Abstract
Population density was estimated monthly by means of direct counts for 16 consecutive months. Capture‐recapture experiments revealed that 9–22% of the population was observed by this technique. Most nymphs appeared in June, and the presence of nymphs in September suggests a second generation. There was a strong positive correlation between summer surface drift and mean annual population density for five out of six sections studied. In July‐August the sex ratio was near equal, while in September females began to dominate. The bias towards females was strong until new imagines emerged in July. From September a marked weight dimorphism was observed between the sexes. Food limitation, and skewed sex and weight ratios implied intersexual competition in this population. Brown trout possibly affected the population density in one section with less shelter. Wing dimorphism was observed in July and August. Macropterous individuals represented a low percentage (<17%) and were only observed in sections with high densities.
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