[en] The sighting frequency (SF), site fidelity, home-range and group composition of bottlenose dolphins (Tursiops truncatus) were assessed during a photo-identification study in 2006-2007. This study aims to provide first data on this unknown austral bottlenose dolphin population to answer to their increasing conservation needs.
Data and pictures were collected during 132 surveys with an average observation effort of 2.8h (SD=1.6) per survey. All clear pictures were analysed using the automatic identification systems FinEx and FinMatch (EuroPhlukes Initiative). The SF was obtained dividing the number of sightings by the amount of effort. Site-fidelity was estimated using the Capture Mark Recapture method and the degree of residency by the re-identification frequency following Culloch (2004).
In total, 377.4h were spent searching for bottlenose dolphins of which 57.3h were spent in the presence of 126 dolphin groups divided in 105 sightings. The SF was 0.28/h with and average duration of 43.2min/sighting (SD=0.76). A total of 43 dolphins were classified into an identification catalogue. These recognizable dolphins were re-identified up to 10 days with 44% (n=43) showing a degree of residency (resighting frequency (RF)¿4). The average group size was 6.1 (SD=7.9) ranging from one to fifty animals. Group size increased significantly with the presence of calves and with the distance from the coast. Nevertheless, all dolphins were seen in inshore waters ¿20m deep. The most frequent group formation was loose (34.5%), variable (29.8%), disperse (20.2%) and tight (15.5%). At least 6 identified dolphins, including one mother with her calf, were photographed in the mouth of the Rio Negro 250km from the study area indicating that their home-range is far from limited to Bahia de San Antonio. Data indicate that it concerns a resident but yet unknown population of bottlenose dolphins with a high commercial potential and an urgent need of conservation measurements.
