Estuarine dolphins, Sotalia guianensis (Van Bénéden, 1864), were identified in Sepetiba Bay (22º35'S, 44º03'W), state of Rio de Janeiro, Brazil, between February 1994 and July 2006 using the photo-identification method. A total of 72 surveys resulted in the identification and cataloguing of 217 individual dolphins. Seventy-nine (36%) were resighted at least once during the study period. The average interval between resightings was 40,6 ± 31,1 months. Large numbers of mother-and-calf pairs were recorded during the 12 years and new individuals were frequently encountered in the photographic records. These results suggest that part of the dolphin population is more consistently found in Sepetiba Bay, probably for feeding and reproduction, while other individuals use the Bay only sporadically.

Applying the photo-identification technique, we took pictures of dolphins' dorsal fins, looking for natural markings on them. A total of 17,969 photographs were taken, from which was created a databank of 462 distinct dolphins. Individuals showed different degrees of residency in the bay. More than 50% of the catalogued dolphins were considered non-residents and/or showed a low degree of residence. The average interval between resightings was 35 + 27.4 days. Animals without markings (calves and juveniles) accounted for 45% (N ¼ 2917) of photographs taken. These results indicate that this population has a fluid structure including different individuals over time. We suggest a possible relationship between the presence of some individuals and local resource availability, such as prey and protected areas.

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