Monitoring a Critical Population of the Bolivian River Dolphin, Inia Boliviensis, Before and after Closing the Floodgates of a Hydroelectric Dam in the Amazon Basin, Brazil: A Quantitative Analysis

Dam constructions threaten aquatic fauna, degrading their habitats and causing population fragmentation. Environmental impact studies were carried out from March 2010 to July 2015 as part of the licensing process concerning the Jirau Hydroelectric Dam construction, located on the Madeira River, Rondonia, Brazil, aiming to assess its impacts on the local fauna. The dam closed its gates in December 2012 and the water level rose from 2013 to 2015, creating a 468 km2 reservoir. Dams may be a source of negative effects to several species, including the Bolivian river dolphin, Inia boliviensis, which have a restricted distribution in northern Bolivian rivers and part of the Madeira River Basin. This study evaluated the presence of I. boliviensis in different sections of the Madeira River before reservoir filling (BRF) and after reservoir filling (ARF) through a quantitative analysis. Dolphins were actively searched along linear transects in four sections of the river: Abuna, Mutum, Caicara and Canteiro, over 125 km (250 km round trip). A total effort of 986 h was carried out along 210 days during the hydrological seasons (falling, low, rising and high-water level). Twenty-nine (29) groups were recorded BRF in the direct dam influence area (N = 62), while thirty-eight (38) groups were documented ARF (N = 111). The highest number of sightings was recorded in Abuna during both periods: 24 in BRF (N = 50) and 33 in ARF (N = 101). The greatest number of dolphins was detected during the falling and low hydrological seasons: 19 in BRF (N = 48) and 30 in ARF (N = 92). This is expected, since decreasing water levels lead to a higher concentration of prey and, consequently, higher numbers of dolphins can be observed. Standardized methodology and experienced staff are essential to ensure adequate dolphin population estimates, especially in these critical habitats, where the main impacts caused by the dam are population fragmentation, isolation and extirpation, and reproductive segregation.