The giant otter Pteronura brasiliensis, categorized as Endangered on the IUCN Red List, is an apex predator in Neotropical freshwater systems with the potential to regulate food webs through density-mediated trophic cascades (Quéméré et al., 2021, Environmental DNA, in press). The giant otter has been extirpated in some countries and extant populations are declining.

Giant otters were once abundant in Argentina but heavy hunting, habitat loss and degradation resulted in their dramatic decline. The last known population disappeared in the 1980s. Since then, records of solitary individuals have been reported in the Iguazú and Paraná River basins, the most recent from 2010, but these records lack supporting evidence such as images or signs. In the Bermejo River basin, in the Chaco region, the last sightings of giant otters date from the late 1800s.

El Impenetrable National Park, created in 2017, protects the declining dry forests of the Chaco region. To the north, the Park is bounded by the Bermejo River. On 16 May 2021 in El Breal, one of several ponds formed by old meanders of this river, SDM observed an adult giant otter and secured a photograph. Surveys later that day revealed two fresh latrines and two recently dug dens on the shore. El Breal pond has steep clayey shorelines 2–3 m above the highest water level and a total surface water area of 0.37 km². Water depth is c. 10 m and potential giant otter prey in the pond include abundant fish, macroinvertebrates and caimans. A mature dry forest surrounds the pond.

The origin of this individual is unknown. The closest giant otter population is in the Jejuí River in Paraguay, close to its confluence with the Paraguay River (Cartes et al., 2013, Paraquaria Natural, 1, 8–11). But an animal dispersing from there would have to travel c. 1,000 km of waterways to reach El Impenetrable National Park. Alternatively, this individual could be part of a previously unrecorded relictual population. Genetic analysis of fecal samples could shed light on this.

Giant otter photographed in El Impenetrable National Park, Argentina, in May 2021. Photo: Sebastián Di Martino.

Using camera traps, we have identified the animal as a male and found no evidence of other accompanying giant otters. To ensure the survival of this individual, it is paramount that deforestation around the Park is halted and any poaching in the Bermejo River deterred. Federal and provincial governments need to work together to increase patrolling in the lands around the Park and along rivers. Conservation translocations of captive-bred and wild individuals could be considered, to increase the likelihood of establishing a founding population.