Abundance estimate and acoustic monitoring of harbour porpoises (phocoena phocoena (L.)) in the blasket islands' candidate special area of conservation

Visual surveys and acoustic monitoring of harbour porpoises were conducted between July and October 2007 within the Blasket Islands' candidate Special Area of Conservation (cSAC) to derive density estimates and acoustic-detection rates. A total of 44 sightings of 102 individual harbour porpoises were recorded on 74 tracks with a total distance of 389km. Density estimates 9 SE, determined using DISTANCE software, ranged from 0.71 to 3.39 porpoises per km ², giving abundance estimates ranging from 162 ± 120 to 768 ± 198, depending on the number of sightings per day. The most robust estimate of 303 ± 76 (coefficient of variation = 0.25; 95% confidence interval 186-494) was calculated using the combined data from each track line. There were indications of a seasonal increase in abundance from July through to September. Passive acoustic monitoring was carried out through the deployment of self-contained click detectors called T- PODs. One month's acoustic data were acquired from two sites, and data were extracted as Detection Positive Minutes per day and per hour for analysis. Detections were logged for every day of deployment at both sites. This survey provides the first information on the spatial distribution of harbour porpoises in the Blasket Islands' cSAC, as well as baseline data from which to monitor abundance and distribution.