Skip to content

Connect with WHOI:

Put the D-tag on the Manatee

Put the D-tag on the Manatee

Digital device is enlisted to help prevent boat collisions with endangered species


When boats hit manatees, the outcome is often grim. Bones are broken and skins sliced. Despite regulations designed to prevent collisions, dozens of manatees in Florida continue to be injured or killed—including a near-record 86 deaths in 2006.

Searching for better ways to protect the endangered species, a team of researchers began a quest to learn exactly how the slow-moving, bulbous-looking marine mammals behaved when boats motored past.

At first, the team used a large tethered balloon equipped with a video camera to hover above manatees and record their reactions. Half the manatees they filmed appeared unfazed by passing boats. The other half left their shallow, seagrass habitat for deeper, darker, or cloudier water. Without a way to see under water, it was frustratingly hard to determine precisely how the boats influenced manatee behavior.

“We couldn’t gauge their reactions,” said Douglas Nowacek, a biological oceanographer from Florida State University. “Were they going deep? Calling to each other? Seeking each other out? Or trying to hide?”

To find out, this summer Nowacek harkened back to his graduate student and postdoctoral days at Woods Hole Oceanographic Institution. In the late 1990s, he was part of a team of biologists and engineers who were devising a small, high-tech device called a D-tag, short for digital acoustic recording tag. It temporarily sticks to whales with suction cups, recording their movements along with the sounds they make and the sounds they hear under water.

With the right modifications, Nowacek and colleagues reasoned the tag could be used on manatees. Once the marine mammals’ behavior was better understood, boaters could be educated and adjust their own behavior on the water to cause less harm to manatees.

The ‘scar catalogue’

While a squeaky-voiced cartoon mouse may capture hearts in central Florida, manatees have charmed people along the state’s coasts. Round-headed and squinty-eyed, their bodies are up to 13 feet long and shaped like an overstuffed pillow. Despite their lumbering appearance, they are graceful swimmers—and to many, a vanishing symbol of wild Florida. Since 1966, they have been listed as endangered species. Wildlife refuges have been established for them, and tourists pay for supervised swims in their company.

Manatees’ spend quiet lives resting and eating grass near the surface, where they share space with an increasing numbers of recreational and commercial boats. Manatee biologists with the Florida Fish and Wildlife Conservation Commission estimate that at least 3,000 animals in the state occupy the same waters as approximately one million registered recreational boats and thousands of additional out-of-state boats that flock in the summer months.

“Granted, not all of these boats are in the water at the same time,” Nowacek said. “Still, it’s a lot of boat traffic when you compare it to the manatee population.”

Boat propellers often leave unique scarring patterns across manatees’ backs, which researchers use to tell the animals apart, and keep track of them. Of the nearly 2,000 manatees in the state’s “scar catalogue,” more than half have scar patterns indicating that they have been struck by boats more than once.

“We’ve had dead manatees come in for necropsies (to determine how they died) that have scars, healed-over broken ribs, or other evidence from 50 boat interactions,” Nowacek said. “That means one animal has been hit 50 times by boats in the course of its life. It’s astounding.”

Does speed kill?

Researchers are hopeful that D-tags will help provide a window into the manatees’ world, in the same way they have helped researchers learn more about whale behavior in an effort to prevent whale-ship collisions. The main purpose of the tag is to record sound, which can be sampled up to 96,000 times a second. Additionally, sensors on the tag record temperatures in the water surrounding the manatees, as well as the animals’ depth, pitch, roll, and direction.

Synchronizing the boat sounds with the manatees’ behaviors allows researchers to learn how manatees react—or don’t react, as filmed observations showed—to boats. It will also allow researchers to determine if manatees react differently in shallow water versus deep water, where they might have more options to hide.

While the tags are on manatees, researchers take notes about local boats approaching or passing the manatees. Researchers want to find out if the speed, and even the type, of boats affect manatees’ behavior. For years, Florida wildlife managers have asked boaters to slow their speed in areas populated by manatees. Reducing speed, they theorized, would give boaters a better chance of seeing and avoiding manatees on the surface, and give the animals a fair chance to hear vessels and move away before a strike.

But as the number of collisions has ballooned, critics have argued that slower speeds actually harm the animals, which may not hear the motors of a slow-moving craft until the vessels are nearly on top of their backs.

The current research will be essential for addressing this and other questions as policymakers create regulations designed to reduce collisions. For example, little is now known about how many times each day a manatee is disturbed by vessels, said biologist Chip Deutsch with the Florida Fish and Wildlife Conservation Commission. Or how much time manatees have to avoid collision when they hear a boat.

“We will have a detailed picture,” said Athena Rycyk, a doctoral student at Florida State University working with Nowacek. “We will be able to ask questions like, what does a 20-foot boat traveling 200 meters from a manatee swimming in six feet of water sound like? This can help us determine what, if any, acoustical cues manatees do or do not react to.”

