On the ecology of the fauna of stones in the current in a South African river supporting a very large Simulium (Diptera) population.
The following is based largely on the author's summary of this account of a study of the fauna of stones in the Vaal River in the north-east of Cape Province, South Africa, where there is a very large population of Simulium chutteri Lewis. Females of S. chutteri feed freely on the blood of cattle and horses and also attack man, and the study was designed to provide background data on which to assess the effects of future control measures against Simuliids. Large numbers of larvae of S. chutteri were found on some, but not all, the biotopes formed by the stones in the current of the river not far below the Vaal Hartz Diversion Weir, where water is diverted to a large irrigation area. The sizes of populations of these larvae and of the other invertebrates occupying the stones were assessed at several sampling stations. An unusual sampling method, involving the collection of the fauna of individual stones, was used. The important variables in the aquatic environment were measured. Stone size, current speed and temperature varied little between the biotopes sampled and could not be shown to account for the differences in fauna from station to station. The largest populations of larvae of S. chutteri were found in a semipermanent biotope and in a biotope where the fluctuation in water level was greater than elsewhere. At these two places, numbers of larvae of Hydropsychids and of other species of Simulium were low. Conversely, permanent biotopes in places where there was little fluctuation in water level had larger populations of Hydropsychids and of other species of Simulium, and few of S. chutteri.
From this, it was concluded that larvae of S. chutteri readily move about in the river and rapidly invade newly covered parts of the river bed, whereas their main predators, the Hydropsychid larvae, and their main competitors, the other Simuliids and the very young Hydropsychids, are less mobile.
There were several reasons for the occurrence of the large population of S. chutteri not far below the weir but not elsewhere. In this part of the river, there were many places where the rate of flow was suitable for Simulium larvae. It seemed likely that microplankton, which would provide food for the larvae, built up in the large body of still water held back by the weir. The flow characteristics of the river were artificial in two respects that evidently favoured S. chutteri. Water was not diverted for irrigation at week-ends, so that there was a sharp weekly change of flow, and consequently of water levels, in the biotope formed by the stones in the current. There was also an artificial and gradual increase in the flow of the river at the end of winter and in spring as water was released from storage reservoirs. The river then spread gradually into previously dry parts of its bed. Fewer pupae of 5. chutteri were present when there were many larvae parasitized by Mermithids, suggesting that the nematodes prevented successful pupation. The principal invertebrate predators of Simulium larvae other than the Hydropsychids, were probably a leech and a Plecopteran nymph. Dense growths of diatoms and algae may have reduced the area of the river bed suitable for Simuliids in the late spring. Hydroptilids of the genus Catoxyethira and Chironomini were found mainly on stones with permanent tufts of algae. Two mayflies and an Ostracod inhabited cavities beneath stones.