Mamiraua Institute and WWF navigate the Tapajos River to estimate dolphin populations and verify environmental conditions

Posted on September, 03 2014

Indicators of river health are fundamentally important to planning species and habitat protection. Research is conducted in area of planned hydroelectric dam construction.
In the same week that Amazon Day is commemorated, the Mamiraua Institute and WWF are publicising the preliminary results of an expedition undertaken to register the distribution of Amazon River dolphin species (tucuxi and pink river dolphins) and estimate their abundance in the Tapajos River basin.

The team of researchers travelled 577 km on board two boats in two different stretches of the river. The final result will establish an important information base making it possible to understand and accompany the health of the rivers and the Tapajos basin, in the Brazilian Amazon, especially in the light of risks posed by alterations to the environment like deforestation and the construction of hydroelectric installations.
The Amazon is the world’s biggest tropical forest and the greatest river system on the planet with over 100,000 km of water courses. It extends into nine countries: Bolivia, Brazil, Colombia, Ecuador, Peru, Venezuela, Guyana, Suriname and French Guyana. The river dolphins are one of the symbols of the Amazon territories but, up until today, little is known about their populations. Gathering reliable information is essential for designing conservation strategies to protect those species and their habitat.
The joint initiative unfolded by the Mamiraua Institute and WWF, with the participation of the
Omacha Foundation (Colombia) is the first scientific attempt to measure the size of Amazon River dolphin populations in the Tapajos River basin.

“Species abundance is one of the basic parameters for any study investigating population viability. It reflects the real status of the population and whether its existence is threatened or whether it is going to survive. Together with other information on mortality and reproduction for example, we can model and estimate the probability of extinction in the short, medium and long terms” declared Mamiraua Institute researcher Miriam Marmontel.

The study will also make it feasible to conduct an analysis of impacts on the fauna generated by hydroelectric construction and facilitate a discussion with the Brazilian government on their environmental consequences by providing better information to support decision making. In the stretches of the river that were surveyed, it is planned to build two large-scale plants: the Sao Luis do Tapajos hydroelectric plant, where work is scheduled to begin in 2018, and the Jatoba Hydroelectric plant, where work may start in 2019. The works will affect the entire ecological system and the social dynamics of the respective areas.

As many as seven dams may be constructed in the Lower Tapajos River Basin and many others in equally important stretches of river upstream. “This study shows the situation at a time when no dams have yet been constructed in the area and so it will serve to provide a baseline for comparison with other regions in the Amazon where dams have been installed and may well indicate the risk for the dolphins and the aquatic ecosystems. In the future, a new tallying effort could demonstrate the degradation of the region” Miriam explained.

Since 2006, several institutions (WWF, WCS, WDCS, Omacha Foundation/Colombia, Faunagua/Bolivia and the Mamirauá Institute/Brazil, among others) have been joining forces to carry out more than 15 expeditions to estimate the abundance of the river dolphin population in South America. Up until now more than 8,000 dolphins of the four species have been registered by expeditions, travelling a total of 6,500 km on the main Amazonian rivers. Such efforts seek to make it possible to cover other river basins and then make comparisons among the dolphin population densities in different watercourses.
Alongside other partners, WWF has accompanied the planning and construction of hydroelectric dams in the Amazon for several years. In the Tapajos River basin in particular, it has been conducting studies and debates and designing models in a bid to evaluate and demonstrate the accumulated impacts generated by the construction of multiple hydroelectric installations in a single river basin.
“The results of this first expedition to the Tapajos basin to estimate dolphin populations and verify environmental conditions will also enable governments, scientists, local communities and indigenous peoples to reflect on the potential impacts of dams on this particular river basin and on the Amazon as a whole. In what is one of the most important natural regions in the world, the hydroelectric planning needs to be of the highest standard, ecologically responsible and democratic, accompanied by a transparent discussion of, and respect for the rights of local communities and indigenous peoples” declares Claudio Maretti, leader of the WWF Living Amazon Initiative.

Logistics and methodology

The Tapajos River Dolphin Expedition was divided into two parts: a boat was used to travel on one stretch of the river from Santarem, state of Pará, upstream as far as the Sao Luiz do Tapajos rapids. The expedition boat also entered some tributaries of the Tapajos for sampling, like the Tuparí, and the Lago Verde as well as smaller water courses feeding the main rivers.

For the tallying done upstream from the rapids, the team first had to travel 250 km by car on the Trans-Amazon highway then by boat from the town of Penedo, the site of many mining activities, as far as the municipality of Jacareacanga.

