Animal Info - West African Manatee

(Other Names: African Manatee, Lamantin d’Afrique, Lamantin du Sénégal, Lamantine, Manatí de Senegal)

Trichechus senegalensis

Status: Vulnerable

Contents

1. Profile (Picture)
2. Tidbits
3. Status and Trends (IUCN Status, Countries Where Currently Found, Taxonomy, Population Estimates and Status, Distribution, Threats and Reasons for Decline)
4. Data on Biology and Ecology (Size and Weight, Habitat, Birth Season, Birth Rate, Diet, Behavior, Social Organization)
5. References

Profile

Picture: West African Manatee (8 Kb JPEG)

Manatees are large, cylindrically shaped mammals, with forelimbs modified in to flippers, no free hindlimbs, and the rear of the body in the form of a flat, rounded, horizontal paddle. The flexible flippers are used for aiding motion over the bottom, scratching, touching and even embracing other manatees, and moving food into and cleaning the mouth. Its upper lip is modified into a large bristly surface, which is deeply divided. It can move each side of the lips independently while feeding. The general coloration is gray. 

The West African manatee weighs less than 500 kg (1100 lb). Adults are generally 3 - 4 m (10 - 13') long. It inhabits coastal areas, estuarine lagoons, large rivers that range from brackish to fresh water, freshwater lakes and the extreme upper reaches of rivers above cataracts. This manatee species is dependent on emergent or overhanging, rather than submerged, vegetation. Populations in some rivers depend heavily on overhanging bank growth, and those in estuarine areas feed exclusively on mangroves. Seasonal movements in response to changes in water level affecting availability of food and/or water salinity have been reported in several areas. An individual manatee may travel 30 - 40 km/day (19 - 25 mi/day) through lagoons and rivers.

The West African manatee occurs  from southern Mauritania to Angola; its numbers continue to decrease. The population decline has been attributed largely to hunting and incidental capture in fishing nets. Despite legal protection, the manatee is still hunted throughout its range. It is sometimes also killed in turbines or control gates of dams. The coastal wetlands that are a major habitat for the manatee have already been heavily damaged and are further severely threatened. Woodcutting is resulting in the extermination of mangrove stands in some areas. Mangrove clearance, as well as erosion due to forest clearance upstream, are resulting in increased sedimentation that silts up lagoons and estuaries.

Tidbits

*** By some writers the animal is said to leave the water entirely, and to search for its food upon the land, but this assertion is now ascertained to be incorrect (Wood 1860). 

*** The West African manatee is the most threatened of all manatee species (Perrin 2001). 

*** The manatee does not have incisors or canine teeth, only cheek teeth (molars). Molars designed to crush vegetation form continuously at the back of the jaw and move forward as older ones wear down. The older ones eventually fall out, while new ones come in at the rear of the jaw to replace them. 

*** The three species of manatees, and the closely related dugong, are unique in that they are the only plant-eating marine mammals in modern times.

*** The West African manatee is protected by national law in every country in which it occurs, although ineffectively in most areas. Awareness of the protected status of the manatee is widespread in all areas surveyed, but there is little perceived fear of arrest and punishment. (Perrin 2001)

Status and Trends

IUCN Status:

Countries Where the West African Manatee Is Currently Found:

2006: Occurs in Angola, Benin, Cameroon, Chad, Côte d'Ivoire, The Democratic Republic of the Congo, Equatorial Guinea, Gabon, Gambia, Ghana, Guinea, Guinea-Bissau, Liberia, Mali, Mauritania, Niger, Nigeria, Republic of the Congo, Senegal, Sierra Leone, and Togo. (IUCN 2006)

Taxonomy:

There are three species of manatees: the Amazonian manatee (Trichechus inunguis), the American manatee (Trichechus manatus), and the West African manatee (Trichechus senegalensis). The West African manatee is externally indistinguishable from the American manatee. On the other hand, both the American and West African manatees can easily be distinguished from the Amazonian manatee, because they lack the distinctive white markings on the abdomen or chest which are characteristic of  the latter species.  

Population Estimates and Status:

[Except where indicated, the following entries are based on Perrin 2001.]

