Abstract

Recent verification of the presence of a freshwater manatee population in the landlocked nation of Chad, raises questions about its genetic distinctness from coastal and riverine populations. The manatees are residing in Lakes Léré and Tréné, part of the Mayo-Kebbi river system, on the southwestern border of Chad. Historically, manatees were described as living in the Chari, Bamingui and Bahr-Kieta rivers which flow from the southeastern border of Chad into Lake Chad.^1,2

Hunting pressure, as well as the fragmentation due to the isolation of the lakes regions, has left the Chadian population decreased in size and possibly isolated, both of which phenomena can lead to population genetic bottlenecks, inbreeding and the loss of genetic variability. This loss can, in turn, lead to the potential for increased disease susceptibility, increased juvenile mortality and decreased fertility. There is a clear need for the determination of existing genetic variability in this population, and similarly, among all the manatee populations stiff present in West Africa.

Recognition of the importance of the problems of inbreeding and loss of genetic diversity has led to the prominence of population studies in conservation efforts. The rigor of genetic analyses, including the use of mitochondrial DNA (mtDNA) sequence analysis, can result in significant contributions to short-term execution and long-term planning of species recovery plans, by providing knowledge of population structure, genetic isolation versus gene flow, and the evolutionary bio-geographical relationships between regions. This information is crucial for the evaluation, and recovery management of the declining Chadian manatee population.

Mitochondrial DNA analysis to-date^5,6 suggests that the West African manatee, T. senegalensis, which is morphologically the most similar to T. manatus, is actually the more sequence-derived of the three manatee species. The least sequence divergence is seen between the Amazonian manatee, T inunguis, and T. manatus, which are morphologically the most distinct.³ To begin the analysis of genetic variability in T. senegalensis, we have sequenced a 289 base pair segment of the mitochondrial genome D-loop (including part of the cytochrome b gene, the genes for tRNA Pro and tRNA Thr and the hypervariable portion of the control region), from representatives of three separate West African manatee populations, and compared these sequences with other Sirenian species. Several specimens were collected from the Chadian population which is currently being hunted by the local human populace, and which may be physically isolated from the other West African manatee populations. These samples were compared with specimens from the neighboring coastal Cameroon and Ghana populations, and compared with T. manatus and Dugong dugon as out-groups. Our preliminary results on the three West African populations suggest that the Chadian population is distinct from the manatees in Ghana, but not those in Cameroon. It is not known whether, and if so, how long the Chadian manatees have been isolated from the coastal populations. Our results to-date indicates that either there has not been sufficient time for genetic divergence among the Chadian-Cameroon populations or that indeed exchange is still occurring. Clearly there are manatee populations inhabiting the entire river system from coastal Cameroon into Chad. These populations, in recent history, have been locally isolated by a series of dams and local hunting pressure. It is possible that these isolations are periodic with occasional exchanges during the rainy season.

While its present range appears to correspond with historical data, the population sizes and relative status of the various disjunct populations have not been sufficiently examined to determine the degree to which this species is threatened.⁴ Manatees, though protected in Chad, continue to be hunted for meat and oil, and incidentally captured in fishing nets. Uncontrolled pesticide use along the lakeshores is common and may be compromising water quality.⁸ Because these populations are essentially landlocked and because of continued hunting pressure, by local inhabitants, and mortality due to strandings during drought years, they are likely to be highly threatened. The lakes should receive immediate protection and more effective enforcement of current laws is necessary. Manatee conservation initiatives aimed at working with the local communities and ethnic groups that emphasize community participation should be implemented as soon as possible.

Acknowledgements

This research was made possible through funding from Save the Manatee Club, a National Institute of Health Summer Research Training Grant, Wildlife Conservation Society, and Tufts University School of Veterinary Medicine's International Programs and Wildlife Clinic. The authors wish to thank the people of Chad, and everyone else who contributed to the realization of this work.