicks (120 kHz to 140 kHz). The icListenHF raw audio data were used to validate C-POD harbour porpoise clicks. A comparison of the performance of both technologies showed that the icListenHF recorded five times the number of C-POD Detection Positive Minutes. We recommend that bottom mounted platforms fitted with cabled digital HF hydrophones be used for monitoring harbour porpoise in and near the FORCE test site.
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