Record-Breaking Mortality of Florida Manatees during Extremely Cold Winters of 2010 and 2011
Manatee cold stress syndrome is a complex disease process that involves metabolic, nutritional, and immunologic compromise due to prolonged exposure to water temperatures below 20°C. Signs include skin lesions, anorexia, serous fat atrophy, lymphoid depletion, and opportunistic infections and disease.1 During average Florida winters, less than 50 cold-related manatee mortalities and rescues are recorded each year. However, during the past three years, the Florida Fish and Wildlife Conservation Commission documented the highest levels of cold-related deaths on record, and, in particular, the extremely cold weather of the 2010 and 2011 winters resulted in hundreds of cold-related mortalities and dozens of rescues of cold-stressed manatees. The winter of 2010 was ushered in with a blast of cold Arctic air, which was followed by a historical cold snap through mid-January statewide and below-average temperatures in February and March. A total of 480 carcasses were verified over an 89-day period from 11 January through 9 April 2010; this count surpassed the record high annual count of 429 carcasses set in 2009. Most carcasses (62%, n = 299) were found during the first month, when ambient water temperatures dropped below 10°C throughout most of their winter range. Manatees suffered unusually high mortality in nearly all regions of Florida, but mortality was highest in the central-east and far southwest (Everglades) regions. An early start of the next winter with record-cold temperatures in December 2010, followed by a cooler than normal January 2011, had further impacts on the manatee population. Between 18 December 2010 and 13 February 2011, 204 manatee deaths were reported statewide, with highest mortality numbers in the central east and central west regions of Florida. Early in both winters, many carcasses demonstrated non-specific signs (e.g., congestion, acute respiratory changes) indicative of shock and possible drowning. In contrast with previously described chronic symptoms of the manatee cold stress syndrome, these findings may represent acute hypothermia associated with exposure to extreme cold over a short period of time. As both cold events evolved, chronic signs of the classical cold stress syndrome became more prominent in the examined carcasses. Overall, the unusually cold winters of 2010 and 2011 taught tough, yet valuable lessons on manatee susceptibility to cold. The sensitivity of ambient waters to cold fronts and the quality of thermal refugia within a region appeared to play important roles in affecting the extent of cold-related deaths, thereby highlighting the significant threat that loss of warm-water habitat presents to this endangered species.
The FWC is very appreciative of the invaluable assistance of our volunteers and our colleagues from FWRI, FWC-HSC, USFWS, MML, NPS, UF, USGS, UNCW, and Jax Zoo who helped support our efforts in these events. Our response would not have been possible without the many dedicated FWC Law Enforcement officers and local cooperators who spent countless hours verifying reports of ill and dead manatees, assisting with rescues, towing carcasses to boat ramps so that they could be transported to the MMPL for necropsy, and transporting FWRI staff to the carcasses for field necropsies. Financial support was provided by the Save the Manatee Trust Fund and by a grant provided to the FWC from the USFWS. Research related to these mortality events was conducted under USFWS permit #MA773494-9.
1. Bossart GD, Meisner RA, Rommel SA, Ghim S, Jenson AB. Pathological features of the Florida manatee cold stress syndrome. Aquat Mamm. 2002;29(1):9–17.