Abstract
Passive Acoustic Monitoring (PAM) technologies are commonly used to monitor echolocating marine mammals around tidal energy devices. However, the detection efficiency of PAM instruments can be hindered by a variety of factors (e.g., signal attenuation, flow noise, ambient noise) inherent to high flow environments that can vary with deployment depth, and can impede monitoring efforts. While previous work indicated that conventional hydrophones that record raw pressure time series data may be preferrable for monitoring harbour porpoise in tidal channels, where these technologies should be deployed for effective monitoring (i.e., at the sea surface or on the sea floor) remains an unresolved issue.
In partnership with the Pathway Program, Sustainable Marine Energy Canada Ltd. and the Fundy Ocean Research Center for Energy assessed the relative performance of a surface deployed and bottom-mounted conventional hydrophone to understand whether deployment location impacted the detection range of the instrument. An icListen HF hydrophone was deployed about 2 m below the surface from a floating tidal energy platform (i.e., PLAT-I) and bottom-mounted on an autonomous subsea platform 65 m from the PLAT-I in Grand Passage, NS. A series of passive drifts were then conducted from a vessel over the platform and in the vicinity of the PLAT-I across a range of tidal flow conditions while playing synthetic harbour porpoise clicks (‘pseudo clicks’) emitted from an icTalk. The drifts measured by the surface deployed hydrophone occurred in August 2020; the drifts measured from the bottom-mounted hydrophone occurred in January 2020. We found that it was possible to detect pseudo-clicks and real harbour porpoise clicks from both hydrophone locations. However, data from the surface deployed hydrophone contained audible interference from waves and the broadband, impulsive, bursting of bubbles associated with wave action that is difficult to differentiate from echolocation clicks. This ambient noise will negatively affect automated porpoise click detectors and could lead to increased rates of false-positive detections. The surface-deployed hydrophone also had substantial electrical noise in the data which could affect automated detectors. The drifting vessel had to stay clear of the PLAT-I whereas it could pass directly over the bottom-mounted hydrophone. These differences in drift geometry made the comparison of detection ranges challenging. Pseudo clicks were detected at greater distances from the bottom-mounted than the surface deployed hydrophone. However, it is important to bear in mind that these results were generated using synthetic clicks generated by an icTalk (nearly omnidirectional), and that real harbour porpoise emit a stronger echolocation click using a directional beam. Further, understanding the effects of current velocity on the quality of the icTalk signal could help with interpretation of results.
The choice of which PAM instrument to use and where to deploy it depends on the scientific question being asked. A primary objective of the Pathway Program is to define, test and validate an environmental effects monitoring solution that can be used by tidal energy developers for monitoring the near-field (0 - 100m) region of their tidal energy device at the FORCE demonstration site. Both hydrophone mounting locations were able to detect low- 4 power pseudo-clicks close to or longer than 100 m, and thus satisfy the near-field monitoring requirement. While it is possible to detect harbour porpoise clicks using a surface deployed hydrophone, the detection range for automated detectors may be smaller than a bottommounted hydrophone due to impulsive ambient noise associated with wave action at the surface. Moreover, harbour porpoise clicks are directional and are typically produced while diving and foraging at depth and are less likely to be detected by a surface deployed hydrophone. However, for a surface-deployed turbine, such as the PLAT-I, having the monitoring hydrophone close to the turbine depth may provide more relevant data than a bottom mounted hydrophone. An important consideration in selecting a monitoring technology is whether near-real-time data are required or if archival results provided several months after collection is sufficient. For real-time data, a hydrophone mounted on the turbine platform is much more economically sustainable than a separate monitoring platform with its own power and data cable. For archival data analysis and reporting, especially for bottom mounted turbines and for prototyping programs, a separate bottom mooring for the monitoring equipment may be a better solution based on cost and performance.
Given these considerations, the results of these measurements did not provide sufficient evidence to strongly prefer one hydrophone position over another. Rather, developers are encouraged to demonstrate that they are able to detect pseudo-clicks in the turbine’s nearfield using a drifting projector. When cabled hydrophones are to be used, developers need to safeguard against acoustic and electronic contamination from equipment on their tidal energy devices.