Florida Manatee Status Statement
Manatee Population Status Working Group
28 April 2000
Years of scientific study of the Florida manatee have revealed both good news and some cause for concern regarding the status of this endangered mammal, according to the interagency Manatee Population Status Working Group.
Long-term studies support division of the Florida manatee population into 4 regional subpopulations: Northwest, Southwest, Atlantic (including the St. Johns River north of Palatka), and St. Johns River (south of Palatka). These divisions are based primarily on documented manatee use of wintering sites and from radio-tracking studies of individuals’ movements. Although some movement occurs among subpopulations, researchers found that analysis of manatee status on a regional level provided insights into important factors related to manatee recovery.
The good news is that the Northwest and St. Johns River subpopulations have steadily increased between 1974 and at least 1998. This population growth is consistent with the lower number of human-related deaths, high estimates of adult survival, and good manatee habitat in these regions. Unfortunately, this good news is tempered by the fact that the manatees in these two regions probably account for less than 20% of the state’s manatee population.
The picture is less optimistic for the Atlantic coast subpopulation. Scientists are concerned that the adult survival rate (the percentage of adults that survives from one year to the next) is lower than needed for sustained population growth. The population on this coast appears to have been growing slowly in the 1980s but now may have leveled off, or could even be declining. In other words, it’s too close to call. This finding is consistent with the high level of human-related and, in some years, cold-related mortality in the region. A possible down-turn of the population in the 1990s is particularly worrisome in light of the fact that many protection measures, such as comprehensive speed zones, were established during this same time period.
Estimates of survival and population growth rates are currently underway for the Southwest region. Preliminary estimates of adult survival are similar to those for the Atlantic region, i.e., substantially lower than those for the Northwest and St. Johns River regions. This area has had high levels of watercraft-related deaths and injuries, as well as periodic natural mortality events caused by red tide and severe cold. However, pending further data collection and analysis, scientists are unable to provide an assessment of how manatees are doing in this part of the state.
Over the past ten years, approximately 30% of manatee deaths have been directly attributable to human-related causes, including watercraft collisions, accidental crushing and drowning in water control structures, and entanglements in fishing gear. In 1999 alone, 39% (105 of 268) of manatee deaths were human-related.
The continued high level of human-related manatee deaths, particularly the increasing percentage of watercraft-related deaths, raises concern about the ability of the overall population to grow or at least remain stable. The Manatee Population Status Working Group is also concerned about the negative impacts of factors that are difficult to quantify, such as habitat loss and chronic effects of severe injuries.
The exact number of manatees in the wild is unknown. The high count was 2,222 manatees statewide, on 27 January 2000. The highest count on record is 2,639 in 1996. Unfortunately, the counts vary widely depending on weather conditions and the raw numbers alone do not provide an accurate picture of population trends.
The group agrees that the results of the analyses underscore an important fact: Adult survival is critical to the manatee’s recovery. In the regions where adult rates are high, the population has grown at a healthy rate. In order to assure high adult survival the group emphasizes the urgent need to make significant headway in reducing the number of human-related manatee deaths.
The interagency Manatee Population Status Working Group comprises biologists from the U.S. Geological Survey, the U.S. Fish and Wildlife Service, and the Florida Fish and Wildlife Conservation Commission. The group’s primary tasks are to assess manatee populations trends, to advise the U.S. Fish and Wildlife Service on population criteria to determine when species recovery has been achieved, and to provide managers with the interpretation of available information on manatee population biology.
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