Tucked away in northwestern Panama, the San San and Changuinola rivers snake their way through banana plantations on their way to the Caribbean Sea. The two rivers, part of the 16,400-hectare (40,500-acre) San San Pond Sak wetlands, are home to the endangered Antillean manatee (Tirchechus manatus manatus). A passing glance into either river may not reveal the burly sea mammal, which can weigh up to 3,000 pounds and reach 10 feet in length, even though the waters are not deep.

That’s because the rivers are so murky. Though the opacity is due in part to naturally present waterborne sediment, thick aquatic vegetation and decaying organic matter, it is exacerbated by runoff from nearby farms and deforested land. For researchers, this means manatee populations cannot be tracked with aerial methods such as drones, as is done in Florida, says Héctor Guzmán, a senior marine biologist at the Smithsonian Tropical Research Institute in Panama (STRI).

So Guzmán and a team of engineers and other experts from the Technological University of Panama (UTP) and the National School of Electronics, Computer Science, Telecommunications, Mathematics and Mechanics (Enseirb-Matmeca) in Bordeaux, France, decided to listen instead of look. They installed four highly sensitive hydrophones in the two rivers to record the sounds manatees make.

Analysis of the sounds, recorded during 2015-18, have allowed them to estimate manatee population sizes and movement patterns in the two rivers, helping to address a dearth of data on the species in Panama and prompting continued hydrophone monitoring and experimentation.

“Estimating manatee populations in Panama is like working in total darkness—you can’t count what you can’t see,” Guzmán says. “When we started the project, we started diving to install the hydrophones, and we used full face masks and all this stuff, and it was awful. The water is terrible.”

Hydrophones are used around the world to detect various types of underwater noise. Marine researchers, for instance, use the instruments to record the calls of dolphins and sea lions in Monterey Bay, California and of orcas in the waters near Seattle, Washington. Now, in cloudy Panamanian river waters, they’re listening in on the Antillean manatee.

Previous attempts to track the species in Panama’s turbid rivers using other techniques such as sonar and even infrared cameras posed logistical challenges and proved costly. Hydrophones can measure underwater sounds with a high level of precision, and the use of several of them in an array can provide an even more sophisticated picture of the underwater soundscape, according to the U.S. National Oceanic and Atmospheric Administration (NOAA). The agency has installed hydrophones in four locations across its national marine sanctuary system, including Gray’s Reef National Marine Sanctuary off the coast of the U.S. state of Georgia, and recently began building a database of acoustically active organisms, including recordings of turtles, fish and invertebrates.

The technology is expanding in other parts of the world. In Mexico, for example, the nonprofit Cousteau Society has teamed up with a group of Mexican scientists to build a network of hydrophones to record and help collect population and movement data on the vaquita porpoise (Phocoena sinus), one of the most endangered marine mammal species on the planet. Researchers from the University of Washington in Seattle and France’s ENSTA-Bretagne Institute of Advanced Technology, meanwhile, have deployed hydrophones in the Indian Ocean to monitor blue whale populations.

Ongoing improvements in hydrophones, and growth in their manufacture and market size, are making the technology more effective and available for researchers looking to conduct acoustic monitoring of marine mammals. At the same time, digital sound analysis and ever-improving machine-learning algorithms to process acoustic data have boosted scientists’ ability to identify marine species and estimate population sizes.

“We produced these algorithms that were able to extract the moment when the manatees give a vocalization and, given previous work that show that each manatee has a certain pattern of acoustics, we grouped the sounds that were similar and were able to identify when a given manatee talks,” says Fernando Merchán, a UTP professor and researcher. “In this project we want to expand this and have better and optimized algorithms that are able to analyze more data, and build our own smart hydrophones with better electronics.”

Manatees in many ways are ideal subjects for hydrophone monitoring. Not only do they frequently produce underwater sounds that distinguish them from other species, but their calls also occupy individual frequencies similar to variations in human voices. This allows researchers to count and identify individuals and follow their movements.

The team over three years collected 300,000 audio samples, each about two minutes long, in a process called “acoustic capture and recapture.” They ran the sounds through an algorithm that groups similar manatee vocalizations in clusters, differentiating each individual, Merchán says. By counting such clusters, researchers can estimate population sizes—and do so without disturbing the manatees.

“With some species you use tags that work with ultrasound, and they’re kind of invasive because you have to put the tag on and so forth,” Merchán adds. “Here, we just listen. It’s very non-invasive.”Recording individual manatees’ vocalizations in different locations of the river allows researchers to track their migration, and to estimate the size of manatee congregations throughout the year. In Panama, March and May are the peak months for manatee gatherings, with numbers dwindling toward the end of the year, Merchán says. Over the course of the study, the population in Río Changuinola remained relatively stable. Researchers identified 42 manatees there in 2016 and 43 in 2018. In Río San San, the population fell from 35 to 26 in the same period. The team currently is working to complete analysis of data they collected this year and in 2019 so they can update the population numbers.

Guzmán says that since 2008, at least 20 manatees have died in the San San Pond Sak wetland, seven of them in 2019 and 2020 alone. “It’s really worrying,” he says.

A subspecies of the West Indian Manatee, the Antillean manatee has been listed as endangered on the International Union for the Conservation Nature’s Red List of Threatened Species (IUCN) since 2008. The population of the mammal throughout its range, which extends from the Gulf of Mexico as far south as Brazil, was estimated at fewer than 2,500 mature individuals in 2008, with a projected decline of more than 20% over the next 40 years.

