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Population Monitoring and Assessment

FWRI scientists use a variety of methods to assess and monitor the current and future status of the Florida manatee population.

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FWRI scientists use a variety of methods to assess and monitor the current and future status of the Florida manatee population.  Population assessments currently include conducting manatee counts at winter aggregation sites, and estimating survival, population growth, and reproductive rates.  Assessments also include estimates of risk to the population, including projected declines in population size and probability of persistence into the future (i.e., risk of extinction).  Staff contributed to a population projection model for the Florida manatee (http://www.pwrc.usgs.gov/resshow/manatee/documents/OFR2007-1082.pdf).  The model is being used to integrate information about manatee life history to predict population changes under various environmental and management scenarios.   A complementary study included a quantitative “threats” analysis conducted with our federal partners (http://www.pwrc.usgs.gov/prodabs/pubpdfs/6731_Runge.pdf).  The analysis suggested that watercraft-related mortality has the greatest impact on manatee population growth and resilience.  In addition, staff developed Measurable Biological Goals that are related to the dynamics of the manatee population as described by the population model.  These goals are a means of monitoring the contribution of management toward recovery by measuring improvements in survival rates, available warm water, and population size.   Specifically, the goals include:  (1) mature population size exceeding 2,500 mature individuals statewide; (2) sufficient regional adult survival rates to ensure a less than 1% probability of population declines greater than 30% (over a 60 year timeframe), given available warmwater resources; and (3) sufficient regional warmwater carrying capacity to ensure a less than 1% probability of population declines greater than 30% over the next three generations (~ 60 years), given estimates of adult survival. 

FWC uses two types of aerial surveys to monitor manatees.  These surveys provide minimum counts and information about habitat use and seasonal distribution.  Statewide synoptic surveys provide a count of manatees at known aggregation sites and other sites in winter.  These surveys are conducted to meet §370.12 (4), Florida Statutes, requiring an annual, impartial, scientific benchmark census of the manatee population.  The counts, conducted 24 times since 1991, are flown after cold fronts, when animals aggregate at warm springs and thermal discharges from power plants.  As currently conducted, synoptic surveys yield minimum counts of the number of manatees using these warmwater sites and cannot be used to estimate population size.  FWC uses distribution surveys to determine the seasonal distribution of manatees.  These regional surveys are usually flown twice monthly in specified counties for a period of two years.

Researchers are developing new aerial survey techniques that will provide precise and reliable estimates of distribution and population size.  These new methods and resultant data will contribute to models that incorporate information about how well observers detect manatees from the air and will relate environmental variables to the number of animals counted by observers.  Preliminary surveys incorporating the new distribution survey methods are being tested in Collier County.  A preliminary survey to test new methods for the statewide synoptic survey will be flown in winter 2008.  Details are described in the “Monitoring Activities” and “Ongoing and Future Research” sections of the Manatee Management Plan (http://myfwc.com/imperiledspecies/plans/Manatee-Mgmt-Plan.pdf).

Information on manatee life history is essential for assessing manatee population dynamics and recovery.  Specifically, long-term data on growth and survival of individuals, reproductive performance of mature females, and health of manatees are important to the development of reliable population models.  These data are gathered using a variety of research tools such as photo-identification of distinctly scarred individuals.  Manatee photo-identification is a research technique that uses the unique pattern of scars and mutilations on a manatee’s trunk and tail fluke to identify individual animals over time.  The scars are usually the result of encounters with boats, but they can be caused by entanglement in fishing gear and by infections.  This research is conducted through a partnership between FWRI, the United States Geological Survey (USGS) Sirenia Project, and Mote Marine Laboratory.  Partners work collaboratively to photograph Florida manatees throughout their range, process images, identify manatees, and manage an integrated sightings database, known as the Manatee Individual Photo-Identification System (MIPS).  Photo-identification data provide insights into manatee movements, site fidelity (i.e., the tendency to return to the same location year after year), adult survival rates, and reproductive parameters such as calving intervals and length of calf dependency.

To view highlights from the 2006–2007 fiscal year, download
Population Monitoring and Assessment 2006–2007 Highlights (PDF File—97 KB)

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