U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2004-3137

November 2004

Predicting the Effects of Hydrologic Restoration on 

Manatees along the Southwest Coast of Florida 

Printed on recycled paper

A significant population of Florida’s manatees (Trichechus 

manatus latirostris) occurs in southwest Florida, including 

extensive protected areas within the Ten Thousand Islands (TTI) 

and Everglades National Park (ENP). In this wilderness area, 

manatees are frequently seen using offshore seagrass beds, 

estuarine bays, tidal creeks, and rivers. Upstream from these 

relatively undisturbed ecosystems, humans have greatly modified the natural hydrologic patterns through extensive systems 

of canals, ditches, weirs, roads, and culverts. Historically, water 

moved across this landscape gradually as sheetflow, but human 

structures now rapidly drain the landscape, resulting in large 

pulses of freshwater discharged into the estuaries during the wet 

season, followed by little or no discharge during the dry season. 

Several restoration projects are planned or are underway within 

ENP and TTI to restore freshwater flow to historic levels, and 

these alterations may have effects on manatees using freshwater, estuarine, and near-offshore areas downstream from these 

projects. 

A major goal of this study is to obtain baseline data on 

manatee distribution, relative abundance, habitat use, and movement patterns within the study area (fig. 1) prior to restoration. 

By collecting data before and after the restoration, we can 

evaluate how manatees respond to altered water management 

regimes, perhaps mediated by changes in availability of freshwater for drinking and seagrasses for foraging. These data also 

are being used to parameterize an individual-based population 

model that simulates the movement patterns of manatees under 

different hydrologic conditions.

Field Methods 

Two primary field methods are being used to 

monitor manatees in the study: aerial surveys and satellite 

telemetry. Monthly manatee distribution surveys are flown 

year round following the same inland and offshore flight path 

(fig. 2). Strip-transect aerial surveys involve summer flights 

along parallel flight paths that are roughly perpendicular to the 

coast line (fig. 2). The strip transect approach has been 

successfully used to estimate manatee densities in the Banana 

River, Florida (Miller and others, 1998). Satellite telemetry 

has been used extensively along the east coast of Florida to 

document seasonal movement patterns, migratory behavior, 

and site fidelity (Deutsch and others, 2003). In combination, 

aerial surveys and satellite telemetry provide complementary 

data on coarse-scale population trends versus detailed 

movement patterns.

Figure 1. Study area showing strip transects (black lines) and 

manatee satellite fixes (17,753 yellow dots) from Argos PTT tags 

(26 animals) from June 2000 to December 2003.

Location 

of Study 

Area

STRIP TRANSECTS

MANATEE SATELLITE 

 FIXES-From Argos PTT 

 tags on 26 animals.

EXPLANATION

0 30 KILOMETERS
Thirty distribution surveys were flown from August 1999 to 

February 2002. Six strip transect surveys were flown in 

summer 2000, eight in 2001, and eight in 2002. The parallel 

transects are 3.4 to 8.4 km in length, 1 km apart, with a survey 

strip width of approximately 250 m. Two observers in the cockpit independently record each sighting according to location, 

manatee activity, and group size. The independent data are used 

in abundance analysis to directly estimate and correct for 

manatees that may be missed by the observer. 

We used two types of satellite systems to acquire geographic 

locations from tagged manatees (fig. 3). Most tagged manatees 

were fitted with Argos transmitters, providing approximately 

four location fixes per 24-hour period. We also used GPS tags 

that provided much more accurate locations than the Argos 

data (approx. 30 m versus 150 m) every 15-30 minutes, but the 

battery life was much shorter (8 weeks versus 6 months). In 

combination, the Argos data provided region-wide, long-term 

coverage suitable for revealing general patterns of habitat use, 

while the GPS data showed fine details of travel pathways and 

time spent in specific areas. Most manatees were captured and 

radio-tagged during the winter months at Port of the Islands, 

Faka Union canal, Collier County.

Aerial Surveys Show Substantial Densities 

of Manatees in Area

The aerial survey data show that the Ten Thousand Islands 

region is an important area for manatees. In the wet season 

manatees were dispersed across the area in small groups. The 

majority of sightings were of manatees in offshore bays feeding 

Figure. 3. Captured manatee being released with 

radio transmitter.

STRIP TRANSECTS

DISTRIBUTIONAL FLIGHT 

 SURVEY

EXPLANATION

0 10 KILOMETERS

Figure 2. Strip transects (green lines) and 

distributional flight survey (yellow lines).
on seagrass. Fewer, but consistent numbers were sighted inland 

in canals, rivers, and creeks with a higher percentage inland in 

dry season than in wet. These patterns were comparable over 

the years of data. 

Satellite Telemetry Shows Heavy Reliance 

on Freshwater Sites

The patterns identified from the aerial survey data were 

substantiated by the telemetry data, with even greater detail and 

insight into the activity patterns of the manatees. Satellite-based 

Argos transmitters were used to remotely track movements of 

26 manatees between June 2000 and December 2003, yielding 

17,753 high quality fixes for 6,793 tag-days (fig. 1). A large 

number of fixes occurred in inland sites, often many kilometers 

upstream in canals, rivers, and creeks. During the dry season 

(March – May) manatees spend nearly 25 percent of their time 

upstream in inland sites, whereas during the wet season (July 

– September) they spend much more time offshore foraging on 

seagrass beds. 

