This exhibit is home to a colony of African penguins, some rescued rockhopper penguins, a Western leopard toad and common platannas. A small selection of freshwater fish, indigenous and alien, to our Western Cape rivers can also be viewed in several pools. A bird hide, overlooking the rockhopper rookery, offers people a place to sit and relax in this peaceful riverine environment.
Rocky shoreline and sandy beach
We begin our journey on a rocky shoreline where the river flows into the sea. A great variety of animals and plants, such as mussels, limpets, anemones, sea urchins and seaweeds, live on and amongst the rocks. The rocky shoreline leads onto a sandy beach. Sand is made up of shells and rocks which have been ground up and deposited by waves. Wave action is the key factor influencing all sandy beaches.
Lower river reaches
Floating plants live in the deep, slow waters of lower river reaches. South Africa has no indigenous floating plants, but a number of alien species such as water hyacinth, Kariba weed and waterfern have invaded our rivers.
Water hyacinth is a particular problem in certain rivers and wetlands around Cape Town and the dense mats these plants form must be cleared regularly to prevent flooding. This is an expensive and ongoing operation and the machines used to clear the plants also destroy the natural shape of the river bed. Not only do these plants cause flooding, they also swallow up precious water and choke habitats for fish and other aquatic animals. They cause further damage by preventing sunlight from filtering through the water surface and provide ideal habitats for mosquitoes and even bilharzia-carrying snails.
How do aliens invade our rivers and wetlands?
- Some species were brought into South Africa for garden ponds and fish tanks, but have made their way into our rivers
- Birds and boats carry pieces of plants from one body of water to another, spreading aliens into new rivers and streams
- Fertilisers as well as large quantities of untreated sewage effluent that run into our rivers promote the rapid growth of alien species. In fact, given enough nutrients, water hyacinth can double its weight in one day!
What can I do to stop the invasion?
- Think carefully before buying plants for your pond or fish tank – find out first if they are invasive aliens and if so, avoid them
- Check your boats, fishing rods and nets before you use them and clean off any plant residue, or seeds
- If you recognise an alien plant in a river, or a wetland, try to remove it
- Plant indigenous species in your own garden – they’re more water-wise than aliens and require less fertilising
Middle river reaches
The middle reaches of a river often have deep pools, gentle currents and warmer waters than the lower or upper reaches. These middle reaches are home to a great habitat diversity and many different kinds of animals can be found here; these tend to grow faster and reach larger sizes than anywhere else along the river. Algae grow well on rocks in the river bed and form the basis of a rich and varied food chain. Insects thrive here as well, grazing on small pieces of plants washed downstream from the upper reaches.
Upper river reaches
A river begins in the mountains. Fast-flowing, cold and clear of silt, only small numbers of plants and animals can survive in these shaded, low-nutrient waters. Alien rainbow trout and brown trout have been introduced into the region over the years and have made for excellent fly-fishing opportunities, but their introduction is in turn responsible for the declining numbers of two of our own indigenous species in mountain streams – the Berg River redfin minnow and Breede River redfin minnow.
Freshwater fishes of the Western Cape
Western Cape rivers were once home to a great number of indigenous fish until alien (exotic) species were introduced, mainly for sport fishing. Unfortunately, the early angling enthusiasts of the Western Cape did not realise the angling potential of the indigenous species. They were familiar with the fish in Europe and North America and thought that they would enrich our rivers if they imported species such as bass, carp and trout. Sadly these aliens are now a major threat to the survival our local species.
Not only do alien fish eat our indigenous fish, but they also compete with them for food and bring disease and parasites.
Eighteen indigenous species of freshwater fish are found in the Western Cape, of which 13 are unique (endemic) to the river systems of this region. All 13 species are threatened with extinction. It is very important that we look after our indigenous fishes and protect them from extinction. Not only do indigenous fishes help to maintain the delicate balance of our rivers, they also add to the valuable biodiversity of the region.
Alien fish, brought into South Africa from other parts of the world, are natural enemies of the local, indigenous fish in our rivers for many reasons. They compete with indigenous fish for food and they eat indigenous fish, their eggs and their young. They destroy the nesting sites of indigenous fish and have parasites and diseases, which are unhealthy for indigenous fish.
The fish that you see in this exhibit are all indigenous fish found only in our Western Cape rivers and nowhere else in the world. Some of these migrate upstream to breed and their survival is threatened by the construction of dams, which obstruct migration routes. Dams also change both the natural flood and temperature cycles in the river, both of which serve as natural cues for breeding and migration.