Porpoise click detectors (C-PODs) and icListenHF hydrophones that record with a high sample rate were deployed on a drifter in fast tidal currents in Minas Passage, Bay of Fundy, on six days during June 2017. Harbour porpoise vocalizations were detected within the icListenHF records by using the Coda algorithm and subsequent processing which included a detailed manual review of each click in each click train. The proportion of harbour porpoise detection positive minutes (DPM) was only 0.04 C-PODs compared to 0.19 for the icListenHF hydrophones. Nevertheless, both methods are an incomplete measure of porpoise vocalizations. DPM obtained by the C-PODs had a 65% likelihood of also being classified a DPM from the analysis of icListenHF measurements. Both methods measure some fraction of porpoise vocalization activity so long-term environmental monitoring with either C-PODs or icListenHF hydrophones should serve to discover any substantial change in patterns of porpoise echolocation activity. Nearby active acoustic devices were associated with 38% of the false-positive DPM obtained by C-PODs but Coda did not register such signals as harbour porpoise vocalizations.
It is convenient to deploy C-PODs on bottom-moored subsurface-floats in order to monitor porpoise in fast currents at sites where in-stream tidal turbines are tested. However, C-POD performance degrades when non-target noise quickly fills the memory buffer, resulting in lost detection time. The drifter measurements demonstrated increases in memory loss as current speed increases above 1.5 m/s. Additional lost time might be caused when a C-POD is moored using an unstable tethered subsurface float. The type of mooring may matter because an icListenHF hydrophone attached to a stable bottom platform gave very similar DPM results to those on the drifter.