What if slow-speed zones for boaters don’t protect manatees after all? What if the giant mammals can’t hear the quieter boats coming?
Edmund Gerstein, a Florida Atlantic University marine biology professor, thinks that might be the case. He has come up with a device he says will offer them better help. But Katie Tripp, director of Science and Conservation for the Save the Manatee Club, says the problem is not the zones, but the boaters.
Gerstein, who has performed hearing tests on manatees for 20 years, says the sea mammals can’t hear boats approaching at slow speeds because the sound of the propeller is on a lower frequency than they can detect.
“The problem is awareness,” he said. “The animal is relying on acoustic information when they’re in shallow waters where they can’t see the boat. … You take away their primary sense of detection by slowing down boats.”
Boat collisions are the leading cause of death for these animals, killing 100 manatees a year.
Using money from the U.S. Department of Defense, which wanted to find a way to keep its ships from hitting manatees, Gerstein invented a device he says sends off a warning sound to manatees when they get too close to a boat. It puts out a constant sound at a frequency only detectable by manatees and dolphins, Gerstein said.
The device attaches to the front of a boat so that when it is within several feet of a manatee, the animal can hear the signal and move out of the way.
But other scientists aren’t so sure Gerstein has found the answer.
“It’s a very significant issue,” said Tripp. “But his device is not the solution.”
Tripp said in order for the device to be effective, manatees would have to be conditioned to move when they hear the noise. The only way to do this, she said, is to hit the manatee with the boat so that it learns to move when it hears the sound.
“The idea that more responsibility should be put on the manatee is ridiculous,” Tripp said.
She added, “We know manatees can hear boats.”
A study from the University of South Florida found that manatees hear 10 times better than humans and are able to locate where sounds are coming from. The study suggests manatees might just be bad at avoiding boats.
Tripp said a possible reason Gerstein’s study indicated manatees can’t hear a boat approaching is that he used a boat with an electrical motor. She said this type of motor is typically used when fishing and is much quieter than the motors on boats that would be passing through manatee habitats.
“We’re not talking about apples to apples,” Tripp said. “It’s not the kind of motor manatees associate with boats.”
Gerstein said his invention — which is not commercially available — has been used by the Defense Department on some of its boats and the government is contemplating what to do with his research.
Gerstein and his wife, Laura, began working on the device more than 10 years ago. He said they were interested in testing why manatees often don’t move out of the way of boats.
Starting in 2007, Gerstein began testing the device in the open water in Cape Canaveral, because it’s protected and other boats aren’t allowed there.
He tested the device on 124 manatees and said about 95 percent moved from the path of a boat when they heard it. When the device wasn’t used, 95 percent of the manatees tested stayed on the same course.
Gerstein said the prevalence of slow-speed zones near manatee habitats has not changed the number of fatal collisions and may be causing more injuries to manatees.
“When we first started, we saw manatees with five or six hits on them,” Gerstein said, referring to the number of scar patterns left after propellers cut through their skins. “Now we see 50 hits on them.”
Tripp blames the problem on more people driving boats without boating licenses and without taking the proper training courses.
Gerstein said another issue is that when a boat moves slower, it’s in a manatee habitat longer.
“When you slow boats down, it takes longer to get through a manatee habitat, which increases the time of exposure and chance of collision,” Gerstein said.
Tripp was skeptical.
“For that same reason, we better speed though school zones,” she said.
crandle@sun-sentinel.com or 954-356-4661
