Abstract

This report presents results of field trials of the POD, carried out in Pembrokeshire, UK, from August to December 1998. The POD is an acoustic data logger, designed to operate unattended and detect the sonar pulses of the harbour porpoise (Phocoena phocoena). The POD stores the number of clicks per logging interval that meet specified detection criteria. Upon retrieval from the sea, data are downloaded to a PC.

The POD successfully detected porpoises and reliably distinguished between porpoise sonar clicks and all other biological and physical sources of sound tested. These included boat noises, echo-sounders, “snapping shrimps” and three species of dolphins.

The POD also detected the sonar clicks of bottlenose and Risso’s dolphins, although the immunity of these detections from false triggers caused by other sources of noise was not tested. The duration of each trial was limited by battery life to approximately 90 hours. However, the POD has sufficient memory to collect data for 40 days or more at the settings used during these trials, given a larger capacity battery which would also require a larger housing.

Porpoise activity was recorded at the study site for 8% overall of 568h monitoring effort between August and December 1998. A total of 261 discrete encounters were recorded, several of more than 2h duration and the longest lasting over 4h.

Porpoises were present at the study site for a significantly greater proportion of time at night than at other periods of the day, and the detection rate of porpoise echolocation clicks (a measure of porpoise activity) was significantly greater at night than during the day. Porpoise encounters were significantly longer at night than during the day; there was a greater proportion of ‘transient’ encounters during the day. Porpoise activity was also significantly influenced by state of tide. Porpoises were present for significantly more of the time during night-time ebb tides than at other times of the day or night.

A striking coincidence of harbour porpoise and bottlenose dolphin activity was recorded during one field trial. The presence of both species at the site for extended periods, strongly influenced by diurnal and tidal factors, suggested the use of the trial site for foraging. POD data and post-mortem results from a harbour porpoise found dead at the study site support the hypothesis that porpoises may occasionally be killed by bottlenose dolphins in response to competition for prey. The proportion of time for which porpoises were present at the study site, between August and December, was highest in early December when porpoises were detected in approximately 22% of successive 1min periods. This may reflect the presence of a seasonally important prey resource (herring) at the site.

Both the proportion of 1min logging intervals in which porpoises were detected and the overall rate of click detections, provide numeric indices of porpoise activity and site use. The former method however, is thought more suitable for long term habitat monitoring, whilst the latter is a more sensitive measure of echolocation activity.

The use of the POD revealed previously unsuspected patterns of porpoise habitat use in Newport Bay by allowing data collection over complete 24h periods and for several successive days, thereby providing information unavailable to visual observers.

We conclude that the POD is a suitable tool for the surveillance and monitoring of harbour porpoise habitats, the investigation of porpoise behaviour in the vicinity of fishing gear and the response of porpoises to human disturbance.