Abundance Estimate of Bottlenose Dolphins (Tursiops truncatus) in the Lower River Shannon candidate Special Area of Conservation, Ireland

An abundance assessment of bottlenose dolphins (Tursiops truncatus) in the Lower River Shannon candidate Special Area of Conservation (cSAC) was undertaken between July and October 2010 using photo-identification. European Union Member States are obliged to designate SACs for bottlenose dolphins to protect important habitats. The Lower River Shannon is the only cSAC in Ireland for this species. A cumulative total of 273 bottlenose dolphins were photographed during the 12 transects, and from these a total of 116 individual animals were identified. They were categorized as follows: 71 with Severity Grade 1 marks, 21 with Severity Grade 2 marks, and 24 with Severity Grade 3 marks. There were 50 dolphins with permanent marks (Severity Grade 1) recorded on both sides of the dorsal fin, 64 on the left hand side only, and 57 on the right hand side only. There was an overlap, with some dolphins occurring in more than one category. Estimates of abundance were calculated using left side, right side, and both side identifications. The proportion of dolphins with re-identifiable marks (Severity Grade 1 only) ranged from 0.60 to 0.63. The estimated abundance of marked individuals was elevated depending on the estimated proportion of marked individuals in the population to give a final estimate of 107 +/- 12, CV = 0.12 (95% CI = 83 to 131). Previous abundance estimates for bottlenose dolphins in the Lower River Shannon cSAC ranged from 114 in 2008 to 140 in 2006; the present estimate was within this range and also within the 95% Confidence Intervals for all surveys carried out to date. This suggests that, within the power of the survey technique, the population of bottlenose dolphins in the Lower River Shannon cSAC is relatively stable.