Though red tide blooms are decreasing off the Lee and Collier county coastlines, the toxic algae still is taking its toll on manatees.
In two months' time, 16 manatees have died in Lee and Collier counties, bringing local deaths for the year to date to 93. Of the 16 animals that died in October and November, red tide is the suspected culprit for eight.
On Thursday, biologists with the Florida Fish and Wildlife Conservation Commission posted a listing of all manatee deaths since March that were because of red tide or suspected to be because of red tide. The commission also sends out periodic manatee mortality updates prior to releasing a yearly summary in January.
Statewide, 79 manatees have died from suspected red tide-related illnesses since March. Almost 40 percent of the dead manatees were found in Lee County. Two were found in Collier. Two also were recovered from the upper reaches of the Caloosahatchee River in Hendry and Glades counties.
When manatees encounter a red tide bloom, they don't necessarily die right away, said Ken Arrison, wildlife biologist with the Florida Fish and Wildlife Conservation Commission.
He said some of the mammals that died in Lee County could have been exposed to the algae in northern counties where algae concentrations were higher.
"Animals can be exposed to it and leave the area and die somewhere else," Arrison said.
Red tide stretched up and down the Gulf coast this summer, leading to fish kills as far north as the Panhandle and as far south as Naples and Marco Island.
The blooms have been receding off the coast, but they still are lingering out there. Several miles off the coast of Fort Myers, high concentrations of red tide were reported last week.
Red tide is a saltwater algae that thrives with sunlight, warmth and nutrients. When the algae encounter the right conditions, they start rapid reproduction and create a bloom. The algae release a poison, called brevetoxin, that in high concentrations kills fish and marine mammals, such as manatees and dolphins.
"Unfortunately we don't know the lethal dosage of brevetoxin for manatees," Arrison said.
Arrison said he is still awaiting toxicology results for the most recent manatee deaths. Once those results are in, the statistics could change.
October and November mortality counts for Lee County show eight out of 13 manatees are suspected to have died from red tide toxin. One animal died in October from an encounter with a flood gate or a canal lock. The others were natural or undetermined deaths, which could still turn out to be related to red tide once toxicology tests are complete.
During the same period in Collier, none of the three dead manatees were tagged as potential red tide victims. Two died as babies and the cause of one death was unknown, meaning it had decomposed to such an extent researchers could not figure out how it died. Samples from the bodies of the three sea cows will be tested, however, for red tide toxin.
Though the Caloosahatchee estuaries are losing seagrasses, manatees aren't dying from lack of food, Arrison said. He said he hasn't seen any emaciated sea cows, but he has seen a few strange things in manatee stomachs. The mammals eat seagrasses and mangrove leaves, but Arrison has also found lawn clippings and artificial plants in their stomachs.
Statewide, 358 manatees have died this year. Most died of natural causes, including red tide, but about 20 percent died after encounters with boats. In Lee County this year, nine manatees have died in boat accidents and in Collier four have died from the same cause. More manatees die from boating accidents in Lee County than any other county in the state, but Lee County also sees more dead manatees in general.
Denise Boyd, marine research associate at the manatee Southwest Field Laboratory, said conscientious boating — obeying speed zones, keen awareness and wearing polarized sunglasses — can help people detect manatees under water.
People who accidentally hit a manatee or who find a sick manatee should call the FWC at 888-404-3922, Boyd said.