Detailed Movements and Habitat Use Patterns of Radio Tagged Manatees in the Western Everglades

Susan M. Butler, James P. Reid, and Bradley M. Stith

Poster presentation at the 15th Biennial Conference on the Biology of Marine Mammals, 15-19 December 2003, Greensboro, North Carolina

ABSTRACT

       A radio tracking study on Florida manatees (Trichechus manatus latirostris) is being conducted to determine their distribution, movements, and habitat use in the western Everglades. We have relied on several technologies to acquire geographic locations from a total of 26 tagged manatees (8 males, 18 females). Most were fitted with satellite-based, Service Argos monitored transmitters, which provided an average of six Doppler-derived locations per day. Newly developed Argos-linked, Global Positioning System (GPS) tags, which relayed GPS locations as sensor data through the Argos system, were recently deployed enabling detailed tracking data to be acquired remotely. The GPS tag provided accurate locations every 15-30 minutes. In combination, the Argos data provided region-wide, long-term coverage suitable for revealing general movement patterns, while the GPS data showed detailed travel pathways and time spent in specific areas. From June 2000 through Feb. 2003, a total of 6,157 tracking days were recorded from 46 tag deployments on the 26 manatees. Most remained within the study area providing the first detailed movement data collected across seasons from individual wild manatees in the western Everglades. Activity patterns for all individuals indicated a preference for foraging on seagrass beds in marine areas with brief trips to inland creeks and canals, which provided a source of fresh water. These periodic trips, undertaken approximately four to eight times per month, reveal the reliance of these marine animals on accessible fresh water for drinking. Spatial distribution of submerged aquatic vegetation, temporal fluctuations in freshwater areas, and bathymetry influenced movement and use patterns of manatees within the region. With the implementation of the Comprehensive Everglades Restoration Plan, we expect that altered water management regimes and resulting environmental changes may affect manatee use patterns. These data are being integrated into an individual-based model that will attempt to predict manatee responses to management actions.

INTRODUCTION and METHODS

       This study fills a significant void in our knowledge of manatee ecology, as there is very little existing information on manatee population biology and habitat use in southwestern Florida. Recent advances in tracking technology have made this project logistically feasible and cost-effective. This is the first detailed radio tracking study conducted to identify manatee use patterns in this region.

       Wild manatees were captured and radio-tagged at Port of the Islands during each of the winters of 2001 through 2003. Three manatees tagged by Mote Marine Lab near Charlotte Harbor moved into the Ten Thousand Islands, and four rehabilitated manatee were released in the study area. A total of 26 individuals (8 males, 18 females) were tagged and tracked in this study through February 2003.

Argos and GPS Tracking Techniques

       From June 2000 through February 2003, a total of 6,157 tracking days were recorded from 46 tag deployments on 26 manatees. Four of the manatees (3 males, 1 female) traveled to areas more than 100 km north of the Ten Thousand Islands. Most remained within the study area, however, providing the first detailed movement data collected across seasons from wild manatees in this region.

       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 inland trips, undertaken approximately four to eight times per month, are longer during the dry season; wet season moves are shorter and less frequent.

       GPS tracks for TNP-04 Megan show details for three individual movements into the Faka Union area.  Most moves to this region by tagged manatees were to drink fresh water in the Port of the Islands canals (top of figure), however Megan's track shows two round trips to fresh water areas deep in the historic Faka Union River channel. Tracks using GPS tags are revealing precise manatee use patterns, which often include areas not previously recognized as typical manatee habitat.

       Salinity of inshore waters fluctuated 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.

       Tracks of several individuals that moved south and inshore during cold winter weather may reflect historic manatee movement patterns prior to the availability of artificial warm water refugia.

       Delineated habitat zones are used to classify Argos satellite locations for comparisons of areas occupied. Argos satellite locations, GPS Tracks, and autonomous VHF data-logging stations are detailing the frequency and duration of inland moves to access freshwater.

CONCLUSIONS

       Tracking data and field observations of tagged manatees revealed that the spatial distribution of submerged aquatic vegetation, availability of freshwater, and bathymetry influenced manatee movements and use patterns within the TTI and northern Everglades. 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 region. These data are being integrated into the ATLSS model that will attempt to predict manatee responses to management actions.

ACKNOWLEDGMENTS

       Manatee research is guided by the objectives of the Florida Manatee Recovery Plan. The studies summarized here are a cooperative effort with 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, the Marine Mammal Commission, and Mote Marine Laboratory.

       The Sirenia Project at the USGS/Center for Aquatic Resource Studies conducts long-term research 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.