No abstract

Abstraet--A method of reducing data on stones-in-current faunal communities to a linear scale of water quality in terms of organic pollution is presented. Using data from extensive river surveys each taxon has been allotted a Quality Value (between 0 and 10) related to its occurrence in variously polluted waters. To arrive at the Biotic Index Value of a community, a faunal sample is taken and the individual animals are recorded by taxa. The number of individuals of each taxon is then multiplied by the taxon's Quality Value. The products of these multiplications are summed for the sample and this sum is then divided by the total number of individuals in the whole sample to give the Biotic Index Value.Quality Values for taxa occurring in large numbers in a wide range of water qualities vary according to the diversity and abundance of the Baetid Ephemeroptera. It is not necessary to identify all animals to the species level and sample size is not critically important. The Biotic Index is compared with indices due to other authors.

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Needham and Usinger's analysis of data on the total numbers of animals from 100 samples, each 1 ft2, from a riffle is shown to be erroneous. Their conclusion that 73 samples were needed to arrive at significant figures on total numbers at a 95% level of confidence is optimistic; 448 samples are required to give a sample mean within 5% of the population mean at a 95% level of confidence.Data from sampling sites where the sampling apparatus is totally submerged are particularly variable.

The aquatic stages of the cattle biting pest, Simulium chutteri Lewis utilize river flow in their dispersal and colonization behaviour. Peaks of drifting activity in S . chutteri larvae occurred in the early morning and late afternoon. It is deduced that female flies scatter eggs onto slow flowing waters upstream of rapids . Larvae hatch from drifting eggs and colonize substrates in slower flowing regions upstream of rapids, while later stage instars move into faster flowing regions within rapids where they complete their development . This microhabitat selection by S. chutteri leads to rapid attainment of large population sizes in suitable sections of river and reduces competition between different stage larvae . In their utilization of a variety of microhabitats the larvae of this species differ from co-existing simuliid species which restrict developmental stages to single habitats .Catastrophic drift was recorded for S . chutteri larvae and could be a mechanism to regulate population size.Drift of simuliid larvae off rapids was not related to benthic population densities in the rapids and was therefore not due to excessive production . It is concluded that larval drift off rapids is related to habitat disturbances associated with water flow fluctuations and the activity of aquatic predators and other animals .The distribution of S. chutteri in the Vaal River is restricted by oviposition requirements of the adult female. Knowledge of drift behaviour and water flow requirements of Simuliidae have been applied to manage the population size of S. chutteri in the Vaal River. Hydrobiologia 133, 143-154 (1986) .