An Overview


Alligator Management Section

Florida Fish and Wildlife Conservation Commission

January 15, 2003

•  More than 420 large alligators (> 5 feet) have died of unknown causes on Lake Griffin since December 1997. The mortality rate increased from 1997 through 2000 (1997: 6, 1998: 65, 1999: 101, 2000: 145) but has been decreasing since April 2001 (2001 total: 75) and continues to decrease in 2002 (32 as of Nov. 15th). Both male and female alligators are affected. Unusual numbers of dead softshell turtles and longnose gar also have been found. Other wildlife and fish species seem to be unaffected.

•  In response, the Division of Wildlife (DOW) formed a partnership of 13 agencies and organizations called the Central Florida Lakes Wildlife Initiative to investigate alligator and other wildlife mortality. This group includes scientists from, among others, the University of Florida, the Florida Museum of Natural History, the U.S. Geological Survey, the St. Johns River Water Management District, the U.S. Fish and Wildlife Service, and the FWC. The DOW has contributed $12,000 to support this group. The Initiative has identified funding needs of over 900,000 dollars to determine the cause. Funding from the Lake County Water Authority, the St. Johns River WMD, and the Wildlife Foundation of Florida has provided partial funding that has complemented DOW efforts.

• Sick alligators on Lake Griffin have been captured by DOW biologists, examined by wildlife veterinarians at the University of Florida and compared to healthy alligators from Lake Woodruff National Wildlife Refuge. Sick alligators were lethargic but otherwise, in good physical condition. However, most alligators had nervous system disease that included a brain lesion. Veterinarians believe that alligators died of complications from this nervous system disease.

•  The causes of the observed nervous system disease are uncertain.  Clinical tests and the type of lesions suggest that some sort of toxin is responsible.  However, levels of toxic metals such as mercury and lead that can cause such disorders have been negligible in Lake Griffin alligators.  Checks for organochloride and organophosphate pesticides also indicated relatively low concentrations in alligator tissue.  Disease specific tests have shown no consistent bacterial (such as botulism) or viral diseases associated with the disorder.  Recently 3 small alligators on one of Florida’s 63 licensed alligator farms tested positive for the West Nile Virus.  During 2001, The Wildlife Foundation of Florida funded a comprehensive screening of Lake Griffin alligators for viral pathogens.  None of the tissue samples screened tested positive for the presence of a virus. Therefore it is unlikely that a virus is the cause of the unexplained alligator mortality on Lake Griffin.  To corroborate the findings of the 2001 virus screening additional Lake Griffin alligator tissue samples are being evaluated specifically for West Nile Virus.

•  Changes in fish and other prey populations due to changes in water quality may have affected nutrition in alligators and led to nutritional disease. Both thiamin and selenium deficiency can cause neurological disorders and studies are currently underway to investigate these possible nutritional problems. The Wildlife Foundation of Florida is currently funding a study of alligator diets on Lakes Griffin, Apopka, and Woodruff. A recent development will complicate the interpretation of the results of these studies. Contractors working for the St. Johns River WMD removed over one million pounds of shad from Lake Griffin between February and April 2002 in an effort to reduce nutrient levels in the lake. The shad harvest resumed in mid-November 2002.  See a slideshow of this study.

•  Blue-green algal blooms are also a possible source of toxins that could cause nervous system disorders. Alligator tissues are being examined for algal toxins, but little is known about the identification and toxicology of these toxins, particularly for Cylindrospermopsis raciborskii, a blue-green alga that has become dominant in Lake Griffin since the mid-1990s. Some strains of Cylindrospermopsis are known to produce the toxin cylindrospermosin which causes liver damage. Recently completed analysis of liver and muscle tissues from Lake Griffin alligators did not detect cylindrospermopsin. The same analysis revealed the presence of anatoxin-a, a neurotoxin, at what are thought to be sub-lethal levels. Anatoxin-a may play a role in the observed alligator mortality however the evidence gathered thus far does not fit with anatoxin-a toxicosis. The St. Johns River Water Management District funded a study to examine the toxic effects of Cylindrospermopsis by dosing captive alligators. 
The results of that study did not show any nervous system lesions or liver damage from the dosing regime utilized.


For more information, follow these links (publication date):

Toxic algae in Florida (9/2001)

Potential Toxicity of Cyanobacteria to American Alligators (5/1/2001)

UF/IFAS news release (6/21/2000)
Central Florida Lakes Wildlife Initiative (1999)
FWC news release (5/1/1998)