Manatees died in record-breaking numbers this year, but not from being hit by boats or poisoned by Red Tide. Instead, the largest group of the 699 manatees that were killed as of Dec. 5 were done in by bad weather.
A total of 244 manatees were killed by last winter's bitter cold snap, according to figures released Friday by the Florida Fish and Wildlife Conservation Commission's marine biology laboratory in St. Petersburg.
The cold deaths occurred across the state, reaching down even into the Everglades and the Florida Keys.
In addition to those 244, biologists say it is likely the cold temperatures also contributed to many of the 203 deaths that they ranked in the "undetermined" category as well as the 68 deaths in the "unrecovered" category, meaning carcasses that were spotted but could not be reeled in for examination. So far no manatees have been killed by cold this winter, state officials say.
Manatees are particularly sensitive to cold water. When the temperature drops below 68 degrees, they seek winter refuge in the warmer water flowing from springs and power-plant outfalls.
Last winter's cold deaths were different from the normal ones that occur, said Martine deWit, who oversees the state's marine mammal pathology lab in St. Petersburg.
Normally the manatees that are killed by cold die slowly. Their skin develops lesions, their metabolism slows down and they stop eating, she said. Generally they succumb to secondary infections.
But last winter's cold spell killed manatees more quickly, she said, causing what she called "acute cold shock" — akin to severe hypothermia.
Because of so many cold-related deaths, the total number of manatees killed this year is nearly double the five-year average for the same Jan. 1 to Dec. 5 time period. And with three weeks to go in the year, the number will undoubtedly break 700 before the new year arrives.
Although the cold weather was a natural event, biologists say this die-off underscores the importance of warm-water habitat for the long-term survival of the species.
"We have to be careful about reading too much into a single year of data," warned Gil McRae, the director of the state Fish and Wildlife Research Institute in St. Petersburg,
However, McRae pointed out that the mortality numbers are a minimum number. 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.
The same cold spell that killed so many manatees also produced ideal conditions for counting the number of live manatees last year. The result: a record number that topped 5,000 for the first time. That number — 5,076 — was also classified as a minimum number, meaning there could be more manatees out there that did not get counted.
In response to a question about whether the higher death rate is merely the result of having more manatees, McRae said, "The growth rate of the population is tracking less than you'd expect compared to the mortality."
As a result of last winter's wave of cold deaths, state officials have worked hard to make it easier for manatees to get to warm-water refuges, McRae said. They opened up a fence at Homosassa Springs Wildlife State Park that had previously kept wild manatees out of the park.
They also are dredging out an access to another spring.
Speeding boats, which are usually responsible for around a quarter of all manatee deaths, have not come close to that percentage this year. Boats were blamed in the deaths of 79 manatees as of Dec. 5, which is a decline from the record of 97 set last year. However, it's still within the range of the five-year average.
Manatees did dodge one bullet this year. Although this summer's Deepwater Horizon disaster has been blamed for the deaths of hundreds of turtles, thousands of birds and even some dolphins and whales, the gushing oil apparently didn't kill a single manatee.
Craig Pittman can be reached at firstname.lastname@example.org.