The effects of fish and pH on the distribution and abundance of backswimmers (Hemiptera: Notonectidae)

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Abstract
Backswimmer populations were sampled across a range of pH in lakes with and without fish. Irrespective of pH, backswimmers were abundant in lakes without fish, and usually absent or scarce in lakes with fish. The observed distribution of Notonecta spp. and Buenoa spp. among lakes does not appear to reflect competitive interactions among species, but is best explained by the presence or absence of fish predators and, in the case of Notonecta borealis, the poor dispersal ability of the species. To separate the effects of fish loss from other pH-related effects, the relationship between pH and backswimmer population densities was examined within the group of fishless lakes. Differences in population densities of Buenoa spp. among fishless lakes were found to be related to pH. However, the form of the relationship was best described by a curvilinear function that was not suggestive of a toxicological relationship.