Using T-PODs to assess variations in the occurrence of coastal bottlenose dolphins and harbour porpoises

1. Assessments of anthropogenic impacts on cetaceans are often constrained by limited data on the extent to which these species use particular areas.

2. Timing porpoise detectors (T-PODs) are autonomous data recorders for detecting cetacean echolocation clicks, potentially providing cost-effective opportunities for monitoring cetacean occurrence.

3. The performance of T-PODs was assessed in three areas off the Scottish east coast, where the relative occurrence of bottlenose dolphins and harbour porpoises was known to differ. Land-based observations in one area compared visual and acoustic detections of dolphins, while direct hydrophone recordings of dolphin echolocation clicks were compared with T-POD detections during boat surveys.

4. Land-based surveys recorded 89 groups of dolphins within 900 m of the T-POD. All groups spending >30min in the area were detected on the T-POD, and the probability of detection declined in relation to distance from the recording site. 5. The number of dolphin clicks recorded on the independent hydrophone system was significantly related to the number detected by a T-POD. Between pairs of T-PODs, there was also significant correlation with the numbers of clicks recorded in each hour, both for channels set to detect bottlenose dolphins and for channels set to detect harbour porpoises.

6. Year-round deployments of paired T-PODs detected significant geographical variation in detections for both bottlenose dolphins and harbour porpoises. This pattern reflected published data from visual surveys, where dolphins occurred most regularly within the Moray Firth Special Area of Conservation, and porpoises were sighted more regularly in offshore waters.

7. T-PODs do not detect all cetaceans in the area, and care must be taken when interpreting data from mixed species communities. Nevertheless, these results confirm that T-PODs provide an effective method for monitoring the occurrence of bottlenose dolphins and harbour porpoises, and provide excellent potential for collecting baseline data from poorly studied areas and monitoring long-term temporal change in key areas of interest. Copyright (C) 2009 John Wiley & Sons, Ltd.