Manatees may get hearing tests

By: 
Jul. 15, 2003

by Richard Prior

Staff Writer

The Jacksonville Waterways Commission is asking for an extra $25,000 as it continues fishing for answers to an unexplained spike in manatee deaths last year.

The City Council tonight will consider the special request to study the acoustic effects of dredging on manatees in the lower St. Johns River.

Half the appropriation would come from the Florida Boater Improvement Fund. The balance would come from the Jacksonville Port Authority.

“We’ve done some work that would indicate dredging may be impacting manatees in Duval County,” said Dr. Quentin White, professor of biology and marine science, and dean of the College of Arts and Sciences at Jacksonville University.

The problem is not the dredging equipment itself, White said, but the noise it creates. If a manatee’s hearing is overwhelmed, it can’t pick up the sound of an approaching boat until it’s too late to get out of the way.

“Natural underwater sounds are pretty phenomenal, which you would hear if you did any snorkeling, particularly in a salt marsh environment,” said White. “Shrimp clicking is loud; drums, the fish, make this drumming noise; and croakers make their croaking sound.

“That can be a lot of noise. Add a large man-made noise on top of that, and it gets pretty loud down there.”

The additional money would pay for an array of acoustical buoys to be placed around dredging equipment, which is clearing the way for the Wonderwood Expressway, White said. The recording devices would register the equipment’s volume and tone to compare them with sounds manatees normally hear.

“That will help us begin to figure out whether the noise of the dredge is masking the noise of approaching boats,” said White.

Periodic dredging is also needed to keep the channel open and clear silt out of the St. Johns’ tributaries.

Dr. Edmund Gerstein, who is affiliated with Florida Atlantic University, is principal investigator on the project.

In addition to studying the acoustics, Gerstein has suggested the possibility of outfitting boats with noisemakers that would project sounds on a frequency that manatees can detect. The extra warning would give the lumbering mammals more time to maneuver out of the way.

“That idea is still in the preliminary stages,” said White. “It’s still not demonstrated that it’s a worthwhile thing.”

Manatees tend to stay close to the shoreline, particularly around the grassbeds where they feed, said Greg Radlinski, who drafted the appropriation request on behalf of the Waterways Commission.

“They generally don’t extend much beyond the docks, maybe 50 to 100 feet off a dock line,” he said. “They have a tendency to stay more on the southbank of the river as it moves through downtown.

“They generally don’t go to the very deep channel.”

But when they do, manatees can run into a lot of trouble from boat traffic.

There was an “anomalous increase” in the number of manatee deaths in the lower regions of the river during the spring of 2002. Ten were killed, as compared with one, or possibly two, in other years.

“The thought was perhaps the dredging somehow affected the manatees,” said Radlinski. “The dredging certainly changed boating patterns. Perhaps that had led to the increase in deaths.”

Too many variables frustrate investigators as they try to determine the size of a boat that has killed a particular manatee, White said. However, indications are that larger vessels were responsible for seven of the 10 “anomalous” deaths in spring 2002.

“Three [of the deaths] more clearly came from smaller vessels,” said White, stressing that the information is based on a “very, very tenuous analysis.”

Resolving the question is more than an academic pursuit. Hearings on expanding slow-speed zones in the river have created several disturbances of their own. And the question of whether small or large boats are primarily responsible “makes a difference in terms of what you can do,” White said.

“Most large vessels are moving slowly, but most regulatory effort has been aimed at smaller vessels,” he added. “Perhaps we’re misdirecting the effort.

“I believe our current plan is working, and we do not need to establish more zones for slower boat speeds.”