  • WORLD
  • Angola
    • 2001: Manatees have been reported from the entire coast, but little information is available on status.
  • Benin
    • 2001: The manatee had been thought to be extinct. However, this is apparently not the case.
  • Burkina Faso
    • 2001: Manatees inhabit all of the nations that surround Burkina Faso, so they may occur there as well.
  • Cameroon
    • 2001: Manatees may still be numerous in some areas.
  • Chad
    • 2001: Manatees were once abundant in the Chad basin but had become rare by 1924.  They occur in rivers and lakes.
  • Côte d'Ivoire
    • Mid-1980's: Less than 750 animals
    • 2000: The population is tentatively estimated at 750 - 800.
  • The Democratic Republic of the Congo
    • 2001: Manatees were once common in the extreme lower reaches of the Congo River below Binda. They may occur in the upper reaches of the Congo as well.
  • Equatorial Guinea
    • 2001: Manatees likely occur in the lower reaches of the Mitémélé River on the mainland.
  • Gabon
    • 2001: Gabon may have one of the highest densities of manatees remaining in Africa.
  • Gambia
    • 2001: Numbers are thought to have declined, but as of 1993 the manatee was still numerous in the River Gambia.
  • Ghana
    • 2001: Manatees continue to exist in Volta Lake and Digya National Park.
  • Guinea
    • 2001: Little information is available on the manatee, although it is known to occur in the area.
  • Guinea-Bissau
    • 2001: Guinea-Bissau at one time was considered to be one of the last sanctuaries of the manatee, because of the relatively undisturbed state of its mangroves, wetlands and river systems. Information on its status is scarce.
  • Liberia
    • 2001: Manatees occur throughout the major rivers of Liberia. No information is available on status.
  • Mali
    • 2001: Manatees are found throughout the entire Niger River system of Mali but may have been reduced by hunting.
  • Mauritania
    • 2001: The manatee occurs in the Senegal River and its tributaries.
  • Niger
    • 2001: In Niger it may occur in the Niger River and the Chad basin.
  • Nigeria
    • 2001: The manatee is found throughout Nigeria but is depleted, due to overhunting.
  • Republic of the Congo
    • 2001: A preliminary survey in 1994 found manatees in lakes, rivers and lagoons.
  • Senegal
    • 2001: The manatee is close to extinction.
  • Sierra Leone
    • 2001: The manatee is declining. It is protected but widely hunted and marketed.
  • Togo
    • 2001: Manatees may still exist in Togo Lake.

Distribution:

Currently, the West African manatee occurs in most of the coastal marine waters, brackish estuaries, and adjacent rivers along the coast of West Africa from southern Mauritania (16° N) to the Loge, Dande, Bengo and Cuanza Rivers, Angola (18° S). Centers of population appear to be Guinea-Bissau; the lagoons of Côte d'Ivoire; the lower reaches of the Niger River, Nigeria; Sanaga River, Cameroon; coastal lagoons of Gabon and the lower reaches of the Congo River. (IUCN 2006)

The West African manatee ascends most major rivers within its range until a cataract or shallow water prevents its progress. In some rivers, such as along the Benue River, it seeks refuge during the dry season in permanent lakes that communicate with the rivers during high water but are cut off when river waters subside. The West African manatee can be found 75 km (47 mi) offshore among the shallow coastal flats and mangrove creeks (with abundant seagrasses and calm water) of the Bijagos Archipelago of Guinea-Bissau as well as Casamance (Senegal). Isolated populations cut off from the sea are found in Lake Volta, Ghana above the Volta hydroelectric dam. An additional population, essentially landlocked above the major rapids, is found in the upper reaches of Niger River in the inland delta of Mali as far as Segou, which is the farthest inland record for this species, over 2,000 km (1200 mi) from the ocean. The West African manatee is landlocked in the Logone and Chari Rivers of Chad. It occurs along the entire length of the Gambia River, penetrating into Senegal where there are records as far upstream as Niokola Koba National Park. In Chad, this manatee is present in Lake Léré and Lake de Tréné along the Mayo-Kebbi, Bahr Keeta and Baningi Rivers. The West African manatee is also reported from the Baningi, Logone and Chari Rivers,  tributaries of Lake Chad. (IUCN 2006)

Distribution Map for West African Manatee (27 Kb JPEG)
Distribution Map for all Sirenia (Dugong and Manatee Species) (22 Kb GIF) (Wildl. Trust)

Threats and Reasons for Decline:

In the past, the West African manatee's population decline has been attributed largely to hunting and incidental capture in fishing nets. Recently, continuing uncontrolled and likely unsustainable hunting is considered the major threat. Despite legal protection, the manatee is still hunted throughout its range for meat, leather and oil, by harpoon, trap, net, and snagline. The few historical data that exist indicate decreases in catch rates - an indication of a decline in the population. The West African manatee is viewed as a pest in some agricultural and fishing areas. It purportedly consumes rice and other crops in the fields and eats small fish caught in gillnets. This can result in the manatee being killed. This manatee is known to die incidentally in shark nets, trawls, set nets and weirs. It is sometimes also killed in turbines or control gates of dams. The coastal wetlands that are a major habitat for this manatee have already been heavily damaged and are further severely threatened. Woodcutting, especially of the red mangrove, for firewood and furniture construction is resulting in the extermination of mangrove stands in some areas. Mangrove clearance, as well as erosion due to forest clearance upstream, are resulting in increased sedimentation that silts up lagoons and estuaries. Reduced water flow due to construction of dams reduces availability of estuarine freshwater and increases overall salinity that affects growth of vegetation. (Perrin 2001)

The level of threats, particularly hunting and incidental catches, appears to be continuing to increase throughout the West African manatee's range, with locally high rates and near extirpation in some regions. (IUCN 2006)

Data on Biology and Ecology

Size and Weight:

The West African manatee may reach up to 4.5 m (14' 9") in length and weighs about 360 kg (790 lb) (Stuart & Stuart 1996).

