Spatial distribution and activities of the estuarine dolphin Sotalia guianensis (van Bénédén, 1864) (Cetacea, Delphinidae) in Pontal Bay, Ilhéus, Bahia, Brazil

Abstract

We analyzed the influence of time, tide and water depth on the activity and spatial distribution of estuarine dolphins Sotalia guianensis in Pontal Bay, Ilhéus, Brazil. Direct observations were carried out at four fixed points from January to December of 2006. Estuarine dolphins were sighted in 11 out of 12 months and in 30% of 181 sampling periods. The average group size, which was 3.75 ± 1.59 (n = 64), did not vary significantly by month (Kruskal-Wallis, H = 10.6729; p = 0.3836). Infants and adults represented 18 and 82% of the individuals whose age we were able to determine, respectively. Estuarine dolphins were more frequently present than expected in areas deeper than 3 m (regression GLM, z = 3.773; p = 0.0002). The animals were more frequently observed between 7:00 and 8:00 AM and between 3:00 and 5:00 PM (X² = 83.815; p < 0.0001). Their presence in the bay tripled between the fourth hour of the flood and high tide, remaining elevated during the ebb tide (X² = 22.152; p = 0.02). The tide also influenced the animals' feeding and travelling direction, mainly toward the open see during the flood and to the back of the bay during others stages. Further studies are necessary to analyze the influence of the hydrographic characteristics of Pontal Bay on the spatio-temporal distribution of estuarine dolphins and their prey.