Distribution and Abundance of the river dolphin, (Inia geoffrensis) in the Tijamuchi river, Beni-Bolivia

Very few studies have been conducted in Bolivia regarding the distribution, behavior, or ecology of the Amazon river dolphin (Inia geoffrensis). The only published studies of bufeo in Bolivia are from Pilleri (1969) and Pillieri &G ihr (1977) and were not quantitative investigations of river dolphin abundance or distribution. The work presented here consists of an estimate of abundance and description of the seasonal distribution of the bufeo in the Tijamuchi River, Beni, Bolivia. The study was conducted during January 1998-September 1999 and represents four hydro-climatic seasons (i.e., low, high, rising, and falling waters). Two hundred and twelve h were spent in survey effort. The total study area was approximately 185 km along the river. Strip transects were used to survey for dolphins. Dolphin distributions among three habitats were compared; these habitats were riverine-blackwaters, riverine-mixed waters (black and white), and oxbow lakes of the river system. Group size and age structure were recorded. Any dead dolphins were necropsied. On average, 208 bufeos were observed in the Tijamuchi River, with an average encounter rate of 1.12 dolphins/linear km. Dolphins were seen most frequently during low and falling water (56% of total observations) and least often during high waters (22% of total observations). These seasonal differences were statistically significant. Dolphins were seen most often in oxbow lakes, and next often in confluences. Differences in dolphin abundance according to water colour were not statistically significant. There was some evidence of associations between group size and season, and group size and water colour. Forty-two per cent of observations were of solitary dolphins, 32% were of pairs, and maximum group size was 19. Calves were seen most often during falling and low waters. Causes of mortality of dolphins in the study area were identified as entanglement in fishing nets, intra-specific aggression, and collisions with outboard motors.