|Regional Management Plan for the West Indian Manatee, Trichechus manatus|
|CEP Technical Report No. 35 1995||All CEP Technical Reports|
A. REVIEW OF TAXONOMY AND GENERAL BIOLOGY
Also called sea cow, the manatee is known in most Caribbean countries as vaca marina or manatí. The word probably derives from the Carib Indian word manati, meaning breast in allusion to the way manatees nurse their young from teats marginally resembling human breasts (Shaul and Haynes 1986). Other names given in the region are lamantin, zeekoe, amerikaanse lamantijn, and sekoe (Husson 1978) palpa (Miskito Indians) and manatin (mayan).
Manatees (Family Trichechidae) are members of the Order Sirenia, a unique group of aquatic mammals that feed exclusively on vegetable matter. The genus Trichechus is confined to coastal and inland waters of the New World and is comprised by 3 species. T. inunguis (Amazonian manatee) is endemic to the Amazon region and lives exclusively in freshwater, T. senegalensis (West African manatee) occurs in rivers and estuaries of West Africa, and T. manatus (West Indian manatee) is distributed from the southeastern United States to northern South America. The Straits of Florida on one side and the cold temperatures of the northern Gulf of Mexico on the other are believed to have promoted the differentiation of the latter into 2 subspecies identified based on osteological characteristics (Domning and Hayek 1986): T. m. latirostris (Florida manatee) inhabits coastal Florida and the northern Gulf of Mexico and T. m. manatus (Antillean manatee) occurs along the coasts and rivers from Mexico to northeastern South America including the Greater Caribbean. Manatees seen in Louisiana and east are believed to be from Florida, whereas those found in Texas probably belong to the Mexican population. The only living member of the Family Dugongidae, the dugong (Dugong dugon) inhabits Indo-Pacific waters.
Manatees have a fusiform, gray to black body, a horizontally-flattened tail, and no hindlimbs. Forelimbs are modified into paddle-shaped fins and present nails at the tips. The snout is blunt and the flexible lip pads are provided with sensory bristles. Finer, widely spaced hair covers the surface of the body. The eyes are small. The ears lack external pinnae. Canines and incisives are absent and the cheek teeth move continuously forward in a conveyor belt-like fashion. Bones are extremely dense, and long bones and ribs lack a marrow cavity.
A sample of 33 Antillean manatees captured in Guyana had lengths ranging from 1.0 to 3.4 m; weight of smallest animal was 27 kg and the largest exceeded 400 kg (Bertram and Bertram 1964). Adult Florida manatees range from 400-900 kg in weight and 2.8 to 3.5 m in total length (O'Shea 1992).
Manatees in Florida are arrhythmic (Hartman 1979), but may follow diet patterns in the winter (Bengtson 1981, Kochman et al. 1985), leaving warm-water sites to forage during the warm hours or between cold spells (Rathbun et al. 1983a, Powell and Rathbun 1984); in Trinidad they may show response to tidal cycles (Boyle and Khan 1993).
Manatees are most active in the evening and early morning, when they are usually feeding (Bertram and Bertram 1963). It is speculated that in certain parts of the Caribbean they became crepuscular or nocturnal in response to hunting (Ackerman 1992, Rathbun et al. 1983a, Reynolds et al. in ms., Reynolds and Odell 1992). Manatees may be seen occasionally lying at the surface in the middle of the day, apparently basking in the sun, especially in cold weather (Bertram and Bertram 1963, ). Despite the large size, manatees are secretive and can disappear quietly, or swim speedily when faced with danger. They usually travel totally submerged, with the help of the pectoral flippers. Flippers may also aid in pushing the body up out of the water as far as the shoulders, while they graze on river bank vegetation (Bertram and Bertram 1964).
When water temperatures drop below about 20oC in autumn and winter, Florida manatees engage in seasonal north-south migrations in search of warm-water sites, where they aggregate during the winter (Moore 1951, Irvine 1983, Irvine and Campbell 1978, Powell and Waldron 1981, Powell and Rathbun 1984, Shane 1983). Movements of up to 600 km have been documented (Rathbun et al. 1983b), with males ranging over wider areas than females (Bengtson 1981). Similar movements might allow exchange between manatee populations in Belize (Bengtson and Magor 1979, O'Shea and Salisbury 1991), Mexico (Colmenero et al. 1990, Benjamin Morales pers. comm.), and possibly Honduras (Rathbun et al. 1983a). During times of flood, manatees move along the large rivers, lagoons and adjacent creeks in Mexico, Costa Rica, Nicaragua, Suriname, Colombia and Venezuela (Mondolfi 1974, Colmenero 1984, Husson 1978, O'Shea et al. 1988, Reynolds et al. in ms. Montoya and Mingucci, unp. data), whereas during the drought they concentrate in the larger, perennial rivers and lagoons, in search of forage and shelter. In contrast, no evidence was found for such seasonal migratory behavior in Panama (Mou Sue et al. 1990).
