Are the Threats to Florida Manatee Changing Over Time?

Michael Belanger, Amanda MacNeill, Nesime Askin, Carin Wittnich

Manatees and dugongs have long been threatened by human activities such as hunting, loss of habitat and boat strikes or collisions. The vast number of Florida manatees being seriously injured or killed by these boat strikes or collisions has resulted in strong legislation, numerous public awareness and education programs and the implementation of watercraft speed limits. However, since the implementation of these new programs or legislation, the population of the Florida manatee has not substantially increased. We were interested in exploring what other factors may be playing a role in preventing the population of the Florida manatee from increasing.

Looking at a 10 year period, from 1995 to 2004, data obtained from the Florida Fish and Wildlife Conservation Commission was examined pertaining to Florida manatee population numbers and their causes of death. Synoptic survey data and annual numbers of carcasses were examined to verify the causes of their deaths. The following information is expressed as the absolute number of manatee deaths, a percentage of the total number of manatee deaths, and a percentage of the absolute numbers of the causes of deaths.

As it can be expected the number of deaths over the 10 years has variability but certain conclusions can be made. Despite having regulations and public awareness programs, the number of watercraft deaths have increased from 42 (1995) to 69 (2004) with a maximum number of deaths of 95 in 2002. Perinatal deaths gradually rose from 56 (1995) to a high of 72 (2004) and cold stress had a dramatic rise from 0 (1995) to 50 in 2004. Other causes have dropped substantially such as flood gate/locks- 8 (1995) to 3 (2004)- and natural- 35 (1995) to 24 (2004).

When looking at the causes of death as a percentage of the deaths within the same year, it can be seen that certain causes form the majority of the deaths. For instance, watercraft (20.9%), perinatal (27.8%) and undetermined causes (26.4%) formed the majority of deaths and were over 20% in each case. However, in 2004, watercraft rose to 25%, perinatal stayed the same at 26.1% and undetermined caused dropped to 18.5%. All other causes had drops in percentages except for cold stress which went from 0% (1995) to 18.1% (2004).

When comparing the causes of deaths as a percentage of change in only the first and last year, it is observed that the watercraft (+39%), perinatal (22%) and cold stress (100%) deaths had dramatic increases. Death due to other causes such as natural, flood gate/lock and other human causes had large percentage decreases. From these results, it is clear that Florida manatees are still facing both human and environmental stresses whose relative impact has changed over time. Watercraft related mortalities continue to play a role in manatee deaths but other causes such as perinatal mortality and cold stress have risen to become significant contributors. Further research should focus on these specific causes (perinatal and cold stress) of death if we are to have a significant and favorable impact on Florida manatee conservation.