Trichechus manatus

West Indian Manatee

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West Indian Manatee West Indian Manatee West Indian Manatee

 

Description

Possibly mistaken for mermaids by Christopher Columbus on a voyage to America, the West Indian manatee is a large, rotund, aquatic mammal. The tough skin is grey-brown with sparse hairs covering the body and many bristles on the muzzle (5). The front limbs are short flippers and the body tapers to a flat, paddle-shaped tail that distinguishes this species from the related dugong (Dugong dugon), which posses a fluke-like tail (6). Two subspecies of the West Indian manatee exist; the Florida manatee (Trichechus manatus latirostris) is usually larger in size than the Antillean manatee (T. m. manatus) (7). A unique feature (amongst mammals) of the manatee is the constant replacement of molar teeth; new teeth enter at the back of the jaw and replace old and worn teeth at the front (5). Recent evidence suggests that manatees may possess a unique sixth sense that enables them to detect pressure changes through sensory hairs (8).

Geography

The West Indian manatee (Trichechus manatus) — sometimes called the sea cow—is found along the coast of Florida and in the Caribbean. Most adult manatees are about 10 feet long and weigh 800 to 1,200 pounds, although some larger than 12 feet and weighing as much as 3,500 pounds have been recorded. These “gentle giants” have tough, wrinkled brown-to-gray skin that is continuously being sloughed off. Hair is distributed sparsely over the body. With stiff whiskers around its mouth, the manatee’s face looks like a walrus without tusks.

Christopher Columbus was the first European to report seeing a manatee in the New World. To Columbus, and other sailors who had been at sea for a long time, manatees were reminiscent of mermaids—the mythical half-fish, half-woman creatures of the ocean. Manatees are not fish, however, but marine mammals.

The manatee maneuvers through the water moving its paddle-like tail up and down and steering with its flippers. It is very agile for such a large animal, sometimes somersaulting and doing barrel rolls in the water.

The manatee often rests suspended just below the water’s surface with only the snout above water. It feeds underwater, but must surface periodically to breathe. Although the manatee can remain underwater for as long as 12 minutes, the average time is 4-1/2 minutes.

Manatees are herbivores, a term that means they eat only plants. They consume 4 to 9 percent of their body weight each day—that’s 32 pounds of plants for an 800-pound animal! To do this, manatees spend 5 to 8 hours a day eating—typically non-native water hyacinths and hydrilla, along with native aquatic plants such as Vallisneria or eelgrass.

Manatees move between fresh- water, brackish, and saltwater environments. They prefer large, slow-moving rivers, river mouths, and shallow coastal areas such as coves and bays. The animals may travel great distances as they migrate between winter and summer grounds. During the winter, manatees congregate around warm springs and around power plants that discharge warm water. During summer months, they have occasionally been seen as far north as Virginia and Maryland.

Manatees reach breeding maturity between 3 and 10 years of age. The gestation period is approximately
13 months. Calves may be born at any time during the year. Usually a single calf is born, but twins do occur. An adult manatee will usually give birth to a calf every 2 to 5 years. The low reproductive rate makes the species less capable of rebounding from threats to its survival. Newborn calves weigh 60 to 70 pounds and are 4 to 4-1/2 feet long. They nurse underwater for about three minutes at a time from a nipple located behind their mother’s forelimb. Born with teeth, calves begin eating plants within a few weeks but remain
with their mother for up to 2 years. Manatees may live for several decades.

Manatees communicate with each other by emitting underwater sounds that are audible to humans. The vocalizations, which sound like squeaks and squeals, are especially important for maintaining contact between mother and calf. One field report described a mother and her calf, separated by a flood gate, calling to each other for three hours without interruption until they were reunited.

Manatees face many threats to their survival throughout their range. Historically, they were hunted for their flesh, bones, and hide. Manatee fat was used for lamp oil, bones were used for medicinal purposes, and hides were used for leather. Hunting is thought to be largely responsible for the initial decline of the species; however, hunting is no longer allowed in countries where manatees are protected.

Today, the greatest threats to manatee survival are collisions with boats and, in Florida, loss of warm water habitat.

Ecosystem

The West Indian manatee (Trichechus manatus) — sometimes called the sea cow—is found along the coast of Florida and in the Caribbean. Most adult manatees are about 10 feet long and weigh 800 to 1,200 pounds, although some larger than 12 feet and weighing as much as 3,500 pounds have been recorded. These “gentle giants” have tough, wrinkled brown-to-gray skin that is continuously being sloughed off. Hair is distributed sparsely over the body. With stiff whiskers around its mouth, the manatee’s face looks like a walrus without tusks.

Christopher Columbus was the first European to report seeing a manatee in the New World. To Columbus, and other sailors who had been at sea for a long time, manatees were reminiscent of mermaids—the mythical half-fish, half-woman creatures of the ocean. Manatees are not fish, however, but marine mammals.