On the trip 160 tucuxi river dolphins (Sotalia fluviatilis) were registered and 112 pink dolphins (Inia geoffrensis). The numbers will be subjected to statistical analysis by researcher Heloisa Pavanato to obtain the density and abundance of the animals.

Counting the animals is done by the distance sampling method in the following way: the boat goes upstream in a straight 100 metres out from the bank. The sightings are made in a strip of water 200 metres wide; 100 metres to the starboard side (right) and 100 metres to the port side (left). The type of information registered includes species identification (pink dolphin or tucuxi), size of the group (which varies from region to region, in this sampling it varied from one to nine individuals), and the distance of the animal from the shore and of the boat from the animal. The bank of the river where each set of sightings and registrations was done was photographed to typify the respective phyto-physiognomy. Data was gathered every 2.5 kilometres to further characterize the local phyto-physiognomy which may influence the number of animals. The numbers registered are corrected by for probability of detection given that it is easier to spot the animals that are nearer to the boat.

Results and recommendations

During the expedition no sightings of tucuxi dolphins were made above the São Luiz do Tapajós rapids. The researchers believe that this species does not have the size or strength needed to go up the rapids and above them only pink dolphins were found, with a low population density.

Another important fact is the corroboration of the occurrence of pink river dolphin upstream of São Luiz do Tapajos rapids until the limit sampling of this study in the municipality of Jacareacanga. According to the maps of the International Union for Conservation of Nature’s (IUCN) Red List of Threatened Species, the pink dolphin’s distribution goes as far upriver as the Itaituba municipality. This recent study has shown that its occurrence goes beyond that region and it may be that it goes even further upstream in the Tapajos and in its tributaries: the Juruena river and the Teles Pires.

The researchers also observed that the number of individuals sighted and registered was low, which may be due to external interference such as the presence of mining activity. “There are many mining dredges working in this area and that may explain the low numbers recorded”, added researcher Miriam Marmontel.

The Tapajos River Dolphin Expedition was conducted by the Mamiraua Institute and the WWF Living Amazon Initiative with the collaboration of the Omacha Foundation and the Humboldt Institute. It involved a team of eight researchers, a photographer and a cameraman as well as a support team, keyboarders and the boat crew.

The direct impacts of dams on the fauna
According to Mamiraua Institute researcher Heloise Pavanato, who coordinated the voyages, dam construction has direct consequences for the aquatic and terrestrial fauna and for the forest itself. In the case of the river dolphins, the problem is that there is no knowledge about the movements of these animals or of how far they manage to go upriver or down. According to the researcher, building a dam creates a barrier to such movements, isolating the individuals in certain stretches. That reduces interactions among the animals leading to a reduction in genetic variability, increasing susceptibility to disease and over the long term it may lead to their local extinction.

About the species

The pink river dolphins, known locally as botos, are a charismatic species and to this day little is known about them. The biggest river dolphins in the world inhabit the Amazon and they can be up to 2.55 metres long and weigh as much as 160 kg. One feature that distinguishes them from marine dolphins is their longer neck and ability to move their heads from side to side, an important difference because it gives them the flexibility they need to seek out their food in among the tree trunks and branches of the flooded forest.

Dolphins are carnivorous cetaceans at the top of the food chain. They can swim for miles in the main rivers tributaries and lakes in search of fish, their main diet. Their presence is always the sign of a healthy river. They also help control the health of the fish populations because they feed on the most vulnerable fish thereby contributing to the process of selection.

In the Amazon there is one species of tucuxi dolphin (Sotalia fluviatilis) and there are three species of pink dolphin (Inia geoffrensis, Inia boliviensis and Inia araguaiensis) distributed in the Orinoco, Amazon River and Araguaia-Tocantins basins. The pink dolphin species Inia boliviensis occurs in the sub-basin in Bolivia and can be found as far downstream as Borba in the lower course of the Madeira River, in Brazil. In 2014, the first discovery of a new river dolphin species for a 100 years was announced: Inia araguaianens, restricted to the area of the Tocantins-Araguaia River basin in Brazil, in the transition area between the Cerrado and the Amazon.

The threats to the dolphin populations are mercury contamination resulting from mining activities, intentional capture to use as bait for catching the piracatinga catfish (Calophysus macropterus), and accidental death from entanglement in fishing nets.


Amazon Day is commemorated on September 5, the date of publication of Act n° 582, which in 1850 created the Province of Amazonas, separating the region from the Province of Para.

Boto rosa no Rio Tapajós e mercado de Santarém, PA
© Adriano Gambarini / WWF