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Southeast Ecological Science Center

Radio Tracking Manatees to Assess the Impact of Hydrologic Changes in Southwest Florida

James P. Reid, Dean E. Easton, Lynn W. Lefebvre, and Susan M. Butler
U.S. Geological Survey, Florida Integrated Science Center, Sirenia Project

Poster presented at the16th Biennial Conference of the Estuarine Research Federation,
St. Petersburg Beach, November  4 - 8, 2001


ABSTRACT

The Sirenia Project is conducting a multi-year project to develop ecological models necessary to understand and predict the effects of hydrologic restoration on manatees (Trichechus manatus latirostris) in Southwest Florida.  As part of this effort, we initiated a radio tracking study in June 2000 to determine manatee movement patterns and habitat use prior to hydrologic alterations. We hypothesize that manatee distribution and movements will be influenced by changes in water flow patterns, particularly in the Ten Thousand Islands downstream from the Southern Golden Gate Estates and Faka Union Canal.

Fourteen manatees have been tracked within the Ten Thousand Islands/Everglades National Park region using satellite-monitored Argos tags from June 2000 through June 2001. Weekly movement patterns suggest a preference for foraging in marine areas with brief trips to inland creeks and canals, which provide sources of fresh water. During the winter months, one adult female migrated from the Pavilion Key/Chatham River area south to Whitewater Bay, where she spent several months before returning to her previous warm season range.  A data-logging GPS tag, which collects locational data at 15-minute intervals, was deployed on 3 manatees for two-week periods during 2000 and 2001. These data will allow us to determine fine-scale habitat use and provide accurate information on travel paths.


INTRODUCTION

As part of a multi-year project to develop ecological models necessary to understand and predict the effects of hydrological restoration on manatees in South Florida, the Sirenia Project initiated a radio tracking study to determine manatee movement patterns and habitat use. Given the reliance of this endangered aquatic mammal on freshwater and aquatic vegetation, we expect that altered water management regimes and resulting environmental changes may affect manatee habitat use and movement patterns.Attaching a satellite monitoring tag to a manatee

The coastal waters of the Ten Thousand Island National Wildlife Refuge (TTI) and Everglades National Park (ENP) provide ample natural habitats for manatees. This is the first detailed radio tracking study of manatee use in the region.


METHODS

During the initial phase of this project, three manatees were tagged using satellite-monitored Argos tags.  These individuals were rescued in southwest Florida during the spring of 2000 because they exhibited symptoms of exposure to red tide. They were treated at Lowry Park Zoo and released during June 2000 in Rookery Bay, near Marco Island.


 


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Satellite-based & Field Tracking

Diagram of the Argos tag - click to enlarge
  • Satellite-monitored Argos tag for remote monitoring
  • VHF and ultrasonic tags for field tracking & tag recovery
  • Application of GPS technology
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A manatee with the satellite monitoring tag in tow
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Research crew with a radio-tagged manatee
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Manatee captures were conducted at Port of the Islands near Marco Island on 07-08 February 2001, and 20-21 March 2001. Eleven manatees were captured and radio-tag assemblies were attached to 9 of these (8 females, 1 male).


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RESULTS and DISCUSSION
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Argos tags provide locations and data on temperature and transmitter activity. Positions include a location class (LC) designating their accuracy; quality locations include LC 1 <1000m, LC 2 <350m, and LC 3 <150m. Tagged manatees relayed an average of six quality locations per day, with a frequency of approximately two per day from each location class.

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Following release, the three individuals tagged in the June 2000 dispersed to areas that they had likely used prior to their rescue.  Two visited Port of the Islands and Faka Union canal during the first month after their release. One of the individuals, Poi,  made several trips between Marco Island and Lake Okeechobee during the summer months.

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The deep canals at Port of the Islands provide a passive thermal refuge for manatees during brief cold winter weather as well a access to freshwater at the spillway.  Seven of the nine manatees tagged in winter 2001 returned to Port of the Islands at least once after they were tagged.

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During the winter months of 2001, one manatee migrated south to Whitewater Bay where she spent several months before returning to her previous warm season range.

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Movement patterns for all individuals suggest a preference for foraging on seagrass beds in marine areas with brief trips to inland creeks and canals, which provide a source of fresh water. These travels, undertaken approximately four to eight times per month, reveal the reliance of these marine animals on accessible freshwater.

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Warm season use areas for some individuals included seagrass beds off Cape Romano and the canals of Marco Island.  Other manatees moved southeast into the northwest region of Everglades National Park, relying on Port of the Islands, canals upstream of Everglades City, and inland creeks for fresh water.

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Movements by tagged manatees were often rapid and direct. Feeding areas were documented among Thalassia, Syringodium, and Halodule seagrass beds along the outer islands.  Spatial distribution of submerged aquatic vegetation, temporal fluctuations in freshwater areas, and bathymetry influence movement and use patterns of manatees within the Ten Thousand Islands. Individual site fidelity for some varied with season and calving events.

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Tracking manatees with GPS tags

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Periodically, a datalogging GPS tag was attached to tagged manatees for two-week periods during 2000 and 2001.  Data from this tag, collected at 15 or 20 minute intervals, allows us to determine fine-scale habitat use and provides accurate information on travel paths. Up to 90 locations (accurate to <10m) can be stored per day.

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Argos tag - click to enlarge

Integration of Argos transmitters to relay locations from GPS receivers shows promise for use in manatee tracking buoys, providing remote access to detailed location data.

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These data provide the first details on manatee use patterns in the TTI/ENP. Salinity of inshore waters fluctuate with winter dry periods and summer rains. Abundance and species composition of submerged vegetation within inland bays may vary with these seasonal changes, thus influencing manatee feeding patterns.

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Seagrass - click to enlarge

Additional studies are planned to assess manatee habitats including characterizing and mapping the distribution of submerged aquatic vegetation in areas targeted by tagged manatees as foraging areas.

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In this comparison of Argos and GPS data collected simultaneously from a tagged individual, precise travel paths, rates, and destinations can easily be identified.

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CONCLUSIONS

Tracking data and field observations of tagged manatees reveal that the spatial distribution of submerged aquatic vegetation, availability of fresh water, and bathymetry influence manatee movements and use patterns within the Ten Thousand Islands.  Manatees routinely traveled from offshore seagrass beds to inland freshwater areas. We expect that altered water management regimes and resulting environmental changes may affect manatee habitat use and movement patterns within the Ten Thousand Islands. These data are being integrated into models that will attempt to predict manatee responses to these management actions.

The Sirenia Project at the Florida Integrated Science Center conducts long-term detailed studies on the life history, population dynamics, and ecological requirements on the endangered West Indian manatee (Trichechus manatus). This research provides managers with information that is essential to recovery assessment and planning.


ACKNOWLEDGMENTS

Manatee research is guided by the objectives of the Florida Manatee Recovery Plan. The studies summarized here are a cooperative effort with one or more federal, state, and private partners, including: Everglades National Park, U. S. Fish and Wildlife Service, Big Cypress National Preserve, Rookery Bay National Estuarine Research Reserve, Florida Fish and Wildlife Conservation Commission, Mote Marine Laboratory, and Save the Manatee Club.

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Sirenia Project USGS
Florida Integrated Science Center
412 N.E. 16th Avenue,
Gainesville, FL 32601 USA.
Phone:(352)372-2571  FAX:(352)374-8080

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Florida Manatee

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