The manatee maneuvers through the water moving its paddle-like tail up and down and steering with its flippers. It is very agile for such a large animal, sometimes somersaulting and doing barrel rolls in the water.

The manatee often rests suspended just below the water’s surface with only the snout above water. It feeds underwater, but must surface periodically to breathe. Although the manatee can remain underwater for as long as 12 minutes, the average time is 4-1/2 minutes.

Manatees are herbivores, a term that means they eat only plants. They consume 4 to 9 percent of their body weight each day—that’s 32 pounds of plants for an 800-pound animal! To do this, manatees spend 5 to 8 hours a day eating—typically non-native water hyacinths and hydrilla, along with native aquatic plants such as Vallisneria or eelgrass.

Manatees move between fresh- water, brackish, and saltwater environments. They prefer large, slow-moving rivers, river mouths, and shallow coastal areas such as coves and bays. The animals may travel great distances as they migrate between winter and summer grounds. During the winter, manatees congregate around warm springs and around power plants that discharge warm water. During summer months, they have occasionally been seen as far north as Virginia and Maryland.

Manatees reach breeding maturity between 3 and 10 years of age. The gestation period is approximately
13 months. Calves may be born at any time during the year. Usually a single calf is born, but twins do occur. An adult manatee will usually give birth to a calf every 2 to 5 years. The low reproductive rate makes the species less capable of rebounding from threats to its survival. Newborn calves weigh 60 to 70 pounds and are 4 to 4-1/2 feet long. They nurse underwater for about three minutes at a time from a nipple located behind their mother’s forelimb. Born with teeth, calves begin eating plants within a few weeks but remain
with their mother for up to 2 years. Manatees may live for several decades.

Manatees communicate with each other by emitting underwater sounds that are audible to humans. The vocalizations, which sound like squeaks and squeals, are especially important for maintaining contact between mother and calf. One field report described a mother and her calf, separated by a flood gate, calling to each other for three hours without interruption until they were reunited.

Manatees face many threats to their survival throughout their range. Historically, they were hunted for their flesh, bones, and hide. Manatee fat was used for lamp oil, bones were used for medicinal purposes, and hides were used for leather. Hunting is thought to be largely responsible for the initial decline of the species; however, hunting is no longer allowed in countries where manatees are protected.

Today, the greatest threats to manatee survival are collisions with boats and, in Florida, loss of warm water habitat.

Conservation

While manatees have no natural predators, their numbers are threatened by human activities. Because of their low reproductive rate, it is difficult for the species to rebound from a decline in numbers. Although the population in Florida has historically been hunted by Native Americans and, later, by the European inhabitants, it was never the victim of commercial hunting. In other parts of their range West Indian manatees have been exploited commercially and, in some cases, this continues. Although protection laws exist in countries such as Costa Rica and Venezuela, illegal poaching still occurs.

One of the main causes of manatee mortality is collisions with motorboats. Manatees are also killed in canal locks and found entangled in fishing nets. They are also threatened by the loss of (or damage to) sea beds due to agricultural and industrial runoff. These same pollutants have been shown to accumulate in the tissues of manatees and some could be toxic to the animals.

Manatee conservation efforts were initiated as early as the eighteenth century, when the English established Florida as a marine sanctuary for the species. In 1893 a state law was established to protect manatees. At the start of the twentieth century fines were established for the killing of a manatee. Manatees are now protected by the U.S. Marine Mammal Act (1972), the U.S. Endangered Species Act (1973), and the Florida Manatee Sanctuary Act (1978).

 

Threats

Threats to the Florida manatee encompass catastrophic natural events and anthropogenic factors that could cause declines in reproductive and survival rates or declines in the carrying capacity of the environment. Much of the following text is modified from the Biological Status Reviews for the Florida manatee (FWC 2002, Haubold et al. 2006).

Anthropogenic Threats
About half of adult mortality rangewide is attributable to human-related causes, primarily watercraft collisions (Ackerman et al. 1995, Deutsch et al. 2002). This is significant because the manatee population growth rate is highly sensitive to changes in adult survival rate (Eberhardt and O’Shea 1995, Marmontel et al. 1997, Runge et al. 2004). The future of the Florida manatee is also jeopardized by the predicted loss and deterioration of warm-water habitat, including retirement or deregulation of aging power plants and reduction in natural spring flows.

