Listening to Amazon River dolphins can help us save them

Two endangered freshwater dolphin species are quietly fighting for their survival in the remote and often inaccessible parts of the Amazon Basin. In a new study published in the journal Scientific Reports, researchers have been investigating the behavior and movement of Amazon River dolphins by using passive acoustic monitoring.

The pink river dolphin (Inia geoffrensis) and the tucuxi (Sotalia fluviatilis) are both victims of increasingly hostile human activities. These species, already battling dwindling numbers, are threatened by conflicts with fishermen, as well as the ever-expanding pressure from agriculture, mining, and dam construction. 

Focus of the study

During the wet season from April to August, tucuxi and pink river dolphins venture into the seasonal floodplain forests (várzea) to search for food, primarily freshwater fish. 

However, the dense vegetation and floodplains pose significant challenges for researchers attempting to survey these dolphins using traditional methods like boats or drones.

How the study was conducted

The researchers, led by Florence Erbs and Michel André, developed an innovative approach. The team deployed a network of five hydrophones, each submerged to depths of between three and five meters, covering an area of 800 square kilometers within the Mamirauá Sustainable Development Reserve in Brazil. This area marks the convergence of the Solimões and Japurá rivers.

The team captured sound data across diverse habitats – river channels and confluence bays, floodplain lakes, and flooded forest. This data collection was carried out at different intervals during the wet and dry seasons spanning from June 2019 to September 2020.

The experts utilized sophisticated deep learning algorithms, known as a Convolutional Neural Network (CNN), to classify the detected sounds. The algorithms, trained on sound data manually collected from boat surveys, were able to distinguish between echolocation clicks from dolphins, boat engine noises, and the sound of rainfall, with remarkable accuracy rates of 95, 92, and 98 percent respectively.

What the researchers learned 

In the course of their research, the team discovered a fascinating pattern: the presence of dolphins in the surveyed areas increased significantly from 10 percent of the day to 70 percent as water levels rose between November and January. This suggests that dolphins were using these waterways to venture into the floodplain. 

It was further observed that boto adolescents and females with calves tended to spend more time in the floodplains than the males, possibly due to an abundance of prey or as a shelter against aggressive male behavior.

Implications of the research 

The researchers are advocating for the wider adoption of this methodology, emphasizing its potential to greatly enhance our understanding of the habitat preferences and requirements of Amazon River dolphins. 

“This study demonstrates the suitability of using CNN-based classification to automatically detect river dolphin echolocation clicks in the complex soundscape of freshwater habitats,” wrote the study authors. 

“The efficiency and speed of the CNN method allow us to analyze the totality of the data collected without having to subsample as usually done for manual analysis, making it possible to detect major movements of dolphins in the study area, and rare passages in specific habitats or seasons.”

Conservation strategies 

The researchers said that passive acoustic monitoring in floodplain forests could help experts understand the habitat preferences and requirements of river dolphins, especially the boto females and calves. 

The experts also noted that forecasting the dolphins’ response to habitat loss and degradation will contribute to the management strategies of the aquatic-terrestrial transition zone, which is critical for the maintenance of habitat connectivity.

“Another area of applications is towards developing and implementing standardised protocols to monitor distribution shifts in relation to the recent amplification of drought and flood events in the Amazon basin,” wrote the researchers. “As sentinel species of the aquatic systems they inhabit, river dolphins can constitute an early detection system of ecosystem unbalance.”

More about Amazon River dolphins

Amazon River dolphins, often known as boto or pink river dolphins (Inia geoffrensis), are one of the few species of freshwater dolphins in the world. As their name suggests, they inhabit the Amazon River and its tributaries across several South American countries. Here’s some more information:

Physical characteristics

Botos are the largest freshwater dolphins, with males reaching up to 2.5 meters in length. They are renowned for their distinctive pink color, which can vary from a dull grey to vibrant pink, depending on age, diet, and exposure to sunlight.

Behavior and diet

These dolphins are generally solitary or live in small groups. They’re known for their exceptional agility, allowing them to navigate the Amazon’s complex river system and flooded forests. Botos have a diverse diet, consuming more than 40 species of fish, and occasionally, crustaceans.

Reproduction

Boto dolphins have a long gestation period of 11-15 months, and usually, only one calf is born at a time. The young dolphin stays with the mother for an extended period, learning necessary survival skills.

Tucuxi 

The tucuxi (Sotalia fluviatilis), another species inhabiting the Amazon basin, is smaller and grey-colored. While they share the same threats as the boto, they also face additional challenges from hunting for bait and human consumption.

Efforts are ongoing to understand and conserve these unique species, and the use of technologies such as hydrophone monitoring and machine learning offers promising new ways to aid in their conservation.

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