Distribution and abundance of bottle-nosed dolphins Tursiops truncatus (Montagu) in the Shannon Estuary

The feasibility of developing commercial whale-watching in the Shannon Estuary was investigated by determining the distribution and abundance of bottle-nosed dolphins in the estuary. A variety of field techniques were employed, including photo-identification, fixed boat transects, scan sampling, minimum number estimates and the collection of incidental sightings over an eighteen-month period. Land-based observations showed that the presence of dolphins was influenced by the state of the tide, with a peak in sightings recorded three hours after high water. Dolphins were observed on 25 (61%) of 41 boat transects surveyed, of which sixteen turned into encounters, with dolphins sometimes present for many hours. A minimum number estimate conducted on 16 October 1994 showed that at least 56-68 animals were present, and photo-identification has recognised 25 animals individually. Dolphins were often recorded in discrete areas, especially off Kilcredaun Head, Beal Bar and off Leck Point. Sighting records from the Tarbert to Killimer ferry showed that dolphins were regularly seen especially in the spring and summer (May to August), being recorded on up to 88% of days in the month. Photo-identification showed that the same dolphins were present in the estuary throughout the study period, suggesting that they were resident. This is the first known resident population of bottle-nosed dolphins in Ireland. The implications of this work for the development of commercial dolphin-watching in the estuary are discussed.