Published online by Cambridge University Press: 27 March 2012
The occurrence of dolphins in the Belém area, northern Brazil, has been poorly surveyed. In addition, there has been much speculation on the identity of cetaceans inhabiting these brackish waters. Vantage point observations conducted from September 2008 to October 2010 resulted in the observation of 44 groups of dolphins and 79 individuals. Among these, 56 were Inia geoffrensis (71%) and 23 Sotalia. In addition, 21 boat transects were conducted and recorded 50 individuals in 19 sightings. Among sighted during transects 36 were Sotalia (72%) and 14 were Inia. In both observation methods, Tasco Offshore binoculars were used and size and group composition and behaviour (feeding, resting, socializing, travelling and not identified) were recorded. All the categories of behaviour were recorded for Inia and Sotalia, except resting. The Inia group size ranged from one to four individuals, the solitary ones prevailing (57% in vantage point and 67% in boat surveys). Groups ranging from one to seven Sotalia dolphins were registered, in both observation methods. Boto calves were observed in September 2008 and 2009, and February 2010. In April 2010 two botos were sighted in courtship behaviour. Sotalia calves were found in March, May and December 2009. Botos frequently came closer to the right river margin, often entering the Igarapé do Tucunduba, usually in feeding behaviour. On the other hand, Sotalia was sighted in the main channel, or closer to the left margin, opposite to the observation point. Sotalia were observed in the rainy season, except for three sightings in the dry season.