ST. PETERSBURG — For a third straight year, cold weather and speeding boats took a serious toll on Florida's manatee population, with 2011 racking up the second most manatee deaths on record.
State biologists working for the Fish and Wildlife Research Institute in St. Petersburg reported finding 453 manatee carcasses last year. They documented a record 766 manatee deaths in 2010 and recorded the third-highest total of 429 in 2009.
Manatees are sensitive to cold water. When the temperature drops below 68 degrees, they seek winter refuge in the warmer water flowing from the springs gurgling up from beneath the earth and from power plant outfalls dumping heated water back into a bay or river.
The bitter cold of last winter killed 118 manatees. "We saw animals that had acute cold shock," said Martine Dewit, the veterinarian in charge of investigating manatee deaths.
The record for cold-related deaths, set in 2010, is 282. Biologists suspected that hundreds more were killed by the cold that year, boosting the number above 500, but some carcasses were so decomposed that they could not verify the cause. Something similar may have occurred last year too, Dewit said, although there is no way to tell for sure.
Cold killed at least 56 manatees in 2009 — more than double the previous year's toll, signaling the start of a trend state biologists find particularly troublesome.
"We are concerned about the number of manatee deaths the past three years, including those resulting from exposure to cold weather," said Gil McRae, director of the state's wildlife research laboratory. "Over the next few years, we will use data from monitoring programs to better understand any long-term implications for the population."
Manatees have been listed as a federally endangered species since the first endangered list was published in 1967. They are a popular icon of Florida, featured on everything from license plates to tourist knickknacks.
But so many have been clobbered by boats that scientists use their scar patterns as a way to tell one manatee from another.
Regulators have tried for decades to cut back on the number of boat-related manatee deaths — using speed limits and other measures. They have focused on that one cause because it's the one aspect of manatee mortality that humans can control, unlike Red Tide and cold, and generally accounts for one-fifth to one-quarter of all deaths.
However, in the past 40 years, only two boaters have been prosecuted for killing manatees.
Boats accounted for 88 of the deaths this past year, which Dewit called high. The number is an increase from 2010, when boats hit and killed 83 manatees. The record for boat-related deaths is 97, set in 2009.
Boaters plying the waters around Pinellas County killed seven manatees, tying for second most in the state with Brevard County. The deadliest waterways in the state were in Lee County, around Fort Myers and Cape Coral, where boaters killed 14 manatees.
Biologists who work with manatees always point out that the mortality numbers are a minimum. In other words, there were undoubtedly more manatees that died that no one saw — say down amid the Ten Thousand Islands, or along the Big Bend area's sea grass beds.
In the same way, they say the numbers counted during their annual aerial survey should not be taken as gospel. The 2010 survey counted 5,076 manatees, and last year they found 4,834. So far there has been no count scheduled for this year, Dewit said.
Craig Pittman can be reached at email@example.com.