Recording sound will also help scientists learn about the sounds that manatees make: what type of vocalizations they use, how often they vocalize, and how vocalizations change depending on their activity. The D-tags will even allow the scientists to learn if the manatees make more or fewer calls at certain periods of the day, or when they are alone or with other manatees.

Fits to a manatee

Nowacek knew the D-tag could work on manatees, after testing the device on two animals during a 2001 study in Belize. But in the tests, the D-tag’s suction cup attachment—intended to adhere easily (and non-invasively) to smooth-sided whales—would not stick to the rough-skinned manatee.  He contacted WHOI research engineer Mark Johnson, who along with his engineering colleague Tom Hurst, teamed up to make a custom-designed device for manatees.

Hurst and Johnson learned that Florida manatee researchers attach their own tracking devices to manatees using a padded rubber tube belted around the animal’s peduncle (the area above the tail). Initially they experimented with a tag that clamped to the tube, but nixed that design approach after finding it could rub and irritate the animals’ skins as they moved through the water.

“We’re trying to learn about their behavior; we don’t want to affect their behavior,” Hurst said.

Still, researchers acknowledge that they expect some behavior modifications immediately after captures, so they delay the start of recording by several days—enough time, they hope, to allow the manatees to return to their normal routines.

“Imagine if someone just attached something to your arm and sent you on your way,” Hurst said. “You are going to need time to get used to it before returning to routine activities.”

Inside a D-tag

The tag’s final design enclosed electronics inside a watertight, black plastic housing that they could mount on the belt with a plate and several screws. (It resembles a BlackBerry attached to a person’s belt). This design didn’t move around—latching securely, like a ski boot into a binding, which avoided skin irritation and ensured the collection of quality data. Yet it would still easily detach, at a pre-programmed time, after gathering data for two days. The belt also has other tracking devices for other scientific studies, including a GPS tag that provides coordinates for both boats and manatees. It is removed by biologists several weeks later.

Inside the tag’s housing is a chunk of syntactic foam that keeps the device afloat when it is released; a radio beacon that beeps to help researchers find and recover the tag; batteries (the same type used in cell phones); a motor that triggers the release at a predetermined time, and the electronic components that record data.

Before trying the tags on wild manatees, Nowacek and colleagues tested the D-tags on captive manatees living at the Mote Marine Laboratory. Then they tested them on a wild manatee undergoing rehabilitation after a boat collision, and another captured for a health assessment.

In August, scientists turned their attention to Lemon Bay in southwest Florida. It teems with watercraft, from motorized fishing boats to luxury yachts, traveling through a channel connected to the Gulf of Mexico. With its warm temperature and lush seagrass beds, Lemon Bay is also populated by manatees.

To catch a manatee

This August and September, it was the job of Florida Fish and Wildlife Conservation Commission biologists Chip Deutsch and Margie Barlas to catch manatees and attach the belt and D-tag. Their work required a team with as many as 15 to 20 people, some who worked from a boat for the tagging, and others who took to the sky to find by plane the right manatees to tag. They looked for adults that were alone or in small groups, and those without calves.

Their team launched vessels into targeted areas then waited for word from spotters in airplanes about an appropriate animal to tag. Once in sight of an identified manatee, they dropped a large, open net behind the boat. With the manatee inside, they pulled the net closed, like strings on a purse, and drew it to the rear of the boat.

Next came muscle power. The team pulled each 600- to 2,000-pound manatee onto the vessel, which had a large, flat bottom and an open stern ideal for bringing large animals on board. Once there, the animals received a full health assessment, including body measurements and back fat thickness (a measure of body condition), and blood, nasal mucus, fecal, and urine samples, even samples of tears.

While on board, the team attached the belt and D-tag, which would start recording movements, depth, and sounds within two to three days after the animal’s release. Two days after that, the tag popped off, was picked up by the team, and shuttled to a lab to download the data.

Chewing, and other sounds

This year, a total of 10 wild manatees were tagged. Nowacek and Rycyk said it is still early to draw any conclusions about what they hear on the tags, though the sound files have proved interesting listening. “We hear a lot of chewing,” Nowacek said.

Roars recorded from some passing boats was startling, Rycyk noted. “When a boat flies by, especially a boat moving fast, it is really, really loud,” she said. What’s more, boats can be heard for up to a minute before actually passing near the manatee.

Nowacek said they will complete a second year of field work using the tags before they spend a year sorting through the data and conducting analysis. “It’s fairly complex data to sort out—boats moving, manatees moving, vessel noise, and manatee behavior,” he said.

This fall, the tags were sent back to Hurst for cleaning and modifications in preparation for the 2008 field season. Among his challenges will be to make the tag ever smaller and lighter, with new sensor features and longer battery operating time. “Tags are like cell phones and video cameras—they keep shrinking in size,” he said.

Meanwhile, Hurst is also turning his attention to D-tags designed for use on other marine species: dolphins, harbor porpoises, and larger whales.

Amy E. Nevala

Funding for this research came from the Florida Fish and Wildlife Conservation Commission, with support from the Disney Wildlife Conservation Fund.