The effects of Bayluscid on gastropod snails and other Aquatic Fauna in Rhodesia

Abstract

Extensive use of the molluscicide, Bayluscid (Bayer 73), in natural waters in Rhodesia led to fears that the aquatic fauna as a whole, including the predators of gastropod snails, would be adversely affected. The purpose of this study was to test the effects of one intensive application of Bayluscid on two stream catchments and in one small impoundment. The two streams were chosen because of their widely differing water qualities, one had soft water, bicarbonates 15 to 40 mg/l as CaCO3, calcium 2 to 6 mg/1 as Ca, and the other moderately hard water, bicarbonates 114 to 163 mg/l as CaCO3, calcium 20 to 45 mg/l as Ca. The water in the small impoundment was similar to that of the first stream. The fauna was sampled for some months before and after treatment. Bayluscid was applied by the Rhodesian Department of Health according to their standard procedures, and the resulting concentrations were determined by laboratory analysis of water samples. Biological results were as follows: Immediate effects 1. Gastropod snails were virtually eliminated. 2. All fish were killed. Fish died more rapidly (five to ten minutes) in the soft water than in the harder water (two to six hours). 3. The rest of the invertebrate fauna, especially insects, was not seriously affected. In the soft water stream some were exited and others became moribund but, at least in some cases, these effects were temporary and reversible. There was no reduction in density of invertebrate predators such as crabs and Odonata. Subsequent effects 4. Recolonisation by snails which are hosts to schistosomes, Bulinus (Physopsis) spp. and Biomphalaria pfeifferi, was slow. In the stream with hard water they began to reappear after 10 months after treatment but, in the soft water stream none was found after 22 months after treatment. Other snails, specially smaller species, returned more rapidly. 5. There were local increases in density of insect larvae, including mosquitoes, during the first three months after treatment, but not subsequently.