Sinking without Trace? Plummeting populations of Amazon river dolphins

Sinking without Trace? Plummeting populations of Amazon river dolphins

Amazon river dolphins are the lesser-known, freshwater cousins of the dolphin world, and comprise the boto and tucuxi species. The International Union for the Conservation of Nature (IUCN) has classified both species as Data Deficient, as insufficient data exist to determine if they are at risk of extinction. In a new PLOS ONE study, Vera da Silva from the Instituto Nacional de Pesquisas da Amazônia, Brazil, and colleagues examined population trends for the two species, which until recently were considered to be abundant in some places. Their analysis of 22 years of survey data uncovered shocking rates of population decline; I interviewed da Silva via email to find out more.

Da Silva (right) weighing an Amazon river dolphin

What drew you to study aquatic mammals, and specifically river dolphins?

VdS: I have been fascinated by dolphins since I was a student. I came to the Amazon to volunteer and encountered the Amazon dolphins, going on to study them for my masters and doctoral degrees. According to legend, Amazon river dolphins have the power to enchant people. It is true!

What are Amazon river dolphins like?

VdS: In many ways, Amazon river dolphins resemble the more familiar marine dolphins. Like marine dolphins, river dolphins have very poor eyesight and use echolocation to hunt and communicate. Unusually, the adult males are pink, and they are larger and stronger than females. Amazon river dolphins are very curious and friendly; even wild dolphins can come very close to boats and floating houses and interact with people.

The two species of Amazon river dolphins are called boto and tucuxi – tell us about them.

VdS: “Boto” is the Brazilian word for dolphin. The boto species is known by locals as “boto-vermelho” (red dolphin) and is also known as “pink dolphin” in English. These dolphins have long, narrow beaks and flexible necks, helping them to catch fish and to weave in and out of flooded forest regions. The tucuxi, or boto-tucuxi, species is in fact more closely related to marine dolphins than to the boto-vermelho. It is one of the smallest dolphins, reaching at most 1.5 meters in length, so looks like a miniature version of Flipper!

Since boto and tucuxi dolphins often live in remote parts of the Amazon basin, how do researchers get data about them?

VdS: The two species are found in almost all main rivers, tributaries and lakes of the Amazon river basin, but it can be a long journey to the different study areas. I live in Manaus, in the north of Brazil, which is over 500 kilometers from my study site at the Mamirauá Sustainable Development Reserve. To get there, I first have to reach the city of Tefé, a journey of 12 hours by fast boat or 50 minutes by plane. Once there, I board a small aluminium boat equipped with a 50 horsepower outboard engine, and travel for another 1.5 hours to reach the reserve. It’s a long commute!

A long commute: The boat journey to the reserve

In your study, you analyzed 22 years of population survey data. What did you find?

VdS: At the beginning of the project in 1994, we thought that these dolphins lived forever. But around 2000, we started noticing instances of killing of the dolphins for use as bait for catfish fishing. In fact, our data showed that populations of both boto and tucuxi dolphins were halving about every decade. The dolphin populations we’d been monitoring almost daily for several years were disappearing in front of our eyes. This was a big shock, and we started campaigning to protect the boto and tucuxi species. As a result, the Brazilian government established a five-year moratorium on this type of catfish fishing.

What else do you hope your study might lead to?

VdS: We hope that our study helps to protect the Amazon river dolphin and provides data to support the Brazilian environmental agency in decisions about the conservation of these important freshwater dolphins.

Research Article: F. da Silva VM, Freitas CEC, Dias RL, Martin AR (2018) Both cetaceans in the Brazilian Amazon show sustained, profound population declines over two decades. PLoS ONE 13(5): e0191304.

Images Credits: Projeto Boto; Tony Martin; Jonne Roriz


Beth works at PLOS as Journal Media Manager. She read Natural Sciences, specializing in Pathology, at the University of Cambridge before joining PLOS in 2013. She feels fortunate to be able to read and write about the exciting new research published by PLOS.

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