Watercraft collisions: Watercraft collisions account for approximately 25% of all manatee deaths and 35% of documented deaths of known cause, and are the single greatest cause of human-related mortality (Ackerman et al. 1995, Wright et al. 1995, FWC-FWRI unpubl.). In 2005, there were over one million registered vessels in Florida (FHSMV, http://casey.hsmv.state.fl.us/Intranet/dmv/TaxCollDocs/vesselstats2005.pdf), and many more out-of-state boaters visit Florida annually. The number of registered vessels in Florida has increased by an average of 2.9% per year over the past 25 years, doubling since 1980 (FWC, unpubl.). Given that about 97% of registrations are for recreational watercraft (Wright et al. 1995), it can be expected that there will be a continued increase in recreational vessels plying the waterways of Florida with a concomitant increase in the human population. In addition to the expected increase in boat numbers over the coming century, there are other factors that may act synergistically to increase the risk of collisions between manatees and watercraft. Modifications to the design of vessel hulls and engines are allowing boats to travel at higher speeds in shallower waters (Wright et al. 1995), thus threatening manatees and scarring seagrass beds. Boater compliance with existing slow speed zones is inconsistent (Gorzelany 1998, Shapiro 2001).

Sub-lethal effects on manatees of increased vessel traffic and a growing human population in the coastal zone are cause for concern. Most adult manatee carcasses bear scars from previous boat strikes and the healed, skeletal fractures of some indicate that they had survived previous traumatic impacts (Wright et al. 1995, Lightsey et al. 2006). Of over 1,000 living individuals in the manatee photo-identification database (Beck and Reid 1995), 97% had scar patterns from multiple boat strikes (O’Shea et al. 2001). Approximately one-third of these individuals were severely mutilated, especially on the tail and the dorsum. It should be noted that the photo-identification database contains only animals with scars or other identifiable features. Non-lethal injuries may reduce the breeding success of wounded females and may permanently remove some animals from the breeding population (O’Shea 1995, Reynolds 1999). Vessel traffic and recreational activities that disturb manatees may cause them to leave preferred habitats and may alter biologically important behaviours such as feeding, suckling, or resting (O’Shea 1995, Wright et al. 1995).

Loss of Warm-water Habitat: Expected changes in the network of warm-water refuges over the next several decades present the most serious long-term threat to manatees in Florida, as noted in the federal Recovery Plan: “one of the greatest threats to the continued existence of the Florida manatee is the stability and longevity of warm-water refuges” (USFWS 2001, p. 28). Ultimately, the discharges from power plants provide unreliable warm-water habitat when viewed over the long term (i.e., next 20 to 100 years) because the once-through cooling technology that creates the large thermal plumes is being replaced by more efficient and alternative cooling technologies (Laist and Reynolds 2005a). Short-term threats to the network of warm-water sites also loom on the immediate horizon. Some aging power plants may be shut down and potential deregulation of the electric utility industry may eliminate or reduce the reliability of warm-water effluents that large numbers of manatees depend on to survive winter cold periods (Rose 1997, U.S. Fish and Wildlife Service 2000). Temporary disruptions in heated effluents during winter have caused changes in local manatee distribution (Packard et al. 1989) and have been implicated in elevated numbers of deaths from cold stress (Campbell and Irvine 1981, Ackerman et al. 1995). The complete elimination of a secondary warm-water refuge in northeastern Florida through diffusion of the heated effluent resulted in a shift in manatee distribution within the area and in substantial mortality of manatees that remained in the region (Deutsch et al. 2000, Laist and Reynolds 2005a). Loss of certain key warm-water sites could result in catastrophic mortality and would likely reduce the environmental carrying capacity for manatees in Florida.

The long-term reliability of artesian springs that provide natural warm-water refuges for manatees is also in doubt because human demand for ground water and loss of recharge areas through development will likely result in diminished spring flows (Reynolds 2000, Laist and Reynolds 2005a). According to the U.S. Census Bureau (2001), Florida’s human population increased by about 23% to 16 million between 1990 and 2000, and projections suggest that the number of people living in Florida will increase by another 10 million people by 2025. In order to meet the increased demand for water that a growth in human population will entail, it is likely that spring flows and water quality will decline, further reducing natural warm-water habitat for manatees. This natural habitat will become even more important in the future as existing industrial sites disappear.

Other Direct Threats to Manatees from Human Activities: Other threats from human activities include entanglement (in fishing gear or debris), entrapment in water-control structures and pipes, exposure to contaminants, incidental ingestion of debris, and crushing (in flood-control structures, in canal locks, or between large ships and docks) (Beck and Barros 1991, Ackerman et al. 1995). Indirect effects from increased vessel traffic include increased water turbidity from wake action and scarring of seagrass beds by propellers (Sargent et al. 1995).

Indirect Threats to Manatees from Human Activities: There is no commercial or subsistence utilization in the USA. However, manatees have become the centre of a large ecotourism industry at certain winter aggregation sites, such as Crystal River. Tens of thousands of people visit these areas to observe and swim with manatees. No-entry sanctuaries provide manatees with havens to avoid swimmers and boats at these sites. Manatees do, in fact, increase their use of these sanctuaries when more boats and swimmers are present (Buckingham et al. 1999, King and Heinen 2004). Manatees also have been found to alter their behaviour in response to the presence of human swimmers, including decreased resting and suckling and increased swimming (King and Heinen 2004).

