The results of a field census of Alaotran gentle lemurs Hapalemur griseus alaotrensis carried out in February and March 1999 are reported. The results are compared with a census carried out 5 years earlier in 1994. Both censuses followed the same methods and were carried out by the same team, using direct observation from canoe. Mean group encounter rates for the lemur were assessed in each location, which allowed us to calculate the relative group density in each site. Qualitative information on habitat condition (on the basis of plant diversity, vegetation height and evidence of burning) was gathered at each site. Additional information, mainly on lemur hunting, was acquired through interviews with local people. In most sites group encounter rate was at least 50 per cent lower than 5 years before. Taking into account unusually low water levels because of a drought at Lac Alaotra in 1999, we estimate that these encounter rates reflect a 30 per cent decline in the total population size over the last 5 years. The main cause of this dramatic decrease was humaninduced fire (habitat degradation) coupled with heavy hunting (poaching of lemurs for food). The importance of assessing regularly both population status and threats, and of adjusting conservation actions accordingly is emphasized.
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