Tucuxi

Introduction

IUCN Red List Conservation status: Endangered

Tucuxis overlap in range with Amazon river dolphins inhabiting the central region of the Amazon river basin. However, tucuxi are easy to distinguish from Amazon river dolphins because they are smaller, uniformly grey in colour (as opposed to pink), occur in larger groups, and swim faster. The tucuxi’s range is limited by natural barriers, such as rapids that prevent access to upstream stretches of rivers.

Tucuxis are believed to be the guardians of rivers and a symbol of good luck, as their ability to avoid rocky and shallow sections of rivers guides local populations and their boats to safe sections of the river.

Distribution and habitat

Tucuxi dolphins are found in the central Amazon river basin, with presence confirmed in Brazil, Colombia, Ecuador and Peru. The species’ range largely overlaps that of the Amazon river dolphin. However, unlike the Amazon river dolphin, the tucuxi does not enter into flooded forests, side channels or lakes, or upstream waters beyond rapids. As such, the species is found predominantly in the centre of large main rivers (>200 m width) with rapid flow and depth ^{1, 2}.

Tucuxi range map provided by WWF Brazil.

Abundance

There is no available estimate for the total number of tucuxi river dolphins in South America. The species appears to have been relatively abundant throughout most of its range at one time, and may still be in some areas ³⁵. Where population estimates are available at different sites, they range from just 147 in Ecuador⁵, to approximately 9,000 in the Purus River, Brazil⁶. Given the species broad distribution and the remote and inaccessible nature of some of the rivers it inhabits, carrying out surveys across the full range of the species is impossible. As such, research teams focus on a few key areas have been resampled to assess trends in abundance. For example, the population inhabiting the upper Amazon River in Colombia appears to be relatively stable ⁷, while in the central Amazon river basin there is evidence of decline⁸.

Threats

Map depicting areas of gold mining. Produced by WWF Brazil.

One of the greatest threats to the continued survival of the tucuxi river dolphin is entanglement in gillnets as bycatch. Although considered the main threat to this species, quantitative data on bycatch rates is not available, making it challenging to design adequate conservation measures. Like Amazon river dolphins, tucuxis are threatened by intensive fishing and other harmful fishing methods, mercury contamination from gold mining, disturbance by river traffic, water-related infrastructure such as dams and large navigation projects, sand and gravel mining, river pollution and unusual drought exacerbated by climate change causing low water levels ⁸. Dam construction leads to shifts in habitat use and movements along the rivers ^{9, 10}. Dams also affect river habitats by reducing water availability and quality, and blocking nutrients, sediments, dolphins and migratory fish (river dolphins’ main prey) from their spawning or feeding grounds. As the price of gold has soared, so too has gold mining and the related pollution it causes in the Amazon. In fact, the small-scale gold mining sector is the main source of mercury emissions in Latin America. Indigenous and traditional communities in the region, as well as river dolphins, accumulate mercury in their tissues over time through their consumption of mercury-contaminated fish¹¹. Although there are no studies of mercury contamination in tucuxis, it is reasonable to assume that the species is affected the same way that Amazon river dolphins are, since they share most of the dietary components (fish). Gold mining along rivers also destroys gravel beds where the gold is found, and that migratory fish depend on for habitat and spawning. These cetaceans are bio-indicators and a sentinel species of water health ^{5, 12}. Changes in their well-being should be interpreted as an (early) warning red flag indicating ecosystem degradation and human health risk.

Biology and ecology

Tucuxi look somewhat similar to bottlenose dolphins, with broad flippers and pronounced beaks. Their bodies are muted blue-grey in colour, while their bellies are much lighter shades of ivory, grey or pink. Tucuxi have low, triangular dorsal fins. They are quite small compared to the other river dolphin species, only growing to about 1.5 metres long and weighing around 50kg at adult maturity.

