Real-time porpoise detection
During the construction of offshore wind farms, noise emissions caused by pile driving for wind turbine foundations regularly exceed the sound level at which harbor porpoises may suffer temporary threshold shifts (TTS). Therefore, animals that are potentially present are to be deterred from the vicinity of a construction site before the start of noise-intense activities. The Seiche ‘Wireless Detection System’ (WDS) provides a new passive acoustic monitoring technique that is currently the only system to verify successful harbor porpoise deterrence in real-time and has been tested successfully in the field.
Typically, the deterrence devices 'Pinger' and 'Sealscarer' are deployed to deter harbor porpoises from the injury zone before pile driving begins. Subsequently, the vicinity of the construction site is monitored for the presence of harbor porpoises, and cetacean-porpoise detectors (CPODs, http://www.chelonia.co.uk/) represent the conventional method. CPODs record porpoise echolocation activity, but their data can only be examined after they have been retrieved. WDS buoys on the other hand transmit their data wirelessly and thereby allow real-time monitoring.
Each buoy contains a hydrophone that transmits its signal wirelessly to the monitoring vessel via a telemetry antenna. Incoming signals are digitized onboard the vessel and are processed using the marine mammal detection software ‘PAMGUARD’. This software recognizes specific parameters and can thereby identify candidate harbor porpoise click trains.
WDS buoy setup. Deployed WDS buoy (top, left). The receiving antenna is positioned on top of a cliff (top, center), connected to a receiver box (top, right) and subsequently feeds the signal into a computer with the ‘PAMGUARD’ software. (pictures: Caroline Höschle)
In an initial study, BioConsult SH tested the single detection range of the WDS buoy on wild porpoises and compared its sensitivity to that of conventional CPODs. For this, WDS buoys were deployed alongside CPODs and the respective study area was also visually monitored from a 20 m high cliff for porpoise activity. Ultimately, this study lead to the conclusion that the WDS buoy is comparable to the CPOD in terms of porpoise detection probability and detection range.
In another project, we used nine WDS buoys to monitor the presence of harbor porpoises during the installation of 48 piles that were driven into the seabed for the offshore windfarm NordseeOst. These buoys were deployed at distances of 750 and 1500 m from the construction site of a jacket foundation and in this case the signals were transmitted to and evaluated on board of a nearby ship.
When a harbor porpoise was detected by one of the WDS buoys, it was possible to immediately administer additional deterrence with the ‘Sealscarer’. Consequently, the fundamental advantage of the WDS buoys is that mitigation procedures can be adapted to the situation in the field.