The tide has killed 241 of Florida’s roughly 5,000 manatees, according to the state Fish and Wildlife Research Institute, and the toll appears certain to rise.
The number of deaths from the tide far exceeds the previous annual record of 151. Most occurred along the lower west coast of Florida near Fort Myers, where an algae bloom that began last fall was especially severe and long-lasting.
“Southwest Florida is an area where a lot of manatees are during the winter months,” Kevin Baxter, a spokesman for the research institute, said Friday. “It’s a warm-water area. The bloom has persisted there for quite a while.”
Although the algae had largely dissipated by mid-March, he said, the manatee deaths are likely to continue for a few months because remnants of the toxin still cling to sea grasses. Manatees can eat 100 pounds of sea grass daily, said Pat Rose, an aquatic biologist and the executive director of the Save the Manatee Club in Maitland, Fla.
The state’s annual red tide affects a wide range of aquatic animals and can cause problems in people. The algae contain a nerve poison known as brevetoxin that is not only found underwater but that is also blown through the air when waves break open the algae’s outer casing.
Manatees, birds, dolphins and other animals can be killed by consuming the poison, either by accidentally eating the algae or by ingesting small organisms clinging to sea grass that have soaked up the poison while filtering seawater.
Residents and tourists regularly have respiratory problems after inhaling brevetoxins while strolling on beaches near red tides. People can also become ill after eating oysters and clams that have absorbed the toxin.
Experts are uncertain why this year’s algae bloom was so lengthy and toxic. Phosphorus runoff from fertilized farms and lawns may have contributed, because algae thrive on a phosphorus diet. The Caloosahatchee River, which runs through rural Florida farmland, empties into the ocean at Fort Myers.
But Mr. Rose and Dr. Martine DeWit, a veterinarian with the state’s Fish and Wildlife Conservation Commission, say a major cause may be an unfortunate coincidence of weather and timing.
Dr. DeWit said a mild, fairly windless winter helped the algae persist far longer than ordinary blooms, which generally die off late in the year. That meant large blooms remained offshore when the manatees, driven by a search for warmer waters, began moving to the Fort Myers area.
Manatees are attracted there every year by a warm-water discharge from a local power plant, Dr. DeWit said.
“We’ve seen in the past that when algae blooms coincide with manatee movement, it results in catastrophic mortality,” she said.
The red-tide deaths come amid what is shaping up as a disastrous year for the manatee, whose numbers have slowly been growing in recent years. So far this year, at least 463 have died from a range of causes, more deaths than had been recorded in any previous comparable period.
At least 80 more manatees have been killed this year in the Indian River in east-central Florida, where a huge phytoplankton bloom in 2011 killed most of the sea grasses. The manatees there appeared outwardly healthy, but autopsies indicated that they had severe intestinal distress and that their stomachs were generally filled with a different strand of algae that they were apparently eating in the absence of the grass they normally eat.
What is killing those animals is not yet known, but Dr. DeWit said it appeared to be related to the algae and could — like the west coast’s red tide — be tied to a poison.