The tremendous growth in the human population in coastal Florida over the past half century has resulted in drastic losses of coastal wetland habitats. Seagrass distribution and abundance in many estuaries have declined as the result of direct human impacts (dredging and propeller scarring) and indirect effects of development (declining water quality and nutrient loading). Within Tampa Bay, for example, an estimated 80% of the seagrass present in the early 1900s was lost by 1980 (Kurz et al. 2000). This decline in seagrass coverage is slowly being reversed through actions to reduce nitrogen loading in the regional watershed, which have improved water clarity in much of Tampa Bay (Johansson and Greening 2000, Kurz et al. 2000). Non-point-source runoff is difficult to control, however, so water clarity declines in years of above-average precipitation. Reductions in optical water clarity cause declines in the health and abundance of submerged aquatic vegetation (Stevenson et al. 1993). Indirect effects from increased vessel traffic include increased water turbidity from wake action and scarring of seagrass beds by propellers (Sargent et al. 1995). It will be particularly important to protect, restore, and maintain aquatic vegetation communities in the vicinity of warm-water aggregation sites. Without conservation measures to secure these winter habitats, manatees would have to travel greater distances, concentrate into smaller areas, and forage in sub-optimal environments.

Natural Threats
Naturally-occurring catastrophic threats to manatees include prolonged periods of very cold temperatures, hurricanes, harmful algal blooms (i.e., “red tide”), and the potential for a disease epizootic. The threat from extended periods of cold weather relates to the availability and quality of warm-water habitat, which has already been discussed above.

Hurricanes: Hurricanes are another type of weather-related phenomenon that can potentially impact manatee populations. In the Northwest subpopulation, Langtimm and Beck (2003) found that adult survival rate was depressed in years with severe storms or hurricanes. The mechanism(s) underlying the lower survival probabilities are unknown, as there has not been a corresponding elevation in the number of reported carcasses. Such events could also result in large-scale emigration out of the affected region. In eastern Australia, for example, the simultaneous occurrence of flooding and a cyclone, combined with poor watershed management practices, resulted in the loss of 1,000 km² of seagrass beds and in the mass movement and mortality of dugongs (Dugong dugong; Preen and Marsh 1995). Given the notice from meteorologists that we have entered a new 25- to 50-year cycle of greater hurricane activity and intensity (Landsea et al. 1996), as well as possible longer-term changes associated with global climate change (McCarthy et al. 2001), storm activity may have a greater impact on manatee populations in the future.

Red Tide: Manatees on Florida’s Gulf coast are frequently exposed to brevetoxin, a potent neurotoxin produced by the dinoflagellate Karenia brevis during red tide events. In 1996, 151 manatees were confirmed or suspected to have died in southwestern Florida from brevetoxicosis (Landsberg and Steidinger 1998, Bossart et al. 1998). This epizootic was particularly detrimental to the manatee population because more adults were killed than any other age class. Other red tide mortality events in 1982, 2002, 2003, and 2005 resulted in the confirmed red tide-related deaths of 37, 33, 86, and 68 manatees, respectively, with an additional 35 carcasses suspected to have died from red tide during those years (O’Shea et al. 1991; FWC unpubl.). Recent studies have determined that brevetoxin can exist outside of the algal cells (e.g., on seagrass) for extended periods of time, thus further increasing the threat to foraging manatees (Flewelling et al. 2005). Red tide represents a major natural source of mortality for manatees in the southwest region. There is no clear evidence that these events have been increasing in frequency along Florida’s coast, but certainly the impact on the manatee population has increased over the past two decades. The role of nutrient loading to coastal systems relative to the intensity and duration of inshore red tides is an active area of research for the FWRI harmful algal blooms group.

Pathogens: Manatees could potentially be exposed to pathogens. Spread of such pathogens could be particularly rapid during winter when manatees are concentrated in warm-water refuges. Large-scale mortality events caused by disease or toxins have decimated other populations of marine mammals, including seals and dolphins, removing 50% or more of the individuals in some events (Harwood and Hall 1990). Manatees have robust immune systems that have, through the present time, provided disease resistance. Since 1997 papilloma virus has been found in captive Florida manatees and there is some evidence that it may also be present in the wild population in northwest Florida (Bossart et al. 2002a, Woodruff et al. 2005). While the consensus is that this virus probably does not present a serious threat to manatees at this time, managers are proceeding cautiously (e.g., by establishing a quarantine on exposed captives) and surveillance for papilloma lesions in wild manatees continues.

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