Seasonal use of Amazon floodplains by the tucuxi <i>Sotalia fluviatilis</i> (Gervais 1853), in the central Amazon, Brazil

Abstract

The riverine tucuxi, Sotalia fluviatilis (Gervais, 1853), inhabits an aquatic environment that changes dramatically along the year. The marked annual floods impose a seasonal rhythmicity on the wildlife on these floodplains. The present study provides an insight on tucuxi seasonal use of floodplain, use of area, group size and the occurrence of calves or juveniles within Mamirauá Reserve, Brazilian Amazon. Tucuxis occur year-round in Mamirauá with a mean number of sightings per monthly survey of 3.8. The fluctuating water level in this floodplain influenced tucuxi use of area by allowing or preventing access to water bodies, and influencing its prey movements. The highest encounter rate recorded in the Reserve, for all surveys combined, was 114.3 tucuxis per km (mean = 24.8 per km) with clear tucuxi preference for deep channels with high turbulence and productivity, where prey density is high. Tucuxis were scattered throughout the channels but avoided the lake Mamirauá. Groups of one to 30 tucuxis were recorded, with 91% of groups comprising one to six dolphins (mean = 3.3). Group size showed significant differences among the five systems studied across visual surveys, between years within Mamirauá but not among months. Tucuxi calves or juveniles were recorded year-round.