Adults are 3 - 4 m (10 - 13') long and weigh less than 500 kg (1100 lb) (Husar 1978).

Habitat:

The West African manatee inhabits coastal areas, estuarine lagoons, large rivers that range from brackish to fresh water, freshwater lakes and the extreme upper reaches of rivers above cataracts. This manatee also inhabits the lakes in these river systems. The basic requirements are - sheltered water with access to food and fresh water. It can be found in marine habitats where there is relatively calm water and a source of freshwater. For example, in Senegal, Gambia, and Guinea-Bissau, the West African manatee is attracted to freshwater seeps or springs that are found in marine habitats. The West African manatee can travel freely between saltwater and freshwater. 

The preferred coastal habitats of the West African manatee are: a) coastal lagoons with abundant growth of mangroves or herbaceous growth; b) estuarine areas of larger rivers with abundant mangroves  in the lower reaches and grasses further upriver; and, c) shallow (less than 3 m (10') depth) and protected coastal areas with fringing mangroves or marine aquatic plants. Where river levels fluctuate seasonally, preferred areas are those with access to deep pools or connecting lakes for dry-season refuge, and with seasonal flooding into swamps or forests with abundant grasses and sedges. The West African manatee may be limited to waters of 18° C (64° F) or higher. (Perrin 2001)

The West African manatee is one of the species that live in the Guinean Forests of West Africa Biodiversity Hotspot (Cons. Intl.) as well as in the Guinean-Congolian Coast Mangroves, Gulf of Guinea Marine Ecosystems, Sahelian Flooded Savannas, and Western Guinea Current Marine Ecosystems Global 200 Ecoregions. (Olson & Dinerstein 1998, Olson & Dinerstein 1999)

Birth Season:

The breeding period is uncertain and may last throughout the year, as is the case with the Amazonian and American manatees. 

Birth Rate:

Usually one calf is produced. 

Diet:

The West African manatee feeds primarily on vegetation.  It is dependent on emergent or overhanging, rather than submerged, vegetation. Populations in some rivers depend heavily on overhanging bank growth, and those in estuarine areas feed exclusively on mangroves. In Sierra Leone,  manatees supposedly remove fish from nets and consume rice in such quantities that they are considered to be pests. (Nowak 1999) In Senegal and Gambia, shell remains of mollusks have also been found in their stomachs. (IUCN 2006)

An adult might be expected to consume about 8000 kg (17,600 lb) of aquatic vegetation in one year (Husar 1978). 

Behavior:

The West African manatee feeds principally at night and travels in the late afternoon and at night. It usually rests during the day in water that is 1 - 2 m (3 - 6') deep, sometimes in the middle of a watercourse or hidden in mangrove roots or under natant vegetation. It makes little disturbance in the water while swimming. (IUCN 2006)

Seasonal movements of the West African manatee,  in response to changes in water level that affect availability of food and/or water salinity, have been reported for several areas (Perrin 2001). In Sierra Leone, it was reported that this manatee is present in the main river channels year-round, but that a migration of some sort may take place, with an influx of new animals arriving in most upriver areas as flooding begins in June and July (Reeves et al. 1988). 

An individual manatee may travel 30 - 40 km/day (19 - 25 mi/day) through lagoons and rivers (Nowak 1999).

Birth supposedly occurs in shallow lagoons (Husar 1978).

Social Organization:

The West African manatee is mostly solitary, with mothers and calves the principal social unit. However, they will often rest together in loose, small groups of two to six individuals. (IUCN 2006)

References

Allen 1942, Burton & Pearson 1987, Cons. Intl., Husar 1978, IUCN 1966, IUCN 1994, IUCN 1996, IUCN 2000, IUCN 2003a, IUCN 2004, IUCN 2006, Nowak 1999, Nowak & Paradiso 1983, Olson & Dinerstein 1998, Olson & Dinerstein 1999, Perrin 2001, Poche 1973, Reeves et al. 1988, Sirenia.org, Stuart & Stuart 1996, Wildl. Trust, Wood 1860

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