GPS tags, placed opportunistically on 15 manatees through 

August 2004, show a pattern similar to the Argos data. The 

GPS data show detailed pathways as manatees move between 

offshore and inland areas (fig. 4). GIS analyses of these movement patterns show that manatees make frequent, regular movements between offshore and inland zones (fig. 5, bottom graph). 

Tagged manatees typically spent less than a day at inland sites, 

but often remained on offshore seagrass beds for several days. 

All tagged manatees showed a similar pattern of alternating 

Figure 4. GPS track for a single animal (Anna) April 24 

to August 1, 2003 showing typical inshore-offshore 

movements.

between the two zones at regular intervals ranging from 

2-8 days throughout the year. While transitioning from one 

zone to another, their rate of travel increased substantially, 

often spiking upwards to 2 or 3 kilometers per hour (fig. 5, top 

graph). Upon arriving at an inland or offshore zone, travel rates 

decreased greatly. These directed movements were not only 

frequent, and rapid, but often involved traveling many tens of 

kilometers, requiring a significant expenditure of energy. Such 

costly movements suggest the importance to manatees of both 

offshore seagrass and inland sites.

For 6 individuals that were tracked for at least one full wet 

and dry season, we compared their use of freshwater access 

points. During the dry season, most manatees (13 of 15 animalseasons) visited the Faka Union Canal freshwater site, which is 

the most reliable source of freshwater in the study area. During 

Figure 5. GIS Analysis of GPS data for one individual (Surfer) 

during August 2000, showing speed of movement (top) and 

regular movement between offshore feeding areas (seagrass) 

and inland freshwater sites (bottom).

AUGUST, 2000

8 10 12 14 16 18 20 22 24 26

INLAND

INSHORE

BAY

TRAVEL

CORRIDOR

OFFSHORE

1,000

2,000

5,000

4,000

3,000

SPEED

0

, IN METERS PER HOUR

SINGLE ANIMAL 

 TRACK-April 24 to 

 August 1, 2003

EXPLANATION

0 5 KILOMETERS
the wet season, most manatees (8 of 12 animal-seasons) shifted 

their home range and never visited the Faka Union site. Water 

monitoring stations in the study area show that during the wet 

season freshwater is readily available at many localities; during 

the dry season these same stations show saline or hypersaline 

conditions.

Getting Restoration Right

Evidence for the importance of freshwater to manatees has 

been provided recently by physiological studies (Ortiz and 

others, 1999). This study provides some of the first evidence 

from manatee movement patterns showing the importance of 

freshwater sites to manatees. Because restoration activities 

are expected to change the timing and quantity of freshwater 

inflow to rivers and canals in the study area, we expect 

manatee movement patterns to change in response to the 

changing availability of freshwater. The abundance and 

distribution of manatees likely will track the increased 

avail-ability of freshwater associated with restoration. 

To further address this issue, we are developing an 

individual-based population model that simulates manatee 

behavioral responses to changes in hydrologic restoration. We 

are also exploring the application of a new statistical approach 

to better estimate abundance and changes in distribution by 

directly estimating and accounting for manatees unseen below 

the surface during an aerial survey. As restoration begins, first 

on the smaller scale Southern Golden Gate Estates project 

centered around the Faka Union Canal, then on the larger 

Comprehensive Everglades Restoration Project (CERP), we 

expect to continue using aerial surveys and telemetry to monitor 

the response of manatees to restoration. 

For more information contact:

Jim Reid or Brad Stith

U.S. Geological Survey

412 NE 16th Ave., Room 250

Gainesville, FL 32601

Phone: (352) 372-2571

Fax: (352) 374-8080

By Brad Stith, Jim Reid, 

Susan Butler, Terry Doyle, Cathy Langtimm

Additional information is available on the 

South Florida Information Access (SOFIA) website: 

http://sofia.usgs.gov/projects/manatees

Terry Doyle

U.S. Fish and Wildlife Service

Ten Thousand Islands National Wildlife Refuge

3860 Tollgate Blvd., Room 360

Naples, FL 34114

(239) 353-8990

This study supports the following tasks for the U.S. 

Department of the Interior (Science Plan in Support of 

Ecosystem Restoration, Preservation, and Protection in South 

Florida, May 2004): Southern Golden Gate Estates Hydrologic Restoration project, by modeling predicted changes in 

hydrology and ecology in the Ten Thousand Islands NWR, and 

providing baseline data and monitoring of effects on a 

federally listed species, the West Indian manatee, within the 

Ten Thousand Islands NWR. The study also supports the 

Landscape-Scale Modeling effort by providing an individualbased demographic model of a threatened species, the West 

Indian manatee, and by providing landscape-scale monitoring 

and assessment for the CERP Monitoring and Assessment Plan.

References

Deutsch, C.J., Reid, J.P., Bonde, R.K., Easton, D.E., Kochman, 

H.I., and O’Shea, T.J., 2003, Seasonal movements, migratory 

behavior, and site fidelity of West Indian manatees along the 

Atlantic Coast of the United States: The Wildlife Society, 

Wildlife Monographs No. 151, 77 p.

Miller, K.E., Ackerman, B.B., Lefebvre, L.W., and Clifton, 

K.B., 1998, An evaluation of strip-transect aerial survey 

methods for monitoring manatee populations in Florida: 

Wildlife Society Bulletin 26(3):561-570.

Ortiz, R.M., Worthy, G.A.J., and Byers, F.M., 1999, 

Estimation of water turnover rates of captive West Indian 

Manatees (Trichechus manatus) held in fresh and salt water: 

Journal of Experimental Biology 202:33-38.