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3.11 Manatees

3.11.1 Trichechus manatus and trichechus inunguis

3.11.1 Trichechus manatus and trichechus inunguis

Vernacular names: Manatee (Guyana and other English-speaking areas, manatí (Mexico, Central America, Colombia, Ecuador, Venezuela), peixe boi (Brazil), sekoe (Suriname), vaca marina (Peru, etc.).

Geographical variation and distribution: The Caribbean manatee (T. manatus) ranges from the coastal areas of Florida and Georgia west to the Rio Grande and south through the Gulf of Mexico to the Caribbean and southern Atlantic down to Maranhao, Brazil, including a small portion of the Magdalena and Orinoco basins, and various rivers of Central America and the Guianas. The Amazonian manatee (T. inunguis) is widely distributed throughout Amazonia and up to Peru, Ecuador and Colombia. Distribution of both species is discontinuous for lack of proper habitat and because of local extinctions. Both species are probably monotypical (49, 53, 92, 159, 293, 294, 403).

Elevational range: Low-altitude brackish waters.

Size and weight: T. manatus: The usual total adult length is 260-400 cm and the weight 200-500 kg, but very large specimens can measure up to 450 cm and weigh over 600 kg. T. inunguis is the smaller of the two at a maximum length of 280 cm and an average adult weight of 200 kg (53, 262, 293, 294).

Habitat: Both species are purely aquatic. T. manatus prefers the quiet brackish warm waters of estuaries, big slow-moving rivers and lagoons and may occasionally be found in coastal waters. It is at home in clear and murky as in white or black water but as an herbivore it does require abundant quantities of submerged or floating vegetation. The manatee is generally found in 2-3 m of water but it comes into waters as shallow as 0.5 m to feed on the shoreline vegetation.

T. inunguis lives in similar environments in the system of rivers, lakes and sea-grass meadows of Amazonia. In tidal estuarine areas the rise and fall of the tides is another factor in the habitat, as are seasonal flows in rivers (49, 53, 56, 159, 262, 294, 403, 426).

Abundance: Both the historical record and the trade statistics for manatee products show that the animal was once abundant in the rivers, estuaries and lagoons of tropical America (204, 250, 289, 311, 545, 627). But both species are now considered very scarce throughout their entire range except for remnant populations of several thousand individuals which are fairly well preserved in Florida and Guyana (576).

Behaviour: There seems to be no clearly defined pace of daily activities. Manatees normally swim a slow 2-3 km/hour propelled by the vertical movements of the flattened tail but they are capable of short bursts of speed up to 25 km/h. They rest intermittently on the bottom or float suspended in the water. They come up for air 20 or 30 times an hour but can remain submerged for 15 to 20 minutes. Though generally solitary, the union between the female and her calf is prolonged and males often congregate around an oestrous female. They disperse as the rivers rise to feed in flooded areas and when the water level drops they congregate in the deepest rivers and lagoons. They are considered cautious and are highly sensitive to noise of any kind (58, 262, 294, 403, 407).

Feeding habits: Manatees are strictly herbivorous, feeding on submerged plants (Ceratophyllum, Elodea, Hydrilla, Thalassia, Vallisneria, Utricularia, Ruppia, Potamogeton), floating vegetation and emergents (Hymenachne, Leersia, Luziola, Echinochloa, Paspalum, Oryza) and other aquatic grasses, some Cyperaceae (Eichhornia, Salvinia, Pistia), the leaves and fruits of some riverine trees growing over the water, green algae in brackish sea-water and periphytons adhering to the submergents (53, 54, 159, 262, 407). They forage for intermittent periods of 30-90 minutes or more for a total of 5 hours a day, consuming 20-50 kg of fodder, some 4-9 percent of their bodily weight. Digestive efficiency and alimentary transit time are similar to those of large terrestrial herbivores (48, 54, 262).

Confined to the deeper rivers and lagoons during the dry season, manatees are forced to forego food entirely, and yet their low metabolism and fat reserves allow them to survive for months in this condition (54, 56, 225, 403).

Reproduction: Manatees are thought to require 5-10 years to reach sexual maturity (262, 435). The gestation period probably exceeds one year and the litter rarely consists of more than one calf. Newborn manatees are 85-140 cm in total length (T. inunguis according to Best (55) and T. manatus according to Hartman (262) and the calf remains with the mother up to the age of two. This means that a female manatee calves roughly once every three years. Manatees breed at any time of the year but T. inunguis exhibits a parturition peak during the flood period of the Amazon River (54, 56, 262, 292, 293). The record longevity in the wild was 32 years (54).

Hunting: The most frequent hunting technique is harpooning from a boat, which is facilitated by the flood period in brackish waters. The hunter waits until the manatee comes up to breathe or to browse on shore-growing vegetation. The harpoon has a barbed detachable head attached by a long cord to a wooden staff some 3 m long. Manatees are hunted by day or by night, particularly on nights when the moon is full. In tidal estuaries, fences of wooden stakes (camboas) are also used to catch manatee which come in on the high tide and are then trapped as the tide recedes. They may also be caught in trawlnets or gillnets, either intentionally or as a by-catch (159, 407, 465).

Products: This species is mainly valued for its meat: served fresh, dried and salted or fried in its own fat, this is a prized food. The fat (up to 100 kg per manatee), processed into oil, is another item in great demand for home consumption. The thick and sturdy hide was formerly used for industrial purposes in Brazil (160, 311, 407, 465, 545, 569).

Manatee meat and oil were once major consumer items throughout this mammal's entire range, and were regional and even international trade items from the 18th to the 20th centuries (and until very recently in some areas), but the now-depleted populations have shrivelled output. Poaching and a certain amount of illegal trading of manatee meat still persist in some isolated regions, spurred by the high unit value of the animal and its vulnerability to capture under certain circumstances (111, 245, 407, 545).

Management: Manatees represent an extraordinary resource but primarily one of historical value. The IUCN (576) classifies both species as vulnerable and hunting is forbidden throughout their entire range. A primary cause of the current scarcity of manatees, in addition to hunting, is the drainage of wetland areas, and mortality from collisions with speedboats, drowning in floodgates or fishing nets, vandalism, etc. (262, 292, 293, 403, 426, 436, 597).

The top manatee management priority should be to guarantee the survival of remnant populations, and their recovery where possible. This is undeniably a slow process given the low reproductive rate and late sexual maturity. One promising management and protection prospect is the introduction of manatees as weed-control agents in reservoirs and irrigation systems in Guyana and Brazil (54, 56, 424, 426).

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