The ecotrophic community structure in the open Lake Turkana, Kenya, has changed since the early 1970s when the system appeared limited by zooplankton production and energy was accumulated in stocks of small pelagic species. Later, the slower growing predator stocks of Lates spp. have proliferated and in the late 1980s energy has accumulated at the top predator level. The result is a strong increase (250%) in predation mortality on small pelagic species. This may explain the fivefold decrease in their biomass which is much more than can be expected fom the relative decrease in secondary and primary productivity between the two periods. The regulatory mechanisms in the open lake ecosystem structure seem to have shifted from bottom-up to down-down "control" between 1973 and 1987. Fishing effort should be directed at the Lates spp. and Synodontis stocks and sustainable yields the present conditions could be strongly increased.