This article provides an overview of manatee deaths during the winter of 2008-2009.
Manatee Carcass Salvage, Necropsy, and Rescue Program Update
Manatee researchers with the Florida Fish and Wildlife Conservation Commission's (FWC) Fish and Wildlife Research Institute (FWRI) respond to public reports of manatee carcasses from around the state. Most recovered carcasses are transported to FWRI's Marine Mammal Pathobiology Laboratory (MMPL) in St. Petersburg, where they are examined (necropsied) to determine cause of death. Through carcass salvage and manatee rescue and rehabilitation, researchers learn about manatee health, causes of death, life history, and general and reproductive biology. Wildlife managers use this information to assess population health and status.
Elevated number of carcasses reported
During winter 2008-2009 (November 1, 2008 to April 30, 2009), 261 manatee carcasses were reported statewide. This number was well above the average number of deaths (188) that occurred during November through April of the most recent five years (five-winter average). Although increased carcass counts across years do not necessarily indicate an actual increase in the death rate, the information can be used to monitor threats to the manatee population. During winter 2008-2009, researchers documented a disproportionately high number of deaths related to cold stress. Loss of warm water is a significant long-term threat to the manatee population, and researchers are carefully evaluating the recent data. (See Quantitative Threats Analysis for the Florida Manatee, http://www.pwrc.usgs.gov/resshow/manatee/, authored by U.S. Geological Survey and FWC researchers.)
Deaths caused by cold stress
In winter, Florida manatees migrate to warm-water habitats such as natural springs and discharge canals of industrial power plants. Exposure to water temperatures below 68oF for long periods can cause a complex disease process called the manatee cold-stress syndrome, which involves metabolic, nutritional, and immunologic factors. Symptoms may include weight loss, skin lesions or abscesses, internal fat loss, dehydration, constipation and other gastrointestinal disorders, internal abscesses, and other secondary infections. Unlike chronic cold stress, little is known about the effects of acute exposure to severe cold. Finding a few carcasses that show nonspecific evidence of shock after severe cold snaps suggests that manatees may die when exposed to such cold. Susceptibility to cold stress appears to be related to animal size, experience, and ability to migrate. Thus, adult manatees can handle the effects of cold better than juveniles or calves.
Statewide, 72 deaths from cold stress were reported during winter 2008-2009. This number is twice as high as the most recent five-winter average. (Eleven of these were classified as perinatal [newborn] because the carcasses measured 150 cm [59 in] or less.) Most (74%) of the cold-stress carcasses were recovered on the east coast (see Figure and Map), which is the highest number on record for a winter season on this coast. Soon after the first cold front passed through during late October and mid-November 2008, east-coast manatees began dying of cold stress. Their numbers continued to predominate in the statewide cold-stress totals throughout the first months of 2009, whereas on the west coast, the majority of deaths due to cold stress were reported in March and April 2009. The vast majority (83%) of manatees that died from cold stress were calves (those less than 236 cm [93 in] long). The "calf" category includes manatees that are still dependent on their mothers as well as small, newly independent animals. In comparison, 69% of manatees that died from causes other than cold stress were calves. These percentages indicate that calves are more susceptible to dying from cold stress than older manatees are. Many manatees that died from cold stress were found in Brevard County, where manatees use industrial warm-water sites. Multiple factors may have been responsible for the relatively high number of deaths from cold stress in this county, including long periods of extremely low water temperatures in the shallow lagoon systems or lower than ideal water temperatures at the power plant discharge.
Farther south, the Riviera Beach power plant was running on a limited basis because the plant is scheduled to be replaced with a more efficient plant. The Riviera Beach plant did operate on four days during winter 2008-2009 when water temperatures dropped. This was consistent with their permit requirements. When the plant was not operating, manatees sought warmer waters in the nearby Port of Palm Beach. The FWC and partner organizations increased monitoring in order to protect manatees in the port during the brief periods of cool weather, and the lack of warm-water discharge at this power plant did not seem to affect the number of manatee deaths in this region.
Number of manatee deaths due to cold stress, statewide and
on the east and west coasts of Florida, over the past fifteen winters. View Larger Image in PDF format
Other causes of death
Deaths of newborn manatees during winter 2008-2009 were almost double the most recent five-winter average. Cold stress accounted for 11 of 54 small-calf carcasses recovered during this winter. In addition to cold stress, other factors that could explain this higher number include an actual increase in deaths of newborns, a higher number of births, or a higher number of calf carcasses recovered. The number of deaths for which the causes could not be determined (because most carcasses were too decomposed) was also higher compared to recent winters. Cold may have been involved in some of these deaths, because the carcasses were generally found in areas where other manatees had died from cold stress. The number of deaths due to watercraft strikes was relatively high statewide (51) during winter 2008-2009, about 33% and 31% of which occurred in the Southeast and Southwest regions, respectively. Statewide, the most recent five-winter average was 34. The number of deaths from this cause in the central-east region was unusually low (4). Perhaps these findings indicate that manatees moved southward from the central to the southern region; ongoing aerial surveys and observations during photo-identification efforts may shed light on this possibility.