Marine Mammals - Manatees And Dugongs

Manatees are large stout mammals that inhabit fresh waters and coastal waterways. They are from the Sirenian order, along with dugongs. There are only five Sirenian species, and all are endangered or extinct. Scientists believe that Steller's sea cow, the only species of cold-water manatee, was hunted to extinction during the 1700s.

The West Indian manatee, also known as the Florida manatee, primarily swims in the rivers, bays, and estuaries of Florida and surrounding states. (See Figure 5.11.) As shown in Table 5.1 this species is listed as endangered under the ESA.

Manatees are often called "sea cows" and can reach weights of up to 2,000 pounds. They swim just below the surface of the water and feed on vegetation. West Indian manatees migrate north in the summer, though generally no farther than the North Carolina coast. In 1995 a manatee nicknamed "Chessie" made headlines by swimming all the way to Chesapeake Bay. Eventually biologists, concerned about his health in cooler waters, had him airlifted back to Florida. During the winter many manatees huddle around warm water discharges from FIGURE 5.11 A Florida manatee nurses its calves. (© Douglas Faulkner/Corbis. Reproduced by permission.)power plants and other industrial facilities. Although this can keep them warm, scientists worry that overcrowding in small areas makes the animals more susceptible to sickness.

Imperiled Manatee Populations

Each year during cold weather biologists conduct surveys to determine the number of Florida manatees remaining in the wild. The numbers are estimates based on surveys conducted at known wintering habitats. The latest survey was performed in January 2005 and found 3,143 manatees living along the Florida coast. This number is up from 1,267 reported in 1991. Many manatees have scars on their backs from motorboat propellers—these allow individual manatees to be recognized. In a press release entitled "FWC Biologists Release Preliminary 2005 Manatee Mortality Data" (January 6, 2006, http://www.floridamarine.org/news/view_article. asp?id=26330), the Florida Fish and Wildlife Conservation Commission noted that "the manatee population appears to be doing fairly well" throughout much of the state. Populations in the northern and central parts of Florida are increasing or remaining stable. However, manatees in southwest Florida face extra threats due to their exposure to red tide incidents (proliferation of harmful algae in the water). Scientists warn that these populations "could decline significantly."

Threats to Manatees

Manatees are imperiled for a variety of reasons. Although they can live for fifty or sixty years, their birth rate is low. Mature females bear a single offspring only every three to five years. Many baby manatees die in the womb or soon after birth for unknown reasons. These are called perinatal fatalities. Disease, natural pathogens, and cold water temperatures are also deadly. However, motorboat strikes are the major documented cause of manatee mortalities. Manatees are large and swim slowly at the surface of the water. They often cannot move away from boats quickly enough to avoid being hit. Environmentalists have tried to protect manatees from boat collisions, and have successfully had several Florida waterways declared boat-free zones. There are also areas where boaters are required to lower their speeds.

During 1995 and 1996 a pneumonia-like virus killed more than 100 manatees in southern Florida. The disease, according to marine microbiologist John H. Paul and his colleagues, was caused by a red tide that occurred when toxin-producing aquatic organisms called dinoflagellates bloomed in large quantities ("A Filterable Lytic Agent Obtained from a Red Tide Bloom That Caused Lysis of Karenia Brevis [Gymnodinum breve] Cultures," Aquatic Microbial Ecology, 2002). The Florida Marine Research Institute reported in 2006 that human-related activity accounted for 30% of all manatee deaths between April 1974 and June 2005, most from watercraft collisions. (See Figure 5.12.) The cause of death could not be determined in 28% of the cases. Nonhuman causes were blamed for 42% of the deaths.

A lawsuit by the Save the Manatee Club and other environmental and conservation organizations in 2000 successfully required the state to implement new low-speed zones for boats and establish safe-haven areas for manatees. The rules were immediately challenged by individual boaters and boating organizations; however, the restrictions were upheld by Florida courts in 2002.

Foreign Manatee and Dugong Species

There are two surviving foreign species of manatees found in western Africa and in and around the Amazon River in South America. Both species are designated under the ESA as endangered and are in grave danger of extinction due to illegal hunting, deforestation, habitat destruction, and water pollution. The only remaining dugongs live in the coastal waters of the Indian Ocean FIGURE 5.12 Causes of mortality in Florida manatees, April 1974–June 2005 Adapted from "Summary Report by County, Month, Year and Probable Cause of Death," in Manatee Mortality Database, Florida Fish and Wildlife Conservation Commission, Fish and Wildlife Research Institute, 2005, http://research.myfew.com/manatees/ (accessed January 20, 2006)and Pacific Ocean. Their populations are also considered imperiled. Dugongs around the tiny island of Palau in the western Pacific Ocean are considered endangered under the ESA.