Florida’s manatees are dying from boat strikes in record numbers this year, but conservationists and boating advocates disagree over the reasons why.

Seventy-one manatees had been killed by boat strikes as of July 22, compared with 58 manatees by mid-July 2009, the deadliest year on record, when 97 of the docile marine mammals died in collisions, according to the Florida Fish and Wildlife Conservation Commission.

Patrick Rose, executive director of Save the Manatee Club, said an increase in boating traffic is to blame.

“With every recession you tend to have somewhat fewer watercraft injuries and mortalities, but with the robust recovery in the economy and really dramatic decreases in fuel costs, the number of boating hours goes up,” Rose said, adding that a mild winter and a hot summer also contributed to more boating this year.

But Jim Kalvin, president of Standing Watch, a boater advocacy group, said that pollution, harmed manatee habitat, and unsustainable manatee management programs are responsible for the increase in boat strikes. He said the manatee population has exceeded its “carrying capacity,” meaning there are more animals than the availability of sea grass, their chief food source, can sustain.

“Sea grasses are being foraged by an ever-growing manatee population, and they’re having to swim farther for food because their traditional grounds are barren,” Kalvin said.

“When they’re in a place with plenty of food, they don’t have to migrate,” he said. “But now they have to swim into areas where boaters haven’t seen them before.”

Kalvin said some animals are entering into travel corridors where boats can navigate at higher speeds, while others are moving into shallower waters with heavy vessel traffic.

“There is not enough room for both to occupy the space,” Kalvin said, adding that boaters in shallow waters must travel more slowly, which lowers propellers beneath the waterline and increases the likelihood of striking manatees.

Kalvin said that algae blooms caused by the release of nutrient-rich water from Lake Okeechobee are blocking sunlight and killing sea grass in certain estuaries, such as the Indian River Lagoon.

Rose agreed that algae blooms spawned by runoff of agricultural fertilizer are killing sea grass but said that over-foraging by manatees was not the problem.

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“Even with the remaining habitat we have, the carrying capacity is still multiple times the number of manatees we have, so there’s no scientific support for what he’s saying,” Rose said.

Carli Segelson, spokesperson for the Florida Fish and Wildlife Conservation Commission, agreed. “We have found no conclusive evidence of manatees reaching carrying capacity,” she wrote in an email.

Segelson also said that sea grass shortages are not causing manatees to migrate through waters that put them at greater risk of boat strikes.

“Our sea grass research staff conducts statewide monitoring, and while sea grass resources do vary from year to year for a variety of reasons, we do not see any evidence of significant changes in manatee behavior or distribution,” she said.

Manatees reproduce at a slow rate, with females giving birth to one calf every three to five years, Rose said. “There’s no way to have a population explosion,” he said.

The latest aerial survey of manatees, conducted in February, found there were at least 6,250 manatees in Florida. The animals are listed as endangered under the Endangered Species Act, although the U.S. Fish and Wildlife Service has proposed to reclassify their status as threatened.

In addition to boat strikes, manatees face cold winters, toxins from red tide blooms, and tourists who swim with the animals.

“Still, the overall single greatest cause of manatee mortality remains watercraft strikes,” Rose said. “The vast majority of manatees already have scars from being struck. Every manatee in Florida has been within an inch or so of losing its life.”

Rose said the solution to boat strikes is better education for boaters.

“We’re not saying boaters are bad people. They’re not,” Rose said. “It’s a rare boater who doesn’t care or feel it’s not worth slowing down for, and I say that as a boater myself.”