Reproductive activity occurs throughout the year (Husson 1978, Rathbun et al. 1985a), with some indication of mating peaks between March and August (Charnock-Wilson 1968, Gibson 1992, Janson 1977, Rathbun et al. 1985a, Quintana 1993). Mating usually takes place in shallow water, in remote and quiet lagoons, inlets and creeks (Bertram and Bertram 1963, Husar 1977).
Manatees in Florida gather in week- to month-long mating herds, where up to 17 males may follow a female for over 150 km, until she mates with several of them in sequence. At times the female may strand herself to prevent male advances (Hartman 1979, Bengtson 1981). Similarly, large groups (12-16) have been observed in Belize and Guyana (Bertram and Bertram 1964, Charnock-Wilson 1968) and described to "fight" for over 2 hours before "beaching" themselves and mating lying on their sides (Bertram and Bertram 1964). Mating herds of about 8 animals were reported for Puerto Rico (Mignucci l989, unp. data).
The gestation period in manatees is approximately 1 year (Hartman 1979, Rathbun et al. 1992), and females give birth in shallow sheltered areas (Hartman 1979, Bengtson 1981, Gibson 1992). Small calves are seen throughout the year (Belitsky and Belitsky 1980, Powell et al. 1981), although some suggest that most births occur during the rainy season, between September and March/April (Charnock-Wilson 1968, Colmenero et al. 1988, Quintana 1993). Usually a single calf is born, but twins have been reported (Charnock-Wilson 1968, Colmenero et al. 1988, Gumilla 1745, Husson 1978). Newborns weigh approximately 27-40 kg and measure about 80-130 cm (Gumilla 1745, Husson 1978, Mondolfi 1974, Zárate 1993). The calf may remain with the female for over a year (Husson 1978). Mother-calf pairs spend most of their time in protected areas in proximity to freshwater sources (Colmenero et al. 1988, Gibson 1992, Morales et al. 1995 unp. report). In certain areas females may leave their calves among others in secluded places, while they go out to forage (Reynolds 1981, Domning 1990). In Chetumal Bay females have been observed with two calves of different sizes at one time, which suggests that one female may nurse at the same time, two calves of different ages (Morales et al. l993 unp. report).
Manatees are mostly solitary throughout the range, but may be seen in pairs or small groups of up to 13 individuals at once (Cerrato 1993 unp. report, Estrada and Ferrer 1987, Irvine et al. 1982, Mondolfi 1974, Powell et al. 1981, Quintana 1993, Rathbun et al. 1985, Zárate 1993). Most associations are temporary and seasonal, except between a mother and her calf (Reynolds 1981), which maintain continuous contact by vocal communication (Hartman 1979).
Food and feeding
Essentially herbivorous, manatees are unselective feeders (Bertram and Bertram 1963) consuming submerged, floating or emergent freshwater plants and grasses (Ceratophylum, Eichhornia, Echinochloa, Hydrilla, Panicum, Paspallum, Phragmites, Pistia, Pontederia, Potamogeton, Vallisneria), and shoots and leaves of mangrove Avicennia, Rhizophora, Laguncularia, moko-moko Montrichardia, and leaves of Ipomoea (Mondolfi 1974, Duplaix and Reichart 1978, Husson 1978, Hartman 1979, Bengtson 1981, Hurst 1987, Lefebvre et al. 1989, Mou-Sue et al. 1990, Augusta 1992, Boyle and Khan 1993, INDERENA 1993 in litt., Reynolds et al. in ms.). In the marine habitats of Puerto Rico, Dominican Republic, Cuba and Florida manatees feed on Ruppia and seagrass beds of Syringodium, Halodule, and Thalassia (Belitsky and Belitsky 1980, Packard 1981; Powell et al. 1981, Rathbun et al. 1983a, Estrada and Ferrer 1987, Colmenero et al. 1988). Episodes of manatees taking fish from fishermen's nets have been documented in Jamaica (Powell 1978).
