USGS Releases Threats Analysis for Florida Manatees

A U.S. Geological Survey report released today projects that if current threats to Florida manatees remain at their present level, the statewide population of manatees is expected to remain stable or increase slightly over the next 10 to 15 years, and then decline as natural and industrial warm-water sources are reduced or lost.

The research also reveals that the probability of outright extinction of the Florida manatee population is low over the next 50-150 years if threats remain at their current levels, but the likelihood of a significant decline in the same time period is high. Over the long term, the report projects that the population will stabilize at lower levels.

The report, “A quantitative threats analysis for the Florida manatee (Trichechus manatus latirostris),” assesses and forecasts the status of the Florida manatee population over 50 to 150 years, and examines the relative roles that different threats play in determining the status of these marine mammals. The report was developed in cooperation with scientists at the Florida Fish and Wildlife Conservation Commission’s Fish and Wildlife Research Institute (FWC).

The threats assessment was developed at the request of the U.S. Fish and Wildlife Service (USFWS) as a decision-support tool for the USFWS listing-review process. This research provides results over a wide range of time frames, a wide range of thresholds at which the population would be unlikely to recover, and a wide range of threat scenarios to give decision-makers useful information for assessing the status of the Florida manatee population.

In its analysis, USGS considered the role of five threats, one-at-a-time and in combination: watercraft-related mortality, loss of warm-water refuges in winter, mortality in water-control structures, entanglement, and red tide. An important point of comparison was to the status quo scenario, which is the population trend forecast by the Manatee Core Biological Model (CBM) that uses baseline parameters and assumes the presence of all threats at their current levels.

“The two greatest threats to the future status of manatees are, in order, watercraft-related mortality and loss of warm-water winter habitat,” said Dr. Michael Runge, lead author on the report and a research ecologist at the USGS Patuxent Wildlife Research Center. “Any increase in watercraft-related mortality substantially increases the risk of the Florida manatee population reaching the point of no probable recovery, even if substantial mitigation of other threats occurs at the same time. Reduction of this single threat would greatly reduce the probability of the population reaching the point of no probable return.”

The research showed that reduction of warm-water habitat also poses a significant threat to Florida manatees, especially over the long term. The model accounted for a substantial expected decline in warm-water sources, especially industrial ones, as technological and economic changes result in the loss of refuge sites at power plants over the next 20 to 40 years. Red tide and entanglement, while noticeable threats, have less of an impact on the status of the manatee population. In addition, the effects of water-control structures on the population may have already been largely reduced. Since 2001, many structures have been retrofitted with safety devices, and state plans are to finish this process.

Runge noted that the research demonstrates that multiple management paths might lead to the same desired recovery outcome. For example, he said, partial reductions in the most important threats manatees face-from watercraft and loss of warm-water refuges-may have nearly the same outcome as huge reductions or elimination of the less-important threats. The model used for this research allows managers to investigate trade-offs among threats to help lead to desired management outcomes.

Other key findings of the report include:

There is less than a 2 percent chance that the statewide population will fall below 1,000 individuals over the next 100 years if threats remain at their current levels.
Although the outright extinction of the statewide total population is unlikely if threats remain at their present levels, the probability of the adult population on at least one of the coasts dropping below a critical threshold within 50-100 years is possibly significant.
The probability of the total statewide population falling below 500 animals was 0.22 percent over 150 years, 0.08 percent over 100 years, and less than 0.02 percent over 50 years. The probability of falling below 1,000 animals was 3.62 percent over 150 years, less than 2 percent over 100 years, and 0.28 percent over 50 years.
Because the statewide total population doesn’t tell the whole story of the possible future status of Florida manatees, the report’s authors also examined the population dynamics on both Florida coasts, and considered the status of the adult, rather than the total, population. The geographic separation between the two coasts-and their manatee populations-means that the loss of one of the coastal populations might be interpreted as extinction in a meaningful portion of the range of the Florida manatee, Runge said.

According to the USGS model, there is a fairly high chance (33 percent) that the population of manatees on Florida’s Gulf Coast could fall below 500 adults within 100 years if threats continue at their current levels. For the East Coast, the probability is 26 percent. The probability that the population on at least one of the coasts could fall below 500 adults within 100 years is 49 percent. If the warm-water threat were completely mitigated and all other threats remained at current levels, this risk would drop to 25 percent; if the watercraft threat were completely reduced and all other threats remained at current levels, this risk would drop to 6 percent.

To conduct these analyses, the authors updated the Manatee Core Biological Model to examine the role of different threats. USGS and FWC scientists also updated manatee survival rates, estimated fractions of mortality due to various causes, developed models for the threats themselves, and designed methods to measure the impacts of the threats. The CBM itself was developed previously by USGS and FWC scientists and was used by the state of Florida for part of its status review of Florida manatees. Federal and state scientists view the CBM as a flexible tool that will evolve over time as better information becomes available about manatees and their habitat and as new assessment needs arise.

USGS Open-File Report 2007-1086, “A quantitative threats analysis for the Florida manatee (Trichechus manatus latirostris),” is available at http://www.pwrc.usgs.gov/resshow/manatee/. The report’s authors are Michael C. Runge (USGS Patuxent Wildlife Research Center), Carol A. Sanders-Reed (IAP World Services, USGS Patuxent Wildlife Research Center), Catherine A. Langtimm (USGS Florida Integrated Science Center), and Christopher J. Fonnesbeck (Florida Fish and Wildlife Conservation Commission, Fish and Wildlife Research Institute).