2012 COURSE DATES: AUGUST 4 – 17, 2012 - Sirenian International

Aerial surveys of manatees (Trichechus manatus) in Lee County, Florida 575
A Garmin GPSmap 76 recorded the survey path in each
aircraft. Observers recorded the number of manatees (adults
and calves), location, habitat and behavior for each sighting.
Manatees within close proximity of each other and displaying
similar behavior were grouped together as one sighting. Wind
speed and direction were recorded based on local airport
conditions. Other environmental conditions such as water
clarity, percent cloud cover, and water surface conditions were
estimated and recorded by the surveyor. To ensure that water
clarity and visibility were acceptable for efficient spotting of
manatees, surveys were terminated when sustained winds
exceeded 15 knots or surface conditions scored four or higher
on the Beaufort Wind Scale.
Spatial analysis
Kernel density estimation calculates home range and animal
density by estimating the probability of locating an
individual at a specific place and time (Horne and Garton
2006; Worton 1995). The method creates a “kernel” or
probability density over each point in a dataset (Seaman
and Powell 1996). The density estimate is calculated by
averaging the densities of all kernels that overlap each point.
Kernel density acts as a decay-function with high estimates in
locations with numerous points (sightings), and low densities
in locations with few points (Seaman and Powell 1996).
Using ArcGIS 9.3 (ESRI), survey data were exported as
point shapefiles, and the Spatial Analyst Extension was used to
calculate kernel densities for the total number of manatees per
time period in the survey area. To reflect seasonal shifts,
consideration of manatee distribution and habitat use is often
divided into two periods: winter (November 15–March 15) and
non-winter (March 16–November 14). These two seasonal
designations were used to create two new point shape files
from the survey point dataset. In addition, kernel densities were
generated for sightings in the Central region (Fig. 1) forwinter
(as usually defined, above) and three other time periods: postwinter
(March 16–April 30), summer (May 1–September 30),
and pre-winter (October 1–November 14). The Central region
is of particular importance because 1) its seagrass beds are the
closest foraging grounds to the primary warm-water refuge at
the FPL power plant (Fig. 2) and 2) most manatees that use
this refuge must travel through the Central region. This region
was analyzed separately and according to shorter time periods
in order to identify fine-scale temporal and spatial changes in
the distribution of manatees in this area.
An output cell size of 90 and a search radius of 2500m 2
were applied to both analyses to generate density estimates
for each survey point. The distribution of data was best
represented on maps by using quantile classifications of
numeric data. This method creates classes that may be close
together in terms of the values, but provides a scale that
groups the majority of the data most appropriately given the
density distribution of manatees at survey points. Color
intensity was used to depict areas of high and low density.
Management
When large numbers or notable shifts in distribution of
manatees occurred, counts and observations were reported to
local and state managers and shared with law enforcement via
the Lee County Marine Law Enforcement Task Force
(LCMLETF). This group was formed in 2003 and includes
members from the Lee County Sheriff’s Office, Fort Myers
Police Department, Cape Coral Police Department, Sanibel
Police Department, U.S. Coast Guard Station Fort Myers
Beach, U.S. Coast Guard Cutter Marlin, USFWS Division of
Law Enforcement, USFWS/J.N. “Ding” Darling National
Wildlife Refuge, FWC Division of Law Enforcement Fort
Myers Field Office, Florida Department of Environmental
Protection Division of Law Enforcement, U. S. Power
Squadrons, and U.S. Army Corps of Engineers. The group’s
Mission Statement specifically addresses resource protection:
“The agencies of the Lee County Marine Law
Enforcement Task Force are committed to providing
the highest quality of marine law enforcement to
protect the users of Lee County’s waterways, safeguard
property, and conserve/protect marine life
along with its environment.” (Lee County Marine
Law Enforcement Task Force 2004)
This well-organized group meets monthly to coordinate
and maximize efforts to enforce boating safety and marine
wildlife regulations. The goal of providing survey information
to this task force was to allow the leaders of state and
local law enforcement to deploy officers in ways that
optimized manatee protection, while also ensuring that
other enforcement obligations were handled as well.
Current speed zone regulations for the county were
acquired (Fig. 3) and compared with the kernel density maps
to evaluate the zonal boundaries, both year-round and
seasonal, and the potential need for regulatory improvement.
Results
Survey counts
A total of 34 flights with both the north and south surveys
completed was considered for both spatial and numerical
analysis; all incomplete flights (n=3) were removed from
the dataset for this analysis. Each point (n=4,000)
corresponded to a sighting of 1–102 manatees. The
majority of sightings were single animals or small groups
(mode=1, mean=3). Total counts among survey dates for
northern and southern surveys combined ranged from 135