The Trichechus manatus latriostris (T.m.latirostris) is an aquatic mammal listed as endangered on the IUCN Red List. The manatee is situated mainly in the United States, and is particularly abundant around the coast of Florida (Figure 1). The West Indian manatee is divided into 2 sub-species; the Florida Manatee (T.m.latirostris) and the Antillean Manatee (T.m. manatus). The West Indian Manatee species as a whole is listed as vulnerable on the IUCN Red List. Ecological conservation is the attempt to protect species on Earth from endangerment and extinction (Allendorf FW, 2007). Most conservation efforts have focused on the Florida Manatee and they are intensely studied in the south-east states of America. The specie's main threat is human activity, along with habitat changes and low reproductive rates (Deutsch CJ, 2008).
The West Indian Manatee is one of the 4 remaining species of the Sirenia, the others being the Dugong (Dugong dugon), the West African Manatee (Trichechus senegalensis) and the Amazonian Manatee (Trichechus inunguis) (Bossart GD, 1999). These other species of Sirenia have similar threats and conservation problems to the Florida Manatee. The Sirenia are believed to have evolved from four footed land mammals, with the elephant being their supposed ancestor. Fossil records found that both Manatees and dugongs were present in the West Atlantic and Caribbean, but the manatees prevailed over the dugongs in these areas due to better adaptive features for eating the more abrasive vegetation (Van Beter VB, 1989).
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T.m.latirostris are large aquatic mammals which can grow to 13 feet long and weigh up to 3000 pounds. They range from grey to brown in colour and are sparsely covered by fine hairs, with thicker whiskers on their faces. Manatees are seal-like in description; they are insulated by great amounts of blubber with large flat tails which they use to propel themselves through the water. T.m.latirostris have 2 flippers used for balance, with 3 or 4 nails at the end of each one. Their skin is finely wrinkled and cleaned by creatures that eat algae and other growths off the skin's surface. T.m.latirostris is very heavy boned with no marrow cavities in their ribs and limbs. It has a bulbous face with small wide eyes with an inner membrane for protection against foreign bodies. It has a large snout like nose with small nostrils consisting of valves which are tightly closed when underwater (Van Beter VB, 1989). The Florida Manatee is unusual in that its teeth are continuously replaced throughout its lifetime. New molars grow at the back of the jaw and are slowly worn down as they move forward to the front and fall out. The Manatee's brain is considered to be relatively small in relation to its body size (Van Beter VB, 1989).
T.m.latirostris herbivorous diet consists of vascular aquatic vegetation from the shallow waters of their habitat. They are avid eaters and consume around 15% of their body weight daily (Van Beter VB, 1989). T.m.latirostris are indiscriminate in their choice of plants, although they appear to prefer submerged plants, eating seagrass, manatee grass and shoalgrass, only resorting to feeding on floating vegetation and algae when these are not available (Hartman DS, 1979). The Manatees ability to replace its teeth is thought to be an adaption to their diet as sand from the sea floor is often mixed with the plants they eat making them abrasive (Van Beter VB, 1989).
The Florida Manatee lives in a very specific habitat fulfilling certain criteria. Studies have shown that the temperature, depth, vegetation, salinity, tides, currents and weather all have to be within certain levels for the Florida Manatee to reside there (Hartman DS, 1979). The temperature of the water is extremely important to Florida Manatees due to their low metabolic rates and high thermal conductance (Marmontel M, 1997). They are seasonal migratory creatures; when waters drop below 20°C they migrate south to naturally warmer waters. They have also been observed migrating to artificially warmer waters, particularly to water outlets near powerplants. The majority of Manatees tend to return to the same warm water sanctuaries each year; the most popular around Florida are Crystal River and Blue Spring. When temperatures rise, most manatees disperse back to their usual dwellings (Deutsch CJ 2008). The depth of the water is understood to be one of the most important factors for the manatee. T.m.latirostris prefer a depth of 2 to 3 metres for both resting and for travelling, though they will venture into shallower waters to get to water of their preferred depth if the distance is short. A manatee will only dive into deep water to avoid danger; the main reason for their abhorrence of deep water is thought to be the higher pressure level. Tides are therefore an influence on the location of manatees; low tides prevent them from accessing areas closer to the shore, but they take advantage of high tides to reach plants which are normally inaccessible to them (Hartman DS, 1979). Manatees move between fresh and saltwater, however, they are believed to prefer fresh water areas as they are most often seen in rivers and estuaries and are never too far from a freshwater source. Manatees tend to avoid areas with fast currents, preferring to swim nearer the shore than against fast moving water, however they have been observed swimming slowly against the current, taking their time rather than employing any more effort than is necessary (Hartman DS, 1979).
