Many protected areas in Africa and elsewhere suffer from several external pressures making it difficult for management to set priorities. For example, aerial censuses show that many mammal populations in the Katavi-Rukwa ecosystem of western Tanzania are declining. Vehicle transect data collected from 1995 to 2011 and presented here confirm many of these declines within Katavi National Park itself. Two factors are believed to be adversely affecting large mammals in this national park: (i) reduced water flow caused by local damming of the Katuma River, for rice cultivation upstream of the park, and (ii) poaching. We used transect data, anti-poaching records and timing of dam emplacements to examine the importance of these factors for 23 species of mammalian herbivores and carnivores as well as two combinatorial measures of mammal species’ abundances. Controlling for rainfall, we found that the number of mammals counted per year was associated with both poaching and dams, the importance of which depended on the species. Both factors appear to be adversely affecting mammal populations in Katavi National Park and we make recommendations to both management and policy makers for tackling these problems. More generally, our study shows that wildlife managers of protected areas in the developing world can readily collect information on wildlife trends and basic ecology and can use them for conservation planning.