Ecological aspects of marine tucuxi dolphins (Sotalia guianensis) based on group size and composition in the Cananéia Estuary, southeastern Brazil

Observations on group size and composition of marine tucuxi dolphins, Sotalia guianensis, found in the Cananéia estuary (25°03'S, 47°55'W), southeastern Brazil, were conducted from May 2000 to July 2003. Correlations among four environmental variables (water temperature, salinity, transparency and depth) and group size were investigated, as well as on the way foraging and feeding behavioral patterns related to group formation in S. guianensis. The surveyed area comprised approximately 132km² and field efforts were conducted in 87 days. A total of 378 groups were recorded. Group size (mean ± SD: 12.4 ± 11.4 individuals) varied from lone individuals, rarely seen, to aggregations of up to 60 individuals. No significant differences were found regarding group size among seasons. Groups with calves (89.7% of all observed groups) were more frequent than groups without calves. Feeding associations with marine birds were common in winter when waters were clearer. A positive correlation was only reported when investigating group size and water depth (Spearman's rank test, r=0.4716, p=0.0000). Marine tucuxis usually form large aggregations (20 to 60 individuals) when engaged in feeding activities in deeper waters. Groups were larger (mean ± SD: 18.3 ±15.0 individuals, n=90) in the main estuary entrance, where water is deeper, and smaller in inland shallower waters (mean ± SD:7.8 ± 6.9 individuals, n=47). Because of these common observed aggregations, this area of the estuary has been used as a dolphin watching site since the late 1990s without specific regulations. Guidelines are urgently needed to conduct dolphin watching tours as calves are seen year-round and uncontrolled tourism may disrupt important social interactions