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Sustainable use and management of resources require regular monitoring. In the case of animals, their numbers and distributions are important in determining future trends. The data can also be used for planning and management needs such as habitat use and design of park boundaries ( Norton-Griffiths, 1978).

Aerial surveys in East Africa have collected data on animal numbers and other environmental parameters coinciding with wet, intermediate and dry seasons and hence determining animal movements in relation to their food supply ( Western, 1975; Norton-Griffiths, 1978; Sinclair & Norton-Griffiths, 1979). Population trends of animals and their food supply has a bearing on their sustained management in an ecosystem as noted by Kalikawe & Kgathi (1993), Lamprey (1984), Kahurananga (1981), Western (1973) among others. The Department of Resource Surveys and Remote Sensing has been conducting aerial surveys in Ewaso Nyiro basin since 1977 and the cardinal objective of this paper is to present and explain an analysis of animal population trends in this basin spanning a period of 20 years, which is invaluable to the sustainable management of the basin’s biodiversity.