In July 1982, the invertebrate drift at Marksdrift comprised 98,7% Simulium chutteri; 0,75% Chironomidae; 0,3% Ephemeroptera; 0,15% Copepoda, and 0,1% Trichoptera. Simuliid eggs were found in only 6 out of 75 samples. A single water-level reduction of 57cm (54%) resulted in a more than sixfold increase of S. chutteri larvae in the drift and a more than 50% decrease of 1st and 2nd instar larvae in the drift after the water had returned to its original level. Larvae found lying in pools after the water-level had dropped belonged mainly to instars 5-7, 70% of them showing symptoms of starvation after 3 days when the river had risen again. The drift of simuliid head capsules decreased when the larval drift increased, as fewer simuliid larvae moulted when they had been disturbed. The low drift of eggs and the presence of very few pupae and adults indicated that most of the S. chutteri population was in the larval stage and that July was therefore an ideal month for water-level manipulation. Its main effect was achieved by irritating larger larvae and thus preventing them from resettling.
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