Giant otter population responses to habitat expansion and degradation induced by a mega hydroelectric dam

- We assess the impacts of vast hydroelectric reservoirs on a threatened apex-predator species, the giant otter.
- Giant otter population size estimates did not scale to the newly available habitat area.
- This population shortfall results primarily from the low habitat quality of this reservoir.
- This study highlights how tropical riparian ecosystems are degraded by major hydroelectric dams.

Large hydroelectric dams are major drivers of habitat loss and degradation in lowland Amazonia. Hydroelectric reservoirs reduce the habitat available for terrestrial species, but create new open-water and shoreline lake habitat that can potentially boost populations of aquatic and semi-aquatic species, such as the threatened giant otter (Pteronura brasiliensis). To assess the impacts of mega-dams on this apex-predator, we surveyed the giant otter population across the 443,772-hectare Balbina Hydroelectric Reservoir of central Brazilian Amazonia between 14 and 25 years after this reservoir creation. We compared changes in habitat area and estimated giant otter population size between the reservoir pre- and post-filling stages. The Balbina dam created ∼3525 islands and increased the open-water surface and total reservoir perimeter available to otters by a factor of 62.7 and 8.9, respectively. Some 25 years after damming, however, the estimated post-filling giant otter population size was only twice greater than that estimated before filling and 4.5 times smaller than would be predicted given the total available habitat area and density of dens quantified at a neighbouring undisturbed area used as a surrogate of the pre-filling phase. The observed mismatch between the proportional increase in otter population size and the much greater newly available reservoir habitat area is likely due to low habitat quality in terms of low fish prey productivity and scarcity of suitable sites for denning and territory demarcation. This should be considered in strategic environmental impact assessments of planned hydroelectric dams and in managing existing and future hydropower development in lowland tropical forests.