Like other dolphin species, tucuxi rely heavily on echolocation to communicate, navigate and find food in the dark and sometimes turbid waters where they live¹³. However, unlike Amazon river dolphins that enter into flooded forests, tucuxis have less pronounced melons (foreheads where echolocation clicks are amplified), since they do not need to navigate through many obstacles.

Tucuxi. Illustration by Uko Gorter.

Reproduction and growth

Gestation is estimated at 11-12 months¹⁴. Calving peaks during the low water period of the Amazon in October and November, and they live to be about 15-16 years old ¹⁵.

Diet

Feeding habits of the tucuxi are believed to be generalist, including a variety of fish and crustaceans, as is the case with Amazon river dolphins. However, studies on tucuxi diet are scarce due to difficulties in sampling this animal.

Behaviour

(c) Jessica Melo IDSM

To human observers, tucuxi appear playful, vivacious and extremely sociable. The tucuxi is the only river dolphin that jumps out of the water, like its marine cousins. This species seems to be more social than Amazon river dolphins, congregating in groups of up to 30 individuals². Tucuxi are fast swimmers, and display short dive times of only 30 seconds to one minute.

External links

For more information see:

References

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Martin, A.R., V. Da Silva & D.L. Salmon. 2004. Riverine habitat preferences of botos (Inia geoffrensis) and tucuxis (Sotalia fluviatilis) in the central Amazon. Marine Mammal Science 20(2): 189-200
da Silva, V.M.F. & R. Best. 1996. Sotalia fluviatilis Gervais, 1853. Mammalian Species (527):1-7.
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Gomez-Salazar, C., F. Trujillo, M. Portocarrero-Aya & H.Whitehead 2012a. Population density estimates, and conservation of river dolphins (Inia and Sotalia) in the Amazon and Orinoco river basins. Marine Mammal Science 28: 124-153
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Pavanato, H.J., G. Melo-Santos, D.S. Lima, M. Portocarrero-Aya, M. Paschoalini, F. Mosquera, F. Trujillo, R. Meneses, M. Marmontel & C. Maretti. 2016. Risks of dam construction for South American river dolphins: a case study of the Tapajós River. Endangered Species Research 31: 47-60.
Paschoalini, M., Almeida, R. M., Trujillo, F., Melo-Santos, G., Marmontel, M., Pavanato, H. J., … & Zerbini, A. N. (2020). On the brink of isolation: Population estimates of the Araguaian river dolphin in a human-impacted region in Brazil. Plos one, 15(4), e0231224
Venturieri, R., Oliveira-da Costa, M., Gama, C., & Jaster, C. B. (2017). Mercury contamination within protected areas in the brazilian Northern amazon-amapá state. American Journal of Environmental Science, 13, 11-21
Trujillo, F., F. Mosquera & N. Franco. 2019. Delfines de rio: especies indicadoras del estado de salud de los ecosistemas acuáticos de la Amazonia y la Orinoquia. Rev. Acad. Colomb. Cienc. Ex. Fis. Nat. 43(167):199-211. doi:http://dx.doi.org/10.18257/raccefyn.765.
May-Collado, L. J., & Wartzok, D. (2010). Sounds produced by the tucuxi (Sotalia fluviatilis) from the Napo and Aguarico rivers of Ecuador. Latin American Journal of Aquatic Mammals, 8(1-2), 131-136.
Da Silva, V. M., Carter, A. M., Ambrosio, C. E., Carvalho, A. F., Bonatelli, M., Lima, M. C., & Miglino, M. A. (2007). Placentation in dolphins from the Amazon River Basin: the Boto, Inia geoffrensis, and the Tucuxi, Sotalia fluviatilis. Reproductive Biology and Endocrinology, 5(1), 1-6.
Flores, P. A., Da Silva, V. M., & Fettuccia, D. D. C. (2018). Tucuxi and Guiana dolphins: Sotalia fluviatilis and S. guianensis. In Encyclopedia of marine mammals (pp. 1024-1027). Academic Press.