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Zusammenfassung:
The quantitative relationship between drift and upstream migration to population size and dynamics in G. fossarum , was investigated in sections of 2 mountain brooks, the Rohrwiesenbach and the Breitenbach in Schlitzerland. In both brooks, upstream migration occurs at a time of high population densities in July and August. Changes in the environment of both brooks are of minor importance for the stimulation of migratory behaviour but drift curves are related to environmental factors. Population densities are thus subject to marked fluctuations as environmental factors can induce higher and continual drift, which is counterbalanced in the summer by upstream migration, but not in winter. Thus G. fossarum can hold brooks in higher positions only with the help of upstream migrations, to increase populations which are thinned out heavily throughout the winter.
