Group size and composition of estuarine dolphins, Sotalia guianesis (van Bénéden, 1864) (Cetacea, Delphinidae), in Paraty bay, Rio de Janeiro, Brazil

Abstract

The occurrence of Sotalia guianesis and their group size and composition in Paraty Bay, Rio de Janeiro, are described for the period October 2000 - September 2001. During this period, 36 cruises and 6718min (45,19%) of direct observation were accomplished. Sightings were made in all seasons (in 94.5% of all cruises), during which time 54 groups (total of 1,754 individuals: mean=32.4 individuals/group) were observed. The size of each group varied from one to 100 specimens. The total number of individuals per unit effort and their group size varied seasonally. Groups with 31-40 individuals were the most often observed (25.9% of all groups). Significant differences between group size frequency and season, type of activity were seen. No significant differences were seen between group size frequency and time of day or tidal phase. The highest number of sightings was made in the mornings. Calves were observed in all seasons, with a peak in summer. The high incidence of calves (present in 88.9% of all sightings) and juveniles (87.0%) suggests that Paraty Bay is a nursery area for estuarine dolphin. The present study suggests the existence of links between the seasonal occurrence of estuarine dolphins in Paraty Bay and their group size, reproductive activity, prey availability and water temperature.