Freshwater dolphins are some of the most unique mammals on the planet. Like bats, the dolphins use echolocation to forage for prey in the cloudy waters of the Orinoco and Amazon River basins.
Now, researchers are using their clicks, buzzes and squeaks to monitor the species' movement within its habitat.
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What's Happening to the Freshwater Dolphin?
A new study published in Scientific Reports looked at two different species of freshwater dolphin, the Amazon river dolphin, also known as the boto, and the Tucuxi dolphin. The boto has a distinct pinkish hue and skinny snout and the Tucuxi is a mostly grey dolphin with a pink belly.
Experts suspect that about 50 percent of the dolphin population declines every 10 years. The population of the Tucuxi freshwater dolphin drops 50 percent every nine years. Models predict that the boto dolphins will be reduced by 95 percent within half a century.
Why are the Dolphins Endangered?
Currently, all river dolphin species are threatened with extinction due to commercial fisheries, dam destruction and river changes due to pressures from agricultural and cattle farms.
Listening to the hydrophones or an acoustic device that collects sound reveals the dolphins' movements in their environments. It gives experts a look into how these river creatures use their habitat.
By eavesdropping on the freshwater dolphins, scientists can track the species in inaccessible parts of the rivers, flooded areas and its interactions with humans, all while getting an accurate count of the marine mammals without too much disturbance.
The data will also help scientists understand what is needed to protect the endangered species.
Changing Water Levels
The boto dolphins thrive in a unique environment that changes seasonally from rainfall. Half of the year, large areas of the riverine forests are flooded, giving the aquatic mammals access to areas rich with flowing fish from the submerged vegetation.
When waters are low, river dolphins stick to the main rivers and only move when fish flow through the floodplains. When river dolphins sense that water levels are dropping, they move toward the main rivers to avoid entrapment.
When water levels rose in the bay and river channel between November 2022 and January 2023, dolphins increased their presence from 10 percent of the day to 70 percent. Researchers could hear the mammal’s chips and echolocation clicks peak at the river’s entrance, consistent with the animal’s seasonal visit to the Várzea or floodplains during the rainy season.
The hydrophones used in the dolphin study also allowed the team to track how the dolphins are distributed along the mosaic of the floodplains. In contrast, tracking tags would be too difficult to collect once the dolphins venture into their habitats.
Female boto dolphins with their calves and adolescent dolphins usually spend more time in these floodplains than males. Possibly because of an abundance of food resources and shelter against aggressive males. If water levels allow, female and younger dolphins will stay here instead of returning to the main rivers.
Acoustic Monitoring
The team used five hydrophones to listen in on the dolphins. Each one was submerged at depths between 3 meters to 5 meters and collected data from the Mamirauá Sustainable Development Reverse in Brazil.
After researchers collected the data from the wet and dry seasons, they analyzed it using an AI algorithm called a Convolutional Neural Network. The algorithm detected dolphin echolocation clicks with 95 percent accuracy, noise from boats with 92 percent accuracy and rain with 98 percent accuracy.
Since the team could gather data on the dolphins' movements in and around the floodplain, they are calling attention to how passive acoustic monitoring, or sound recordings to study wildlife, can help monitor vulnerable species and understand how they interact with their habitats.
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