The Cichlid fish Tilapia grahami (=Sarotherodon alcalicum grahami) was introduced into Lake Nakuru (Kenya) in about 1960 and is now one of the main herbivores. Spatial distribution and biomass changes were estimated from lift net catches from 1972-1974 which were partly continued until 1976. The length/weight relationship is represented by the equation W= 0.008. l 2"9s ( W= dry weight = 24 % of fresh-weight; l = standard length = 85.1% of total length). The fish distribution is very patchy (aggregation coefficient 5.2-12.2). The density decreased and the mean fish size increased from in-shore to off-shore regions. At noon the fish concentrate near the shore and at night they move off-shore, a migration pattern probably reflecting a preferance for higher temperatures. 70% of Tilapia concentrate in the top 50 cm and 80% in the top 100 cm. The total ichthyomass of the lake had a mean of 90 t dry weight (=2.1 g/m 2) in 1972 and it increased to a mean of 400 t dry weight (= 10.2 g/m 2) during 1973. Possible causes for the spatial distribution and the biomass variations are discussed. The high density of Spirulina platensis makes nutritional competition among the herbivores unlikely. The main impact of Tilapia grahami on the lake's ecosystem is a substantial increase in diversity by extending the food chains to fish eating birds, of which the Great White Pelican is dominating. The breeding of Pelicans at a neighbouring lake causes a considerable nutrient export (~ 13 t phosphorus/year).
1 Both species have now been assigned to another genus, Sarotherodon, and they are now calk'd S. alealicum and S. alcalicum grahami (Boutenger, 19t2) (Trewavas, 1973