The Amazon river dolphin, known throughout South America as the boto or pink river dolphin, is the largest of all freshwater cetaceans and one of the most distinctive predators of the Amazon and Orinoco basins. Where most dolphins live in open water, the boto hunts among the trunks of seasonally flooded forest, threading its long beak through submerged roots after fish. After decades in which sparse data obscured its situation, long-term monitoring has revealed a species in sustained, basin-wide decline [da Silva et al. 2018b; Martin & da Silva 2021]. This profile examines what makes the boto biologically remarkable, the pressures driving its losses, and the measures working to reverse them.
Biology and Identification
The boto is the largest river dolphin in the world. Males reach roughly 2.5 m and masses exceeding 150 kg, substantially larger than females — sexual dimorphism unusual among cetaceans [Best & da Silva 1993]. It is best known for its coloration: calves are dark grey, but adults often develop a pink to rose hue, most pronounced in mature males, whose pinkness is thought to be intensified by scarring [Best & da Silva 1993].
Unlike marine dolphins, the boto retains unfused cervical vertebrae, giving it a flexible neck that turns the head through a wide arc [Best & da Silva 1993]. Combined with broad, paddle-like flippers and the ability to swim backward, this flexibility lets the animal maneuver between trunks and submerged vegetation in the flooded forest during the high-water season, exploiting feeding grounds inaccessible to other dolphins. Its long, slender beak bears differentiated teeth suited to grasping and crushing hard-bodied prey, and the diet is exceptionally broad — roughly 50 fish species plus river turtles and freshwater crustaceans [Best & da Silva 1993].
Botos rely on echolocation in turbid water where vision is of limited use. They are generally solitary or found in small, loose groups, and their reproductive cycle is slow — long calving intervals and extended maternal care make populations slow to recover from losses [Best & da Silva 1993; Martin & da Silva 2021].
Habitat and Range
The boto occupies the great river systems of tropical South America, across the Amazon and Orinoco basins and their major tributaries in Brazil, Bolivia, Peru, Ecuador, Colombia, Venezuela, and Guyana [da Silva et al. 2018]. It ranges through main and side channels, lakes, and confluences, and during the annual flood pulse moves into inundated forest and floodplain. This dependence on the flood cycle ties the species to the basin's seasonal hydrology and makes it sensitive to anything that alters water levels, connectivity, or temperature.
Three subspecies are recognized across this range [da Silva et al. 2018]. Because the boto's distribution follows continuous river networks rather than discrete patches, fragmentation — by dams or by drought-driven isolation of floodplain lakes — has outsized consequences for the species.
In accordance with NRWL sensitive-species policy, specific site locations, congregation points, and seasonal movement details are not disclosed in this article.
Conservation Status
The Amazon river dolphin is listed as Endangered on the IUCN Red List, assessed under criterion A in 2018 [da Silva et al. 2018]. This represented a significant change: the species had earlier been categorized as Data Deficient because data on its numbers, ecology, and trends were too limited to support a threatened classification. By 2018, improved long-term data made the case for uplisting clear [da Silva et al. 2018].
The evidence behind that listing is among the strongest for any Amazonian mammal. A 22-year monitoring program in Brazil's Mamirauá Reserve — one of the longest continuous cetacean datasets in the region — recorded a sustained boto decline rapid enough to halve the population roughly every ten years, with the co-occurring tucuxi declining even faster [da Silva et al. 2018b]. A later population viability analysis projected a 95% reduction within 50 years across all modeled scenarios unless killing and bycatch are reduced, warning that the boto risks following the functionally extinct Yangtze river dolphin if current pressures continue [Martin & da Silva 2021].
Threats
Killing for use as fishing bait is among the most acute pressures. Botos are deliberately killed and their carcasses used to bait the piracatinga (Calophysus macropterus), a scavenging catfish. Researchers documented harpoon and machete scars on captured animals, and interviews with fishers confirmed targeted killing for bait [da Silva et al. 2018b]. Brazil imposed a moratorium on the piracatinga fishery in 2015 to relieve this pressure, but enforcement has been inconsistent and killing has continued [IUCN-CSG 2015; IUCN-CSG 2020].
Gillnet bycatch drowns dolphins that become entangled in fishing gear, falling heavily on calves where dolphins and fisheries share the same productive waters [Martin & da Silva 2021].
Dams and habitat fragmentation sever the river networks the species depends on; hydroelectric development disrupts the flood pulse, blocks movement, and isolates subpopulations [Martin & da Silva 2021].
Mercury contamination, largely from artisanal and industrial gold mining and forest burning, accumulates through the aquatic food web. Mercury has been measured in boto muscle tissue across the Amazon and Orinoco basins, identifying it as a growing threat to the species and to people who eat the same fish [Mosquera-Guerra et al. 2019].
