Northern Prairie Wildlife Research Center
Beth Wright, Bradley L. Weigle, Florida Department of Environmental Protection, Florida Marine Research Institute, 100 8th Avenue S.E., St. Petersburg, FL 33701 USA, and Scott D. Wright, Marine Mammal Pathobiology Laboratory, 3700 54th Avenue South, St. Petersburg, FL 33711 USA
The Florida Department of Environmental Protection (FDEP) captured and tagged 59 wild manatees on the west coast of Florida between 1991 and 1996. The tagging assembly, developed by the U.S. Geological Survey Sirenia Project, Gainesville, Florida, included a rubber peduncle belt and a tethered floating platform transmitter terminal (PTT). The 39 x 9 cm-diameter transmitter housing contained satellite, VHF, and ultrasonic transmitters. Tags remained on individuals from 2 weeks to 5 years, with breaks in the tagging period.
Two techniques were used for catching free-ranging manatees, land sets and open water captures. Using land sets, manatees were captured in the discharge canal of a power plant during winter using a 150 m long x 10 m deep net. This method required baiting manatees to a specific site with freshwater from a hose, manually pulling a net behind the animals, pursing them in the net, and hauling them to shore. It required 15 people and worked better for catching random animals rather than specific individuals. This method was compromised in the summer when freshwater was widely available and manatees were widely dispersed. The open water capture technique was developed by FDEP in 1992, first as a means of rescuing injured manatees and them to catch specific individuals for research. The net was deployed from a 6-m open transom mullet boat while circling manatees in open water. The manatees were then pursed in the net and pulled on board. This method required 8 people and enabled researchers to catch random animals as well as specific individuals multiple times. The challenges of the open water technique included: "blind sets" and animals due to poor water quality; poor bottom types such as mud and oyster beds; catching very large manatees, multiple animals, and/or mother/calf pairs; catching sting rays or debris along with the manatees; and trying to catch previously captured animals that learned how to escape capture. The use of an airplane and spotter helped in locating manatees in a large area and in making "blind sets." In February 1997, 21 manatees were caught over 4 days by open water captures with assistance from an aerial spotter.
There were numerous obstacles and impediments to keeping manatees tagged over the course of the study. Tag exchanges to replace transmitter batteries every 6 months required a snorkler to quietly approach a manatee and change the transmitter on the distal end of the tether. Depending on the skill of the researcher and disposition of the manatee, individual animals varied on their tolerance to tag exchanges. Damage to transmitter housings occurred from boat strikes and alligator bites, causing complete tag loss or loss of satellite function from water intrusion. Tethers broke at the designed weak-link due to mating activity, boat strikes, alligator bites, and entanglement in crab trap line and floats, monofilament line and fixed structures. People with both good and bad intentions pulled tags off. During warm months, rapid algal and barnacle growth interfered with satellite function and caused some tag loss. Manatees that loss their tags were often retagged by FDEP snorklers clipping tethers with temporary VHF tags to the peduncle belt until they could be caught by open water capture to securely attach the PTT tagging assembly.