River dolphins are strongly affected by the construction of hydroelectric dams. Potential isolation in subpopulations above and below such dams and the resulting low genetic variability of these subpopulations can cause extinction at a local level. Here we aimed to estimate density and population size of South American river dolphins (boto Inia geoffrensis and tucuxi Sotalia fluviatilis), map their distribution, and estimate potential biological removal (PBR) limits in order to evaluate the effects of population fragmentation between planned dams in the Tapajós River, Amazonian basin, Brazil. Boat-based surveys were conducted following a line transect sampling protocol covering different dolphin habitats in 2 stretches of the river divided by rapids. The mark−recapture distance sampling method was applied to account for animals missed on the trackline. After the estimation of population sizes by habitat, PBR was calculated. The farthest upriver sighting of tucuxis was close to the São Luiz do Tapajós rapids, whereas the farthest upriver sighting of botos was upstream of the rapids, suggesting that botos move upstream through the rapids. Estimated abundance of tucuxis (3372 ind., CV = 0.38) was twice as high as that estimated for botos (1815 ind., CV = 0.4). The PBR ranged from 11 to 18 ind. for boto and 21 to 34 for tucuxi. Throughout this study, we identified low abundances of river dolphins compared to other Amazon rivers. Boto may not be sustainable at a population level, due primarily to population fragmentation which would result from the construction of the proposed dams. Precautionary measures are urgently needed before construction of dams begins in the Tapajós River.

Populations of freshwater dolphins are declining in response to increased human pressure, including habitat degradation, overfishing, bycatch, poaching and obstruction of free-flowing river corridors by dams. At least three river dolphin species occur in South America: the Amazonian river dolphin, or boto (Inia geoffrensis), the Bolivian river dolphin (Inia boliviensis) and the tucuxi (Sotalia fluviatilis). A fourth species, the Araguaian boto (Inia araguaiaensis), been proposed for the Tocantins-Araguaia, a large river basin in northern Brazil. Here we show that the Araguaian boto population in the Tocantins River is relatively small (N = 1083, CV = 0.52). During a survey to estimate density and abundance, 138 groups (198 individuals) of botos were observed along a~600 km stretch of the Tocantins River in five different habitats (river margin, river channel, channel, island margin, and a dam reservoir). Overall, lower densities of the Araguaian boto were registered downstream of the Tucuruí dam, the world's fifth largest hydropower dam. Density was 68% lower in the river margin habitat downstream (0.23 ind./km 2 , CV = 0.92) than upstream (0.72 ind./km 2 , CV = 0.53). In addition, density within the Tucuruí reservoir decreases from upstream areas towards the dam. Geographic post-stratification of data into sub-regions (downstream, reservoir, upstream) in relation to the Tucuruí dam helped to reduce CV by~70%, which illustrates the high variability in the encounter rate in these areas. Our findings suggest that the Araguaian boto population has been impacted by the construction of the Tucuruí dam. The construction of other dams proposed for the Tocantins-Araguaia basin should be planned strategically to a1111111111 a1111111111 a1111111111 a1111111111 a1111111111

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