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Abstract

Competing hypotheses that try to relate population size, individual weight, and water temperature in running waters were tested. Female and male dry weight and specimen numbers of eleven caddisfly species (Trichoptera) collected from 1983 to 1991 in four emergence traps along the Breitenbach (Germany) were related to site specific water temperatures. Water temperature along the stream showed the classical pattern, almost constant near the source and increasing annual variability downstream. Temperature sum (dd = degree days) increased downstream; sums of the last three, six and nine months had a clear seasonality, twelve months sums were almost constant. Greatest upstream–downstream differences occurred in autumn and winter. Comparison of Trichoptera was based on yearly abundance and weight data. Females were heavier in all species but Potamophylax cingulatus. Among sites, weight differences were not detected for Apatania fimbriata, Silo pallipes, Tinodes rostocki, Plectrocnemia conspersa and for P. cingulatus (♂) and Rhyacophila fasciata (♂) at least in stream sections where they occurred regularly and abundantly. Weight differences were usually significant only between the uppermost or the lowermost, and the remaining sites along the stream. Sericostoma personatum exhibited an almost site by site weight increase downstream. Drusus annulatus (♂,♀ ), R. fasciata (♀ ) and P. cingulatus (♂) were lightest at the downstream site, where only R. fasciata occurred regularly. C. villosa (♂,♀ ), Potamophylax luctuosus (♂), S. personatum (♂) and Agapetus fuscipes (♂) were lightest at the uppermost site. No single hypothesis explained all results. High individual and population success (high weight or abundance) as predicted by the 'Thermal Equilibrium Hypothesis' occurred in D. annulatus, P. cingulatus and P. luctuosus. In contrast, high abundance and low weight occurred in C. villosa and S. personatum. Individual success seems to depend on a specific fit of genetic pre-adaptations, appropriate physiographical and ecological conditions, and competitive ability; it is not necessarily connected with population success.