LAKE ACIDITY, FISH PREDATION, AND THE DISTRIBUTION AND ABUNDANCE OF SOME LITTORAL INSECTS

Densities of Corixidae (Hemiptera), larval Odonata, and large larval Trichoptera were estimated in the littoral zone of small lakes in an acid-stressed area near Sudbury, Ontario. Fish were present in some lakes and absent in others, and fishless lakes occurred across a wide range of pH. Corixidae were significantly more abundant in lakes without fish than in lakes with fish, and their numbers were not related to the pH of fishless lakes. Anisoptera (Odonata) larvae tended to be more numerous in benthic samples from fishless lakes than from lakes with fish, and their exuviae were significantly more abundant around fishless lakes. In most lakes, the assemblage was dominated by three species; Leucorrhinia glacialis, Libellula julia, and Cordulia shurtleffi. In lakes containing white sucker, Catostomus commersoni, Gomphus spp. were most numerous. In the most acid fishless lakes, L. julia was uncommon, and L. glacialis was extremely abundant. In fishless lakes, numbers of Anisoptera larvae and exuviae were negatively correlated with pH, though species richness was positively correlated with pH. Exuviae of Zygoptera (Odonata) were more abundant around fishless lakes, irrespective of pH. Larvae of Limnephilus (Trichoptera) were most abundant in non-acid fishless lakes, and absent at pH<5.2. Abundances of Banksiola (Trichoptera) were negatively correlated with the pH of fishless lakes.