Harbour porpoise surveys in Rocakbill to Dalkey Island SAC, 2016

A visual survey of harbour porpoises (Phocoena phocoena) was carried out in the summer of 2016 within the Rockabill to Dalkey Island SAC, Co Dublin in order to derive local density and abundance estimates. Single platform line-transect surveys were carried out according to a standardised design on four days between June and September 2016. Surveys were conducted between the 7th June and 15th September 2016. Distance sampling was used to produce a detection function based on the observed distribution of harbour porpoise sightings. Abundance estimates were calculated using the day as the sample and the sighting as the observation: (i) for all survey days, (ii) stratified by sea state and (iii) for all surveys combined Surveys were carried out in favourable weather conditions (i.e., sea-state ≤2, with visibility of at least 6km) on all four survey days. A combined total of 506km of track-line was surveyed, which resulted in 152 distinct sightings totalling at least 246 individual harbour porpoises. The observed proportion of young porpoises (juveniles and calves combined) to adults was 9.8% and the proportion of calves to adults was 5.7%. No other cetacean species was recorded on any of the surveys, but a number of grey and harbour seals were sighted during on-effort periods.