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Florida Manatees spend their time eating, resting and travelling and are very laid-back, slow-moving mammals. Being mammals, T.m.latirostris have to surface to breathe air every 3 to 5 minutes, although when resting they can stay underwater for up to 20 minutes and when they expend lots of energy they submerge around every 30 seconds (Save the Manatee, 2010). Manatees travel at speeds of 3 to 5 miles per hour, although they can reach 20 miles per hour for short distances. They are very graceful, often performing acrobatic somersaults, twists and turns underwater (Taylor D, 2004). Manatees regularly travel alone during summer, though it is not unusual to see larger groups, particularly in migration refuges (Deutsch CJ, 2008). Manatees have a specific niche within the aquatic community as they help to spread plants and wildlife through ingestion and excretion and they also aid conservation of creatures which depend on them for food and protection (Goedeke TL, 2004).
Female manatees reach sexual maturity at around 3 years old and can give birth at 4. However females at this age are often too inexperienced to look after the calf and most manatees do not procreate until they are 6 to 10 years old (Van Beter VB, 1989). Male manatees reach sexual maturity earlier than females, between 3 and 4 years old. When the female's oestrous cycle begins, males form a mating herd which can last for up to 4 weeks. Males join and leave the herd daily and intense competition in undertaken to mate with the female (Deutsch CJ, 2008). The female is persistently pursued and attempts to flee until she has selected a suitable partner. Successive copulation is understood to occur during this period, with the older and stronger manatees fighting off the competition for their mating rights. Manatees do not form permanent bonds with partners and copulation is undertaken only briefly (Van Beter VB, 1989).
The manatee gestation period is between 12 and 13 months. T.m.latirostris give birth to a single calf, twins are extremely rare but do occasionally occur. Manatees are not thought to have a specific mating period and therefore calving occurs throughout the year. They give birth at intervals of between 2 and 5 years (Van Beter VB, 1989). Calves are born measuring 4 to 4.5 feet in length and weighing roughly 66 pounds. The female manatee has very rich milk containing lots of proteins, fats and salts. The calves initially suckle milk from their mother's teats, feeding underwater for up to 3 minutes at a time. Calves are born with molars and start feeding on plants a few weeks after birth when their teeth begin to move forward at a rate of 0.03 inches per month. Florida manatee calves are dependent on their mothers for up to two years, and tend to reside close to her throughout their life; research suggests this proximity allows the young to learn migration routes and good winter refuge areas. T.m.latirostris can live for up to 50 years (Van Beter VB, 1989).
Populations of T.m.latirostris are not easy to estimate due to tracking difficulties, but the number of Florida Manatees in 2001 was estimated at around 3,300. Having less than 2,500 adult individuals in a population of a species is considered as endangered on the IUCN Red List. The Florida Manatee's status changed from vulnerable to endangered in 2007. The population number was estimated using range-wide synoptic surveys which have been carried out every year since 1991. The abundance data is collected during winter when manatees tend to gather in the same warm water locations (Deutsch CJ, 2008). The numbers estimated only includes manatees within the testing area therefore the abundance data cannot be fully relied on (Craig BA, 1997).
The Florida Manatee can be divided in 4 subpopulations, and the synoptic survey of 2001 estimated the numbers within these different populations.
Estimated numbers 2001
East coast of Florida
Upper St. Johns River
South of Palatka
Gulf coast between Pasco-Hernando county line and Louisianna
Between Pasco-Hernando county line and Whitewater Bay in Monroe
(Deutsch CJ, 2008).
A Population Viability Analysis (PVA) Model, which is the most suitable theoretical model in this instance, was applied to the Florida Manatee population in 2006, which allows an approximation for future numbers to be established. The statistical model indicated that there is a 55.5% chance of a 20% decline in the next 2 generations (a generation equals around 20 years) and a 12.1% chance of a 50% decline within the next 3 generations (Deutsch CJ, 2008). Annual mortality of T.m.latirostris is extremely high and although death rates are thought to be decreasing, a record number of manatee deaths have already occurred in 2010. The number of deaths recorded this year in Florida by the 30/09/10 was 656, higher than the record of 429 deaths in 2009, which is an estimated loss of 13% of the Florida Manatee's minimum population (Trip K, 2010).