Manatees may consume approximately 8% of their total body weight in aquatic plants every day (Best 1981). They have an elevated digestive efficiency, between 45 and 80%, and a very low rate of food passage for a hindgut digester(Best 1981, Lomolino and Ewel 1984, Burn 1985). Hurst (1987) calculated the energy requirements of a 600-kg non-lactating adult manatee at over 4,000 kcal/day or approximately 29.5 and 45 kg Ceratophyllum per day.
Manatees use rivers, estuaries and coastal areas, moving freely between fresh, brackish and saltwater areas (Bengtson and Magor 1979, Lefebvre et al. 1989); however, they seem to require access to freshwater (Crombie 1975 unpubl. report, Campbell and Irvine 1975, Powell et al. 1981, Colmenero et al. 1988, Augusta 1992). Additional requirements include abundant aquatic vegetation for feeding, proximity to deep channels for travelling, and quiet coves for shelter (Charnock-Wilson 1968, Hartman 1979, Belitsky and Belitsky 1980, Powell et al. 1981, Gallo 1983, Rathbun et al. 1983a, 1985, Powell and Rathbun 1984, Cerrato et al. 1993, Estrada 1993 in litt., Ferrer 1993 in litt., Zarate 1993, Reynolds et al. in ms.). In Florida they further require access to warm water during the winter (Hartman 1979).
In Suriname manatees occur in flooded swamps and swamp savannah belts (Duplaix and Reichart 1978). In Venezuela and Colombia available habitat increases significantly during the rainy season, permitting access to tributaries and lagoons. During the drought manatees may become entrapped in deep water bodies (O'Shea et al. 1988, Montoya unp. data).
Interactions with other organisms
Manatees in Mexico share habitat with green sea turtles (Chelonia mydas), crocodiles (Crocodilus acutus), river otters (Lutra longicaudis annectens) and dolphins (Tursiops truncatus), picudas or barracudas (Sphyraena barracuda), sharks (Ginglymostoma cirratum) and rays (Aetobatus narinaris). Remoras (Eheneis neucrotoides) commonly adhered to manatee bodies (Colmenero et al. 1988). In Puerto Rico most manatees have a number of remoras associated with them (Mignucci l989, unp. data). In Venezuela manatees share habitat with giant river otters (Pteronura brasiliensis) and river otters (Lutra longicaudis), river and estuarine dolphins (Inia geoffrensis and Sotalia fluviatilis), capybara (Hydrochaeris hydrochaeris), cayman (Cayman crocodilus) and river turtle (Podocnemis expansa) (Ojeda et al. 1993).
Manatees allegedly serve as shark prey in Costa Rica, Dominican Republic, Guatemala, Honduras, Mexico, and Panama (Belitsky and Belitsky 1980, Colmenero et al. 1988, Colmenero and Zarate 1990, Crombie 1975, Gallo 1983, Janson 1977, Mou Sue et al. 1990, Reynolds et al. in ms.). However there are no documented records of manatees attacked by sharks, crocodiles or barracudas in Chetumal Bay, Mexico (Morales and Olivera 1993). Bertram and Bertram 1964, believed that caiman and big or aggressive fish could take manatees, especially injured specimens.
Life history traits
Manatees are long-lived animals and may reach over 50 years of age. Females mature between 3 and 4 years of age. Early mortality rate is high, but decreases after maturity (Marmontel 1993). These parameters contribute to a low maximum potential rate of population growth in Florida manatees (Packard 1985), and presumably the same can be said for other populations as well.
Forrester (1992) reviewed the causes of mortality and morbidity among Florida manatees and pointed out that reported bacterial, viral, and fungal agents, and internal and external parasites are relatively unimportant in terms of pathology and do not cause epizootics. Manatees, especially subadults, are vulnerable to extremely cold winter temperatures and "red tide" organisms have been implicated in Florida in one large die-off of manatees (O'Shea et al. 1985, 1991).
B. GENERAL STATUS IN THE REGION
The West Indian manatee ranges from northern South America to the southeastern United States, including Trinidad and the Greater Antilles. Normally is not found in the Lesser Antilles. Temperature constraints restrict the dispersal of the species between the northern and southern 24oC isotherms (Whitehead 1977). Present distribution is fragmented due to local extinction or habitat unsuitability (Thornback and Jenkins 1982, Lefebvre et al. 1989).
