Les rencensements des hippopotames (Hippopotamus amphibius L.)

Hippopotamuses spend most of the day resting in shallow water, sleeping under the surface's level and appearing periodically above the surface in order to breathe. In order to work out an index of density which would allow to evaluate the variations of hippo's densities in Virunga National Park (Zaïre), the author has studied the main factors affecting the counting accuracy. The main results are as follows : - The fraction of time spent above the surface (= R) varies during the day . R is maximum early in the morning, minimum at midday and intermediate in the late afternoon. The reasons of this variation are unknown. - In presence of waves, even small ones, R decreases. - The impact of disturbance on hippopotamuses censuses is great but varies according to the type of resting place and to the time of day. Consequently, hippopotamuses censuses in Virunga National Park must be made between 6h30 and 8h30 a.m., in absence of wind and by using a method which disturbs hippopotamuses the least. Broadly speaking, this study shows the necessity for each manager of National Park of making preliminary study of the factors affecting the accuracy of countings before adopting a new method of census.