Photo-identification methods reveal seasonal and long-term site-fidelity of Risso’s dolphins (Grampus griseus) in shallow waters (Cardigan Bay, Wales)

A photo-identification study on Risso’s dolphins was carried out off Bardsey Island in Wales (July to September, 1997-2007). Their local abundance was estimated using two different analytical techniques: 1) mark-recapture of well-marked dolphins using a “closed-population” model; and 2) a census technique based on the total number of iden-tified individual dolphins sighted over the study period. The mark-recapture estimates of 121 (left sides; 64 - 178, 95% CI; CV 0.24) and 145 dolphins (right sides; 78 - 213, 95% CI; CV 0.24) closely matched the census technique estimates (population size of 90 - 151). It was found that the dolphins showed a degree of long-term and seasonal site-fidelity. A first long-distance match was made for Risso’s dolphins (319 km) between Bardsey Island and Cornwall, confirming they can be wide-ranging animals. This study demonstrates that the combination of systematic and opportunistic photo-ID studies has complementary value as a population assessment tool in generating the first local abundance esti-mate for Risso’s dolphins in UK waters. From the conservation perspective, these studies confirm the regular presence of Risso’s dolphins in these waters and the presence of calves shows breeding. Bardsey Island may be part of a network of localities that are important habitats to this species where it may take advantage of prey abundance in shallow waters. As such, results of this study may provide assistance to include the Risso’s dolphin in future regional conservation strategies including the envisaged marine protected areas