Abundance and legal status
Small manatee populations exist in 19 countries of the Wider Caribbean. Although data on past abundance is unavailable, in most areas numbers are perceived to have decreased in the past two decades. O'Shea and Salisbury (1991) provide a table of maximum counts of West Indian manatees throughout the Caribbean region. Aerial surveys over selected areas in Belize produced the highest counts (n=102) of any Caribbean country.
West Indian manatees are listed as endangered under the "U.S. Endangered Species Act of 1973, as amendad", and vulnerable to extinction by The World Conservation Union (IUCN) (Thornback and Jenkins 1982). Manatees are listed in Appendix I of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) and consequently trade of any manatee product is prohibited. Annex II of the SPAW Protocol includes all the Sirenia species as requiring total protection under Article 11, which prohibits the taking, possession, killing and commercial trade of the species, their parts or products. Additionally, other relevant international agreements serve to protect the manatees and their habitats. The Ramsar Convention for the conservation of wetlands which entered into force in 1975, promotes the designation and management of national wetlands as important species habitats, in particular for waterfowl. Eleven States of the Wider Caribbean are Parties to the Ramsar Convention. The recently adopted Convention on Biological Diversity has as objectives the conservation of biodiversity and the sustainable use of its components. The Convention entered into force on 29 December 1993, it has been signed by twenty-five States of the Wider Caribbean and ten of the manatee-range states have ratified. Table 1 includes the list of manatee-range countries which have signed or ratified the SPAW Protocol and the Ramsar, CITES and Biodiversity Conventions.
Folklore and legends
The most widespread myth related to manatees is that of mermaids, possibly derived from the way manatee cows nurse their calves. Manatees are the subject of 2 very similar Suriname Carib Amerindian legends explaining the origins of manatee, tapir and porpoise (Amazonian gray dolphin) (Duplaix and Reichart 1978).
History of exploitation and utilization
Manatees have been hunted by native peoples and exploited by Europeans. Their high quality meat is said to have 3 flavors (beef, fish, chicken, pork and others), and the oil has multiple uses, from lighting fuel to medicine. In Colombia and Honduras manatee hide was used to manufacture whips. Archaeological excavations of prehistoric sites in Belize have yielded a wealth of manatee remains (Bradley 1983, McKillop 1985); and they have been reported recently in Dominican Republic and Mexico (Mignucci and Morales unpbl. data). The 17th and 18th centuries witnessed a large scale commercial exploitation of manatees in the Guianas and Caribbean region (Craig 1966, Bertram and Bertram 1973). Disillusioned with the search for gold, Spanish, Portuguese, English, French and Dutch sailors found the manatee meat a lucrative substitute. According to de Jong (1961), salted manatee meat was transported from the Guianas to the sugar plantations in the West Indies or northeastern Brazil (Bertram and Bertram 1973).
During his travels, Bartram (1761) came upon Indians in Florida who used manatee ribs as ivory and called them by a name that meant "big beaver". Manatees were hunted by the Seminole Indians in Florida, but the importance of manatee meat in the diet of these indigenous peoples is unclear (Reynolds and Odell 1992). Grinding of the bones is also used for medicinal purposes, menstrual regulation, arthritis, muscle pain, general body pain, epilepsy and whooping cough.
C. ECOLOGICAL IMPORTANCE
Manatee grazing as a form of weed and mosquito control has been investigated in semi-captive conditions (Allsopp 1960, MacLaren 1967). Although manatees help the situation without pollution (Domning 1992, Reichart 1993 in litt.), sole systematic use of manatee power has not produced encouraging results (Etheridge et al. 1985). It has also been suggested that manatees be raised for meat production (Bertram and Bertram 1968) but their slow reproductive rate, depressed population levels, and endangered status render the idea unfeasible. A role in the recycling of limiting nutrients in the ecosystems seems to be manatees' greatest ecological contribution, by stimulating primary productivity (Lomolino 1977, Best 1981, Domning 1992). Furthermore, manatees may serve as indicators of the general ecological health of the ecosystem they inhabit, and in the long run, their physiology may provide humanity with substances or mechanisms to combat certain types of disease (Domning 1992).
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Preface and Objectives | Summary | I. Introduction | II. National Status | III. Short and Long-term... | IV. References | Appendix I | Appendix II | Appendix III | Table 1 | Manatee Map