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The main threats to T.m.latirostris are both natural and anthropogenic. Human activity, habitat loss, climate change and slow reproduction rates are the biggest threats currently facing the Florida Manatees. The biggest threat from mankind is watercraft collisions. Estimated figures for the amount of manatee deaths that can be attributed to collisions with boats are more than 30% of annual mortalities. Boats can cause both blunt and sharp traumas depending on which area of the boats the manatee impacts on, but both can be fatal (Nowacek SM, 2004). Strict speed restrictions in Manatee abundant areas have reduced the amount of boat-related deaths as the manatees have more time to move out of the oncoming boat's path and fines are issued to any vessel breaking the speed limit. The manatee refuges of Barge Canal and a 4.5km section of Sykes Creek were observed before and after speed restrictions were put into place. Only one death was recorded in the refuges during the 42 months following the speed restriction, compared to 8 deaths in the previous 42 months (Laist D, 2006). Other anthropogenic threats are entanglement in fishing lines and nets, crushing or suffocation by becoming trapped in flood control measures such as flood gates, water pollution and ingestion of or entanglement with discarded litter (Bossart GD, 1999).
Climate change would have a huge effect on Florida manatees, particularly due to their temperature requirements. Climate change is also linked to the increasing severity of coastal storms. Hurricanes are already a regular occurrence on the south-eastern coast of the U.S and cause a large number of manatee deaths each year, directly as well as indirectly. Indirectly, the manatee's habitats are often disrupted along with their food sources (Langtimm CA, 2003), and general changes in vegetation could cause starvation or forced migration to more plant abundant areas. The slow reproductive rates are another threat to the Florida manatee numbers, along with high mortality rates in calves. The death of the mother leaves the calf defenceless resulting in malnutrition, boat collisions or opportunistic disease. Disease and cold related deaths also contribute to annual mortality; red tide events in particular have wiped out large proportions of the Florida manatee population in the past (Bossart GD, 1999).
Social perception of Florida Manatees has significantly changed over the years. Historically, manatees were hunted and killed for sport and meat until interest in them grew and people began to fight for conservation. State Laws were passed protecting the Florida Manatees and the harvesting of manatees was banned in 1893, however this law was virtually ignored and the killing continued. Historic myths about the creatures were the cause for a lot of poaching; for example, one myth claimed manatees ate large amounts of fish, and therefore fisherman thought they were pests (Goedeke TL, 2004). More laws were passed and Florida Manatees became protected under the Marine Mammal Protection Act of 1972 and the Endangered Species Act of 1973. Unfortunately, these laws were also ineffective. It was not until the 1970s when the Save the Manatee Club was founded and the Florida Manatee was named as the State Marine Animal by the FMSA (Florida Manatee Sanctuary Act of 1978) that the laws became effective. The main priority became to educate and increase public awareness of the Florida Manatee (Goedeke TL, 2004). By the 1990s fundraising by merchandising manatee products was making a significant difference. The social perception of T.m.latirostris changed from mysterious monster to being a gentle intriguing creature. Manatee 'adoption' programs were introduced and are very popular, with people sponsoring and naming an individual manatee, raising funds for manatees in the process. Greater public awareness led to the deaths of individual manatees being highly publicised and focused upon in newspapers and television reports, highlighting the high death rate of Florida manatees (Goedeke TL, 2004).
Figure 2. Photo of a Diver with a Young Florida Manatee (sourced from Tampa Bay, 2009)
The increasing popularity of wildlife tourism and 'swimming with animals' has also helped to raise money and awareness for Florida Manatees. Close to 100,000 tourists visit Crystal River in Florida every year to swim and interact with the manatees. Florida manatees are very approachable and allow visitors to get up close and interact with them (Figure 2). Manatee encounter boat tours have also become very popular in the past few years (Sorice MG, 2006). Studies have looked at both the positive and negative effects of this type of tourism. The primary concern was against changes in the manatee's behaviour and routines. Human interaction was observed to stimulate the manatees and cause excitement and playful behaviour. This could have a positive effect as it could lead to greater interactions within the species and therefore increased reproduction. However too much energy expenditure could have negative impacts and affect the manatee's behaviour. Studies have also discovered that manatees are more sociable in the morning and dislike scuba equipment due to the noise and bubbles. Therefore some management has to be undertaken to decide on appropriate amounts of interaction between the manatees and people, as well as the times and regulations about equipment that can be used (Sorice MG, 2006).
The conservation of Florida manatees has come to be of paramount importance. Manatee populations will continue to decrease if nothing is done to protect them. MPPs (Manatee Protection Programmes) have been developed in 13 counties of Florida. These programmes include restrictions and laws put into place to protect Florida manatees. They state strict rules on any further disruption to manatee habitats including the expansion or development of boating facilities. The plans include aims to significantly decrease and eventually stop the fatalities due to collisions with watercrafts. Increasing public awareness is another part of the MPPs, initiating the display of posters informing of the protection practices of Florida Manatees. The plan also aims to ultimately decrease the amount of manatee deaths in general and to conserve and protect these endangered creatures (Indian River County, 2004). These plans are a starting block for the future of Florida Manatees, and although protection has been put in place, mortality rates of T.m.latirostris are still at a record high and more must be done to prevent their total extinction