ABSTRACTPOPULATION ESTIMATES, a computer program designed to analyse fish populations in fluctuating streams, is described. Its special feature is to calculate population density (number and g m−2) as a dynamic function of variable stream area. Other output includes estimated population size, mean length, mean weight and mean condition factor. Simulated flow changes of 12.5–400% changed stream surface area 6–37% for straight channels, 11–80% for meandering channels and 18–182% for braided channels. Fish population density, however, may not change inversely proportional to stream size. The main use of POPULATION ESTIMATES is to evaluate the effects of fluctuating flows on fish po