Aquatic avifauna of the Middle Vaal River
Ornithology should be considered as an essential part of ecological studies. Birds provide an easy way of monitoring the state of the environment, because they are easier to detect, simpler to locate than many other taxonomic groups, and bird taxonomy and distribution are better known than for any other reasonably large group of animals or plants. Bird diversity and density are considered good indicators of overall environmental health, since birds occur in most habitats on land throughout the world, and are sensitive to environmental change. Recreation is an inseparable part of the Vaal River. It will not be advisable to undertake ecological studies without at least considering the effect of recreational activities on the environment. Although still largely unprotected, the Middle Vaal River is a declared Important Bird Area (IBA) on sub-regional level and the Vredefort Dome area was recently promoted to World Heritage status in 2005. Bird species data were gathered at the Middle Vaal River from April 2002 to April 2003. Point surveys were conducted at monthly intervals at fourteen sites. Relative densities and Shannon indices were calculated from the data and certain feeding, nesting and habitat guilds were identified. Based on an existing model used to determine the occurrence of droughts, amendments were made to predict the seasonal occurrence of bird species. During the survey period, 59 water-associated bird species were recorded of which 51 species were recorded during the actual counting times, with an overall total of 5 428 individuals. The major influences on bird numbers and species richness are human activities and some habitat features at sites. According to their relative densities the top fifteen species contributed 90% to the total bird numbers, while the remaining 34 species only contributed 10%. Overall, it was determined that all study sites had a high diversity and evenness of birds as measured by the Shannon index. According to the model used for seasonal occurrence, a pattern of succession of three breeding bird species was observed during the study period at the heronry situated at Limerick. The importance of heronries in sustaining a large part of the aquatic avifaunal population is emphasised by this study. In future the model can be used to make predictions about the bird status at a given time or season. When making management and conservation decisions, it is important to combine as many different factors as possible. In this study not only diversity was used, but also abundance, species richness and certain guilds. It is also recommended to use more than one surveying method, depending on the aim of the survey as some species may largely be overlooked by using only one surveying method. Important points to consider when making management and conservation decisions based on a census survey of aquatic avifauna are discussed. These include taking into account density, diversity, abundance and guilds, as well as the presence of heronries and other breeding colonies. Some conservation and management recommendations are also made specifically for the Middle Vaal River of which the following are the most important: the control of pollution and boating, management of mallards not in captivity that may interbreed with the Yellow-billed Duck, protection of heronries along the river, conservation of landscapes and riparian vegetation surrounding the river. Observations made on the White-backed Night Heron and some other rare species, as well as the high numbers of the Goliath Heron and waterfowl, show that the Middle Vaal River should maintain its status as a Sub-regional Important Bird Area. Ecotourism is an important part of the Vredefort Dome and it can be used to make people more aware of the aesthetic value and beauty of aquatic avifauna.