For centuries the greatest threat to the animal was hunting, initially by pirates centuries ago, and later by employees of banana plantations, which expanded in Panama in the 20th century and used the mammals’ meat as food for workers. Complaints about the manatee’s plight began to reach the Panamanian government in the 1970s. At the same time, a worldwide movement to save the manatee began, helping to slow hunting.

Now, injuries from boat propellers and entanglements in fishing nets, which can drown young calves, pose the greatest threats to the sea cow. Guzmán’s team has found 17 fishing nets in one of the rivers so far this year. “It’s like a mine field for the manatees,” he says.

Pollution and habitat degradation have also contributed to declining numbers, according to the IUCN. In northwest Panama, banana plantations often conduct aerial agrochemical spraying of crops, which introduces toxins into the farm runoff entering local rivers and streams.

Scientists have proven a genetic connectivity between Antillean manatee populations in Panama and those in Mexico and Belize. With lifespans ranging from 40-60 years, they feed on sea and river plants, which helps control seagrass and keep waterways open, says Sofía Pastor, coordinator of the Sea cow manatee program of the Costa Rica Wildlife Foundation.

Pastor has spoken with Guzmán and his team about expanding the program into Costa Rica, where waters are also murky. Manatees have been sighted in northern Costa Rica’s Barra del Colorado Wildlife Refuge, near the border with Nicaragua, but the few attempts to research populations there have not produced conclusive results. Reports of encounters with sea cows and photos taken by community members are all researchers have to go by. No one knows for certain whether they are permanent resident manatees, or whether they migrate from country to country throughout the year. The foundation hopes hydrophones will help provide answers.

“The species is a mystery,” Pastor says, adding funds are being sought to help buy more equipment. “We have a lot to research,” he says. “Gaining more knowledge about the manatee will help us put the pieces of the puzzle together. This species is a focal point for conservation.”

Researchers at Honduras’ Cuero y Salado Wildlife Refuge have also expressed interest in joining the expanding network of acoustical manatee research in the region.

“The more equipment we deploy, the better” says STRI’s Guzmán. “Let’s say after a few months or a year, if we can manage to detect one that was recorded in Honduras or Costa Rica and then find it in San San, that will be a really good story.”

The team hopes to install hydrophones in the waters of the Panama Canal as well as other areas throughout the Americas. The growing datasets could be used to help the IUCN update its population numbers and determine whether to change the conservation status of the Antillean manatee from endangered to a greater or lesser level of protection.

The research team, which has received funding since January from Panama’s National Secretariat of Science, Technology and Innovation (Senacyt), recently launched #MisionManatee to spur hydrophone technology and manatee conservation. As part of that work, it is designing signs to reduce boat speeds and producing manatee-conservation documentaries in the native tongue of the Ngäbe-Buglé, Panama’s largest indigenous group.

The Ngäbe-Buglé’s territory is located near the San San and Changuinola rivers, and researchers say involving local indigenous communities will be key to the animal’s conservation. By providing these communities with information about the mammals, they aim to encourage them to develop ecotourism projects and protect manatee habitat.

They hope decision makers will take notice. For years, conservationists have been calling on the government to expand the limits of the protected San San Pond Sak wetland, which was designated as a site of international importance under the Ramsar Convention in 1993. This, they say, would provide additional safety for manatee mothers and calves in the wetland, which is also home to monkeys, sea turtles and sloths.

“We can produce our signs, we can publish our papers and our bosses will be happy with us, but we want to push it further,” Guzmán says. “We want to use our data to inform policymakers.”

He and other researchers also plan to produce their own hydrophones, with casings for the equipment that are suitable for local waterways. UTP’s Merchán says new models are already being prototyped. The hydrophones they have been using up to now are made in New Zealand and cost around US$6,500 each, a price that includes a special housing designed to protect the electronics down to depths of 650 feet (198 meters). But the rivers being studied in Panama are only about 15 feet (4.6 meters) deep at most. Manufacturing the technology locally for shallow waters could reduce costs by 60-70%, they estimate.

Guillaume Ferré, a researcher at Enseirb-Matmeca, says adding hydrophones to the monitoring network will allow researchers to detect the angle of arrival of a given manatee vocalization and thus reveal the animal’s location in real time. This data could be uploaded to the internet, sent to computers on nearby boats or fed into cell phone applications for the benefit of local communities, researchers and ecotour operators.

The website Orcasound.net, for example, allows visitors to listen live to a series of underwater hydrophones in either Port Townsend or Bush Point, both in the state of Washington, where killer whales are often recorded. The University of Victoria’s Ocean Networks Canada uses hydrophones to monitor Canada’s coasts and the Arctic, delivering real time data for scientific research, governments and industry.

Project organizers hope eventually to capture manatees in the San San River and temporarily hold them in cages in order to measure them, determine their sex and record their vocalizations for a week. They will then use the recordings to help differentiate the sound pitches males and females make and create algorithms to identify them in the wild.

“We hope to establish the rules of the game for local and regional monitoring, and evaluate threats, habitats and migration patterns of this species,” Guzmán says. “In our case, hydrophones are the only way to go. For the manatee population, it’s quite important at an international level.”