Endangered Florida Manatee Clinging to Survival

RESTON, Virginia, August 6, 2004 (ENS) - Manatee populations are growing at healthy rates in two of four regions off Florida's coast, but the endangered marine mammal is stalled or declining elsewhere in Florida, according to the first comprehensive population analysis published in nine years.

The new report by the U.S. Geological Survey (USGS) features three studies led by USGS scientists, who documented survival, reproduction, and population growth rates of the Florida manatee.

The Florida Fish and Wildlife Conservation Commission, Mote Marine Laboratory, and the Florida Park Service collaborated in the research, published in the July issue of the journal "Marine Mammal Science."

Michael Runge, lead scientist for the USGS explained, "Over the past 10 years, the annual population growth rate in two of the regions is healthy, one region appears to be declining, and one region is probably holding steady. That's what the science says."

The Florida state mammal, the West Indian manatee, Trichechus manatus latirostris, can grow up to 13 feet long and weigh 3,000 pounds. (Photo courtesy U. Florida)

The estimates that the annual growth rate for the manatee population in Florida's Atlantic region is 1.1 percent, but the margin of error in this estimate is large enough that it is possible the population is stable or slightly declining.

In the Southwest region, the estimated population growth rate is negative - minus 1.1 percent per year - but the scientists say the margin of error in this estimate is larger than in the other regions.

"The estimates for the Atlantic and Southwest are of concern, because these regions make up most of the manatee population," said Runge.

Slow-moving vegetarians, manatees are killed by exposure to toxic red tides, by watercraft injuries, floodgate and canal lock injuries, cold stress, disease, and injuries during birth.

Biologists with the Florida Fish and Wildlife Conservation Commission’s Florida Marine Research Institute have confirmed that, based on 2003 necropsy reports, 380 manatees died in state waters that year.

Team at the Florida Marine Research Institute conduct a necropsy on the carcass of a manatee to determine the cause of death. (Photo courtesy Florida Marine Research Institute)

The USGS estimates track annual population growth rates over the past decade, using survival and reproductive rates from the two companion studies. The modeling work indicates that these growth rates are largely driven by adult survival, which has implications for both monitoring and management.

"First, we need better estimates of adult survival, especially in the Atlantic and Southwest regions, this involves a strong commitment to the continuation of long term monitoring," Runge said.

"Second, recovery of the species may largely depend on achieving and maintaining high adult survival rates," he said.

One companion study, led by USGS scientist Catherine Langtimm, estimated survival rates of adults and calves.

Every year, scientists from several federal, state, and private agencies photograph individual manatees. Many manatees can be uniquely identified by their scar patterns, and this identification provides the information to estimate survival rate in a statistically sound manner.

The estimated annual adult survival rates over the last decade were 95.6 percent in the Northwest region, 96 percent in the Upper St. Johns River region, 93.7 percent in the Atlantic region, and 90.8 percent in the Southwest region.

Manatee flipper cut by discarded monofilament fishing line. (Photo courtesy UNEP)

Langtimm and her team were able to make formal statistical estimates of calf survival for the first time. In the Upper St. Johns River region, the only region where scientists could reliably identify calves, annual survival rates were 81 percent for first year calves and 92 percent for second year calves.

The other companion study, led by USGS scientist William Kendall, estimated manatee reproductive rates from photo-identification data in the Northwest and Atlantic regions.

When field personnel take photographs, they record whether a female is with a first year calf. Because scientists can identify individual adult females from scar patterns, they can reconstruct the female's reproductive history and estimate reproductive rates.

Kendall and colleagues found that adult females in the Northwest region without a dependent first year calf have a 43 percent probability of producing a calf over the next year, this probability is 38 percent in the Atlantic region. Similar estimates are not yet available for the Upper St. Johns River or Southwest regions.

Scientists say that this new analytical method will apply to a large number of marine mammal populations.