Extreme drought and heat struck in 2023, when more than 200 freshwater dolphin carcasses were recovered from Lake Tefé during a historic drought. Measurements and modeling documented lake water reaching about 41 °C — beyond the animals' thermal tolerance — as shallow, turbid, sheltered lakes overheated under intense sun [Fleischmann et al. 2025].
What Is Being Done
Long-term population monitoring. The multi-decade Mamirauá program is the empirical foundation for the species' conservation, supplying the trend data behind the IUCN assessment and the population viability analysis [da Silva et al. 2018b; Martin & da Silva 2021]. Continuing standardized counts let declines be detected and measures evaluated against real trajectories.
Fishery regulation. Brazil's 2015 moratorium on the piracatinga fishery was introduced to reduce the killing of dolphins for bait and address the mercury hazard the catfish poses to consumers [IUCN-CSG 2015]. Conservation bodies have repeatedly called for it to be maintained and enforced, since weak enforcement lets the bait trade — and the killing — resume [IUCN-CSG 2020].
Mass-mortality investigation. The 2023 Lake Tefé event was documented and analyzed by researchers with the Mamirauá Institute, producing peer-reviewed evidence linking dolphin deaths to extreme lake warming and a basis for monitoring and early warning as droughts intensify [Fleischmann et al. 2025].
Cultural protection. Across much of the Amazon the boto is embedded in the legend of the encantado — a dolphin that transforms into a person — and traditional belief that harming a dolphin brings misfortune has historically discouraged killing in some communities [Best & da Silva 1993]. These values remain a real, if uneven, form of local protection conservation programs can build upon.
How Readers Can Help
Citizen science. Photograph and log wildlife observations through platforms such as iNaturalist. Verified occurrence records contribute to range mapping and the assessments that underpin conservation status.
Policy engagement. Support measures that maintain and enforce restrictions on the piracatinga bait fishery, regulate gillnet use, curb mercury pollution from gold mining, and require rigorous environmental review of Amazon dam projects.
Informed consumer choices. Avoid fish of uncertain origin in regions affected by the bait trade, and support tour operators that demonstrably avoid harming river dolphins.
Education outreach. Share accurate, science-based information about the boto's decline and its drivers — bait killing, bycatch, mercury, dams, and drought.
References
[Best & da Silva 1993] Best, R.C. & da Silva, V.M.F. (1993). Inia geoffrensis. Mammalian Species, 426, 1–8. https://doi.org/10.2307/3504090
[da Silva et al. 2018] da Silva, V., Trujillo, F., Martin, A., Zerbini, A.N., Crespo, E., Aliaga-Rossel, E. & Reeves, R. (2018). Inia geoffrensis. The IUCN Red List of Threatened Species 2018: e.T10831A50358152. https://doi.org/10.2305/IUCN.UK.2018-2.RLTS.T10831A50358152.en
[Fleischmann et al. 2025] Fleischmann, A.S. et al. (2025). Extreme warming of Amazon waters in a changing climate. Science. https://doi.org/10.1126/science.adr4029
[IUCN-CSG 2015] IUCN SSC Cetacean Specialist Group. (2015). Amazon dolphins as fish bait: Brazil introduces a moratorium on piracatinga fishing. https://iucn-csg.org/amazon-dolphins-as-fish-bait-brazil-introduces-a-moratorium-on-piracatinga-fishing/
[IUCN-CSG 2020] IUCN SSC Cetacean Specialist Group. (2020). Amazon River dolphins in Brazil continue to be killed for fish bait. https://iucn-csg.org/amazon-river-dolphins-in-brazil-continue-to-be-killed-for-fish-bait/
[Martin & da Silva 2021] Martin, A.R. & da Silva, V.M.F. (2021). Amazon river dolphins Inia geoffrensis are on the path to extinction in the heart of their range. Oryx, 56(4), 587–591. https://doi.org/10.1017/S0030605320001350
[Mosquera-Guerra et al. 2019] Mosquera-Guerra, F., Trujillo, F., Parks, D., Oliveira-da-Costa, M., Van Damme, P.A., Echeverría, A., Franco, N., Carvajal-Castro, J.D., Mantilla-Meluk, H., Marmontel, M. & Armenteras-Pascual, D. (2019). Mercury in Populations of River Dolphins of the Amazon and Orinoco Basins. EcoHealth, 16, 743–758. https://doi.org/10.1007/s10393-019-01451-1
[da Silva et al. 2018b] da Silva, V.M.F., Freitas, C.E.C., Dias, R.L. & Martin, A.R. (2018). Both cetaceans in the Brazilian Amazon show sustained, profound population declines over two decades. PLOS ONE, 13(5), e0191304. https://doi.org/10.1371/journal.pone.0191304