What Do We Learn by Tracking Manatees?
Research on manatee use of Florida’s coastal and riverine habitats is essential to understanding the resources required to recover and sustain a healthy population. By tracking the movements of individual manatees through their aquatic environment, FWC biologists obtain valuable information about manatee seasonal and daily movements, migratory behavior, site fidelity, diving behavior, and habitat requirements. Studies have also included manatee responses to boats and to changes in warm-water habitat.
Satellite-linked radio-telemetry is a system by which data recorded by an instrument are transmitted from a radio-tag via satellite to receiving stations and ultimately to the scientist. Manatee satellite tags collect and transmit accurate GPS locations, as well as data on water temperature, activity, and diving behavior (see article, How Do We Track Manatees?). Unlike other means of studying wildlife, such as aerial surveys and photo-identification (in which data are only collected when biologists are in the field), satellite telemetry provides information on the animal’s movements, behavior, and habitat use 24 hours per day and in all weather conditions. Satellite tags have allowed biologists to document the seasonal migrations of Florida manatees over hundreds of kilometers. These aquatic mammals show strong site fidelity to warm-water refuges in winter, as well as to summer home ranges; in other words, individuals generally return to the same location year after year. See this FWRI Technical Report (Weigle et al. 2001) for tagged manatee movements along Florida’s Gulf coast and this Wildlife Monograph (Deutsch et al. 2003) for seasonal movements, migratory behavior, and site fidelity on Florida’s Atlantic coast.
As tracking technology has evolved over the past several decades, each innovation has opened up a new window into the lives of manatees. A sampling of the discoveries that have been made through tracking manatees is given below.
Manatee Habitat Use and Central-place Foraging
Manatees rely on different habitats to meet their physiological needs. During winter, for example, they seek out warm-water habitats, such as springs and power plant thermal discharges, to keep warm when ambient waters drop below 20 ᵒC (68 ᵒF). This is because they are adapted to tropical and subtropical waters, so they cannot survive for extended periods in colder temperatures. Since there is typically no food at these warm-water sites, the animals must travel into cooler waters to feed. As a result, manatees commute back and forth between warm sites where they can meet their thermoregulatory needs and seagrass beds or other habitats with aquatic vegetation, upon which they can graze. This pattern of behavior, known as “central-place foraging,” is common in animals.
The map illustrates the movements of an adult male manatee (GPS locations in blue) over a 20-hour period, including a trip to feed about 5 km north of the power plant’s warm-water refuge. Seagrass beds are shown in green. There is considerable variation in how far individual manatees travel from a warm-water refuge; some travel 35 km (22 miles) or more to feed, while others (like the one shown here) remain close to the power plant. Watch an animation of the manatee's movements.
Manatee GPS tags generate a large amount of high-quality data on manatee movements and habitat use. At a 15-minute fix interval, nearly 10,000 locations are obtained for a single individual over the course of just a few months. Properly interpreting these data for multiple animals can be challenging because the symbols lie on top of each other, often making it difficult to discern the areas of highest use. FWC researchers convert points to densities in order to better identify and visualize habitats of importance to manatees, as shown in this map.
Areas of relatively high use by tagged manatees in the northern Indian River (near Titusville) during winter 2011-2012 are illustrated here, with orange and red colors depicting the highest densities of GPS locations. Manatees used a warm-water refuge at the site of a power plant (FPL) and generally crossed the lagoon to graze on seagrass beds on the east side of the Indian River.
For more information on these density maps and to download FWC’s final report (2016), visit the publication record, Manatee Response to the Conversion of the FPL Cape Canaveral Power Plant: Movements, Warm-water Habitat Use, and Thermal Regime of Satellite-tagged Manatees during Winters 2010-11 through 2014-15.
Travel Paths and Corridors
The ability to obtain accurate locations at closely spaced time intervals through GPS technology allows the delineation of travel paths, something past researchers could only accomplish through intensive time spent following tagged manatees in the field. The figure below is an example of a travel path over a 12-hour period as the manatee left the warm water of the power plant discharge canal and traveled ~10 km (6 mi) across cooler waters to reach seagrass beds along the MacDill peninsula of northeast Tampa Bay. GPS locations were obtained about every 20 minutes, except for a short segment from the shipping channel westward to shallower waters, when the individual spent more time underwater.
This manatee traveled at a steady rate of 1.8 km per hour (1.1 mph) on a fairly straight track across the bay. Movement rates of about 1.5-3.0 km per hour (0.9 – 1.9 mph) are common during directed travel behavior. Obviously, manatees do not show blazing speed, but the stamina shown during their long-distance movements is impressive!
When travel paths of an individual manatee are displayed over a longer time period, along with the paths of other manatees, the locations of frequently used travel corridors emerge. This same section of Tampa Bay is shown with the movement paths and GPS locations for two adult males. Two heavily used travel corridors (one east-west and one north-south) intersect with dredged shipping channels, as shown by the white ovals in the figure.
How deep can manatees dive? How much time do they spend in shallow water where they are potentially vulnerable to boat strikes? By attaching a small time-depth recorder (TDR) to the base of the tagging assembly, FWC biologists have answered these questions for the first time. In Tampa Bay during winter, manatees spent an average of two-thirds of their time at depths less than1 m (3.3 ft); this simple statistic illustrates how vulnerable they are to collisions with watercraft, which occur in this surface zone. But manatees are capable of diving much deeper: the maximum recorded depth was 16.2 m (53 ft), corresponding to the bottom of the dredged shipping channel at the entrance to Hillsborough Bay.
An example of such a dive profile over the course of one hour is shown here (blue line); depth readings were taken every five seconds. Note that the manatee appeared to be diving to and following the bottom contour (red line), including the shipping channel, while traveling across this open stretch of the bay. During this hour, the average dive duration was 2.4 minutes separated by short dives (20-45 seconds) during which the manatee stayed near the surface. When resting, manatees can stay underwater for up to about 15–20 minutes.
For more information on diving behavior, see this peer-reviewed article, "Influence of Manatees' Diving on Their Risk of Collision with Watercraft."