The ecology of Lake Nakuru (Kenya)

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Abstract
The cichlid fish Tilapia grahami (= Sarotherodon alcalicum grahami) was introduced into Lake Nakuru (Kenya) in about 1960 and is 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 was represented by the equation W = 0.008 .cntdot. l2.98 (W = dry weight = 24% of fresh weight; l = standard length = 85.1% of total length). The fish distribution was 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 concentrated near the shore and at night they moved off-shore, a migration pattern probably reflecting a preference for higher temperatures. Of Tilapia, 70% concentrated in the top 50 cm and 80% in the top 100 cm. The total ichthyomass of the lake had a mean of 90 t [ton] dry weight (= 2.1 g/m2) in 1972 and it increased to a mean of 400 t dry weight (= 10.2 g/m2) during 1973. Possible causes for the spatial distribution and the biomass variations were discussed. The high density of Spirulina platensis made nutritional competition among the herbivores unlikely. The main impact of T. 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 dominant. The breeding of pelicans at a neighboring lake caused considerable nutrient export (.apprx. 13 t P/yr).