The giant otter Pteronura brasiliensis is endemic to South America and categorized as Endangered on the IUCN Red List. The Amazon basin is one of the last strongholds for this species, yet recent human expansion across Amazonia has led to an increase in the number of hydropower dams, which negatively affect both freshwater and terrestrial biodiversity. Although areas with low human population density that held giant otters before dam construction may have potential for the conservation of the species (Rosas et al., 2007, Oryx, 41, 520–524), there has been no previous robust documentation of the impacts of hydropower developments on the species.

We conducted a before–after control–impact study to evaluate the effects of a new run-of-river hydropower dam on giant otters in the State of Amapá, eastern Brazilian Amazon. This state has the lowest deforestation rates and the highest territorial coverage of protected areas among all states of the Legal Brazilian Amazon, and has low human population densities. In surveys before (2011–2013 and 2015) and after (2016–2020) the construction of a large run-of-river hydropower dam (219 MW, height 20.6 m, reservoir covering 47.99 km²) we surveyed a total of 9,356 km along 139 km of rivers by boat.

There was a relatively constant detection rate in the control zone > 70 km upstream of the dam (0.11 and 0.10 detections/10 km before and after, respectively). In the directly impacted zone (< 50 km upstream of the dam, as established in the environmental impact assessment) the detections declined (0.03 and 0 detections/10 km before and after, respectively), with no evidence that giant otters remained in the impacted zone. Considering this region has a high coverage of protected areas, low deforestation rates (forest cover > 90% in the control zone) and a low human population density, it is imperative to emphasize the importance of mitigation to minimize the negative effects of new and planned hydropower dams. We expect that, in the absence of effective mitigation actions, the increasing number of new hydropower dams across Amazonia are likely to cause further reductions in giant otter populations. Contrary to portrayal in the media, run-of-river dams are not necessarily eco-friendly. Government efforts for more sustainable energy generation using a combination of technologies (e.g. in-stream turbines, solar and wind power) need to be reinforced.