African manatees omnivorous

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The African manatee (Trichechus senegalensis) is unique among sirenians as it is the only omnivore among the species. It occurs across 21 countries in Western and Central Africa, and has adapted to living in both freshwater ecosystems and marine environments.

The African Aquatic Conservation Fund (AACF) has conducted the first dietary studies on the African manatee in Gabon and Senegal and found some unexpected results.

It turns out they are what they eat. By studying the carbon and nitrogen signatures in different foods and comparing them to dead manatees’ ear bones, Lucy Keith-Diagne, executive director of the AACF, was able to determine the percentages of different foods consumed during their average lifetime.

In Senegal, up to 50% of the manatee’s diet is protein derived from fish, clams and mussels. Comparatively, in Gabon only 10% of the manatee’s diet is molluscs.

“During the West African dry season there are very few aquatic or emergent plants in places like the Senegal River,” Keith-Diagne explains. “We think the manatees rely more on molluscs and fish during this time.” Further south in Central African waters, plant and aquatic species are available all year round and the manatees feed on both. “This suggests the nutrients acquired from protein are important components of their diet,” she adds.

“African manatees feed on more than 90 species of freshwater and shoreline plants, from seagrasses and mangroves to freshwater plants and shoreline grasses,” Keith-Diagne says. “Because they live in freshwater rivers, estuaries, lagoons and nearshore oceans, their diet can vary based on their surroundings.”

Ocean-dwelling manatees are known to consume heavy African ark, a species of saltwater clam. In
a freshwater environment, they feed on freshwater molluscs, sucking the shellfish out of the mud.
“For fish, they seem to prefer catfish or fish without scales. So far we have recorded 17 fish species in their diet,” she adds.

During the rainy season, African manatees that live 2 500km inland feed on fruits and tree leaves – in some areas tall trees are completely submerged in flood water.

African manatee. Sketch: Graham Kearney

It’s amazing that a forest elephant and a manatee can eat fruit off the same tree. Manatees eat the fruit during the wet season while elephants can reach the fruit during the dry season, Keith-Diagne says.
In Cameroon, riverside farmers have reported manatees raiding their crops.

Marine and freshwater fish stocks are declining across Africa and as a result manatees can come into conflict with fishermen. “Again, this is regionally specific,” she says. “In some places, manatees are killed in retaliation for damaging nets, while in other areas, like the Ivory Coast and parts of Senegal, the fishermen
share their catch with the manatees.”

 

Written by Georgina Lockwood

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