MARINE MAMMAL COMMISSION - Annual Report for 1998 



(4 m) and weights of 3,500 pounds (1,588 kg), they 

 are gentle, slow-moving animals with few natural 

 predators. Numbering perhaps 2,800 animals (rough- 

 ly the combined maximum winter counts along the 

 east and west coasts of Florida), Florida manatees are 

 one of the most endangered marine mammals in U.S. 

 waters. Their principal habitat, estuaries and rivers in 

 one of the nation's most populous states, places 

 Florida manatees in particularly close association with 

 human activity and development. A consequence of 

 this has been that a third of their deaths are due to 

 human causes, primarily collisions with watercraft. 



Most Florida manatees, particularly calves, are 

 unable to survive long periods in waters colder than 

 about 65 °F (20°C). As a result, their winter range is 

 restricted principally to the southern tip of Florida and 

 to waters near localized warm-water sources, such as 

 power plant outfalls and natural thermal springs, in 

 the southern two-thirds of the Florida peninsula. A 

 small number of manatees also have wintered at 

 artificial warm-water outfalls in northern Florida and 

 Georgia. As water temperatures rise in spring, 

 manatees disperse from wintering areas. By summer 

 most of the population is scattered among rivers, 

 canals, and estuaries throughout Florida. A few 

 manatees also use estuaries along the Atlantic coast of 

 Georgia and South Carolina. The northernmost 

 record is an animal seen in Rhode Island in the 

 summer of 1996. A similar summer dispersement 

 pattern occurs along the Gulf of Mexico coast, where 

 a few animals range as far west as Texas. 



To improve information on the number of Florida 

 manatees, the Florida Marine Research Institute, part 

 of the Florida Department of Environmental Protec- 

 tion, began conducting statewide aerial surveys in 

 1991 during winter cold periods when most manatees 

 aggregate at warm-water refuges in Florida. Because 

 of uncertainty and likely variability in the number of 

 manatees not present at warm-water refuges during 

 cold periods, it has not been possible to extrapolate 

 survey counts into a reliable population estimate; 

 neither is it appropriate to compare individual counts 

 to assess population trends. However, the counts 

 have provided a better understanding of the minimum 

 population size, which before 1991 was conservatively 

 estimated at 1,200 animals. 



Between 1991 and 1998 statewide surveys pro- 

 duced maximum counts ranging from 1 ,465 manatees 

 in February 1991 to 2,639 in February 1996. In 

 1998, 2,022 manatees were counted during a 29-30 

 January survey. Photoidentification records and 

 telemetry studies suggest that manatees rarely move 

 between the east and west coasts of Florida, and the 

 statewide surveys indicate that roughly the same 

 number of animals occur on each coast. The highest 

 count for the Atlantic coast is 1 ,457 manatees record- 

 ed in 1996; the highest count for the Gulf of Mexico 

 coast is 1,329 manatees counted in 1997. 



The record high single survey count of 2,639 

 animals in February 1996 was followed by a record 

 death toll of 416 manatees in 1996, which included a 

 die-off of unprecedented size during a spring red tide 

 in southwestern Florida (i.e., 149 deaths, the vast 

 majority of which appear to have been caused by red 

 tide-related toxins). Trends in population size over 

 the long term are difficult to assess. Most scientists 

 familiar with recent statewide counts and previous 

 winter counts at selected warm-water refuges general- 

 ly agree that the size of the Florida manatee popula- 

 tion is now larger than 20 years ago, although how 

 much larger is unknown. However, there also is 

 serious doubt that recent mortality levels are sustain- 

 able over the long term; the 1996 mortality level of 

 416 almost certainly exceeds the population's recruit- 

 ment potential. In addition, a recent study has found 

 low adult survival rates along the east coast. 



Although a good measure of overall abundance 

 and long-term population trends has been elusive, 

 manatee mortality has been well documented, thanks 

 to a manatee salvage and necropsy program begun by 

 the Fish and Wildlife Service late in the 1970s and 

 transferred to the State of Florida in the mid-1980s. 

 The results document a substantial increase in mortali- 

 ty since the late 1970s (see Table 9). Even excluding 

 the 149 manatee deaths recorded during the spring 

 1996 red tide, annual death tolls for the past three 

 years have risen to roughly twice the levels recorded 

 early in the 1980s. Part of the increase probably 

 reflects an increase in overall population size; howev- 

 er, there has been a disproportionate increase in vessel 

 collisions (nearly three times greater than early in the 

 1980s) and perinatal deaths (nearly four times greater 



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