Occurrence of bottlenose dolphins (Tursiops truncatus) in the Río Negro Estuary, Argentina, and their mid-distance movements along the Northeast Patagonian coast

A systematic study on the presence of bottlenose dolphins (Tursiops truncatus) in the Río Negro Estuary (RNE), Patagonia Argentina, was carried out between the months March – July from 2008 to 2011. Data on the dolphin’s activity patterns were gathered via an Ad Libitum focal group sampling mode accompanying dorsal fin images taken for identification and re-identification of individuals. A total effort of 188 h resulted in 58 h of positive observation of 124 dolphin groups (sightings per unit effort (SPUE) = 0.66 groups/h). Data analysis showed two main activity states for the observed groups, travelling (65%) and foraging (26%). The remaining 9% of the groups were involved in other activity states.
The photo-identification effort, which started opportunistically in 2006, resulted in a catalogue of 21 individual dolphins, with a total mean re-identification rate of 9 days (max = 24 days). When comparing these pictures to the existing catalogue of Bahía San Antonio (approximately 200 km west from the study area) dorsal fins of 20 individuals could be positively matched and most (n = 17) could be subsequently re-identified in both areas, indicating their long distance movements along the North Patagonian coast during the austral autumn months. This season coincides with the lowest amount of feeding activity observed in Bahía San Antonio.
This study suggests that bottlenose dolphins enter RNE, mainly during autumn, to forage. It appears that the search for food resources may be the trigger for their movement patterns along the North Patagonian coast during this season, at least for certain individuals. More research is needed to accurately confirm this hypothesis.