Poster presentation at the Thirteenth Biennial Meeting of the Society for Marine Mammalogy, Maui, HI, 28 November - 3 December 1999.

Human activity in the coastal zone threatens the existence of the West Indian manatee (Trichechus manatus), an herbivorous aquatic mammal that inhabits the coastal waters, estuaries and freshwater rivers of the Caribbean and tropical Americas. Radio-tracking studies, using field-monitored VHF radio-transmitters and Argos satellite-monitored platform transmitter terminals, have addressed questions on seasonal movements, site fidelity, and identification of high-use areas. However, managers often require precise information on habitat use for site specific management policies. The application of the NAVSTAR Global Positioning System (GPS) shows promise for obtaining frequent and detailed location positions (< 5m after differential correction) from tagged manatees. The Sirenia Project collaborated with Lotek Marine, Inc. in the development of a prototype GPS tag for manatees. An 8-channel GPS receiver, independent VHF tracking beacon, sensors and a computer/memory module were enclosed in a floating housing which attaches to a manatee using a belt and flexible tether. Schedules for GPS position fixes (as frequently as every five minutes) and duty cycling of the VHF beacon are programmed by the user. Although brief surface times can limit the use of GPS tags on marine mammals, location fixes can be obtained as quickly as 18 to 22 seconds. In tests on manatees, up to 95% of attempted location fixes were successful during one 24-hour deployment period. Four thousand position fixes, with pseudoranges for differential post-processing, can be acquired per deployment. The attachment method must allow for tag recovery in order to obtain data. Location data, integrated with GIS display of seagrass habitat maps, bathymetry, and other detailed map coverages, allow for fine-grained analyses of habitat use. This tag, best suited for brief, intensive tracking bouts, may be applicable for use on dugongs, the only other marine mammal routinely tracked using tethered floating tags.

The West Indian manatee (Trichechus manatus) is an herbivorous aquatic mammal that inhabits the coastal waters, estuaries and freshwater rivers of the Caribbean and tropical Americas. Development and human activity in the coastal zone threatens the existence of this species.  Managers require accurate information on habitat requirements and movement corridors in order to make appropriate decisions about how best to protect this endangered mammal and its habitat. Radio-tracking, using conventional field-monitored VHF radio-transmitters and Argos satellite-monitored platform transmitter terminals, has been employed in the US and elsewhere to address questions relating to seasonal migratory movements, site fidelity, and identification of high-use areas. Although findings from these long-term radio-tracking studies have been useful in implementing management policies, additional information on precise habitat use is often required for site-specific management actions. 

The application of the NAVSTAR Global Positioning System (GPS) shows promise for obtaining frequent and detailed location positions from tagged animals. The high degree of accuracy provided by GPS tags (< 5 m after differential correction) can provide continuous tracking of individuals. Fine-grained analyses of habitat use and better estimates of travel paths could be determined with GPS data collected frequently over time.

The Sirenia Project collaborated with Lotek Marine Inc. in the development and testing of a GPS tag for use on manatees. A Motorola GPS engine, coupled with a VHF transmitter, was enclosed in a floating housing. This was attached to a manatee using a belt and flexible tether that had been developed by the Sirenia Project for attaching floating radio transmitters.  A floating housing, which allows the radio antenna to remain at the surface, is required to effectively track manatees in saltwater habitats. The durable housing, consisting of a polycarbonate tube with delrin antenna cap and nosecone, is designed to survive minor boat strikes and alligator bites. Range of the VHF beacon averages 7 km.

Operation parameters, programmed by the user with Lotek's GPS2000 host software, include duty cycling (weekly, daily, and hourly) of the GPS receiver and VHF beacon. The beacon, equipped with an independent power source,  is used for manual tracking and feedback on unit activities. The GPS receiver can be programmed to attempt location fixes as frequently as every 5 minutes.  Although surface times are critical for GPS tags to function on marine mammals, location fixes can be obtained within 18 to 22 seconds.

Using a lithium battery pack, about 8000 location attempts can be performed in the solved or inverse differential modes. Tag recovery is required in order to download locations although utilities for remote data recovery could be developed and applied.

Lotek Engineering developed an Inverse Differential Correction software that allows for post-processing correction of solved-location GPS data. Correlation with base-station reference data enables about 70% of locations to be corrected; uncorrected solved locations also are provided. Although a larger percentage can be corrected for data from a differential GPS receiver, this software allows for the use of a robust solved-data GPS receiver. This engine uses less power and can fix a location within a 20-second period, thus extending battery capacity.

Location Accuracy Tests

Figure 1 – In a test to determine location accuracy and precision, the tag was positioned at a known location in the Banana River, Brevard County, FL, and allowed to record locations every ten minutes over a 49-hour period. The resulting data (100% of fix attempts) fit descriptive parameters typical for solved-data acquired with selective availability "on." (Scatter plot of locations with circle radii of 95% and 50% from centroid of all locations)

Figure 2 – The first test was repeated at the same location for 50 hours.  Data from this test (also 100% of fix attempts) fit descriptive parameters typical for solved-data acquired with selective availability "off."

Figure 3 – Using Lotek's Inverse Differential Correction software, 229 locations (76%) from accuracy test #2 were corrected to more accurate values. Slight improvements could be achieved using normal post-processing of data from differential GPS receivers, but without the advantages of efficient solved-data engines. These accuracies are more than adequate for addressing most manatee/habitat correlations. As with any spacial study, elements of scale are critical when defining objectives and evaluating results.

Animal Deployments

Tests of the GPS tag on manatees resulted in frequent locations enabling detailed insights into the movements of the tagged animal. GPS tags also will be valuable tools in intensive investigations into how manatees utilize their habitat. Locations, when integrated in a GIS, can be correlated with seagrass habitats, bathymetry, and other detailed map coverages.

 For 18 days, the tag was deployed piggyback to an Argos PTT on a previously tagged adult manatee using the protected waters of the Banana River at the Merritt Island NWR, Kennedy Space Center.

Argos Data from Knicky – The distribution of Argos locations (mean # loc./day = 3.7) reflects the animal's persistent use of an 18 sq. km portion of the Banana River, however specific use patterns are not discernable.

GPS Data from Knicky – The GPS tag, programmed for 10 minute GPS fix intervals, recorded locations (mean # loc./day = 122) in shallow water along shore at night and 1.2 to 1.8 km offshore within seagrass beds during the day. This diel use pattern, which was not evident from the Argos track, had not been identified previously. Manatee use close to shore is rarely observed during frequent daytime aerial surveys in this area.

GPS Data from Crystal – The GPS tag was later attached to Crystal, a manatee in Kings Bay, Crystal River. During the five days she was tracked, this young individual repeatedly moved among several specific use areas. A noteworthy finding was the previously undocumented nighttime use of the upper portions of residential canals, including one trip through a culvert to a flooded roadside ditch. She also once made an 8km move during the night to the mouth of Crystal River where she remained from 2220h until 0050h; she returned to Kings Bay by 0636h.

Application to Other Marine Mammals - This custom housed data-logging receiver has been deployed successfully on dugongs (pers. comm. Nicholas Gales, Australian Department of Conservation & Land Management), the only other marine mammal now routinely tracked using tethered floating tags.  His use of actively triggered detachment devices show promise to ensure tag recovery.  Innovations in miniaturization, data recovery, and adaptable housing and attachment designs could allow this technology to be deployed over a wide variety of marine animals.

Special thanks to Alicia Windham-Reid, Sharon Tyson, and Susan Butler (USGS/Sirenia Project) who assisted with field operations.  Howard Kochman and Chip Deutsch (USGS/Sirenia Project ) helped with this presentation.