Diet of the Nile Tilapia, Oreochromis niloticus, in lake Muhazi (Rwanda)

The diet of Oreochromis niloticus, the mainly exploited species in Lake Muhazi, was studied from December 1989 to September 1990. The diet was investigated in terms of sites, seasons and size of the specimens caught. O. niloticus feeds on phytoplankton, and Cyanobacteria make up an important fraction in the stomach contents analyzed. In the 118 stomach contents, belonging to fishes larger than 90 mm, the index of occurrence reaches 100 % for some Cyanobacteria (Merismopedia tenuissima). For this dominant group, the mean abundance index is 64,3 %, the mean volumetric index is 83 % and the dominance index of Lauzanne is 58,1. Two species, Microcystis aeruginosa and Ceratium hirundinella represent more than 90 % of the biomass in the stomach contents analyzed from the two sites and in all seasons. For the period of study, we established that O. niloticus feeds only during the day (from 6 h to 21 h 30) and has a high daily ration (26,5 g.d(-1)). This may explain the good growth of this species in an environment where phytoplankton is abundant. However, the population of O. niloticus of Lake Muhazi has a relatively small biomass and feeds on a small fraction of the phytoplankton primary production (6 %).