Abstract

Harbour porpoises (Phocoena phocoena) are notoriously difficult to observe and become very difficult to see in moderate or rough sea
conditions. Passive acoustic monitoring provides a complementary study method to visual observation which is less weather dependent
and can be carried out 24 hours a day. Recordings of free ranging harbour porpoise vocalisations indicate that they emit narrow band
ultrasonic clicks with the centre frequency between 115 and 145kHz. We have been deveIOping equipment to monitor these
vocalisations in the field since 1992. Ultrasonic clicks are filtered and made audible using envelope tracing circuitry. Work on this
automated monitoring system hosted by a portable computer was started in 1992. This equipment was designed to automatically detect
and log acoustic features extracted from small cetacean echolocation click signals. The data are then processed to allow porpoise
vocalisations to be distinguished from other sources of noise including the vocalisations of other cetacean species. Recording the data
directly to a computer allows the system to be deployed from platforms of opportunity by small survey teams, and eliminates problems
of inter-observer bias. Acoustic data have been collected from several areas and the equipment has been deployed off a number of
vessels, including those used for the Small Cetaceans Abundance in the North Sea (SCANS) survey. Results obtained suggested that
acoustic detection rates were comparable to those of an experienced team of visual observers in sea state three, and the frequency of
detections was largely unaffected by sea states of four or below. Field trials have shown that the automated detection system can detect
harbour porpoise vocalisations at sufficient ranges to be a useful survey tool. The equipment has been improved since the SCANS
survey and further developments are considered. These include a multiple hydrophone system to measure bearings to vocalising animals
and application of the equipment to studies of other small cetacean species. So far. the equipment has mainly been used for population
assessment. However, it should also have many applications in behavioural studies, especially those investigating the process of
entanglement.