Porpoises and dolphins (Odontoceti) use echolocation to extract information from their surroundings. Dolphins produce clicks in a wide frequency range and are typically short and loud while porpoises are producing longer and weaker clicks in a narrow frequency range (120 - 145 kHz, mode 132 kHz). These clicks can be recorded by the passive acoustic device, C-POD (Chelonia Limited), when a marine mammal is swimming in the vicinity of the recorder. The C-POD can record clicks between 20 and 160 kHz including ambient background noise, sonar and other biotic underwater sound. The key to the performance of the C-POD is detection and classification of series of clicks, so-called click trains. Click trains have distinctive features which are used by the classification algorithms to identify the occurring cetacean species.
Currently (August 2016) eight C-PODs are attached to buoys or anchored to ship wrecks or to artificial hard structures positioned along an east-west gradient covering coastal, midshore and offshore zones of the BPNS.
Left: C-POD anchored to a ship wreck (©VLIZ) - Right: Harbour porpoise (©Kustfotografie.be/Misjel Decleer)