Generally its pom pom weed, verbena and black wattle that keep Klipkop busy in summer. But an ever increasing mass of water grass encouraged plot 30 residents to get to the (rather cold) bottom of the problem.

For some time, we had noticed dwindling levels of water birds and aquatic life in our dams. A visiting lecturer pointed out what he felt to be the problem - the weed which was progressively taking over the water body, and causing large tracts of water to stagnate. The weed he was referring to is an aquatic invasive species commonly known as 'grass'. His suggestion was simply to get in there and pull it out, or purchase some grass carp for a longer term solution to the problem.

Our knowledge of land-based invasive species is well developed, through far too much practice, but we know little about the removal of their water-based relatives.

A little bit of research tells us that mechanical removal will work as a temporary measure but the plants will grow again. Poisons are available, but they will kill everything in the dam, and can work their way up the food chain. Our policy on Klipkop is to use poison as a last resort, and only where their use is authorised (e.g. Brush-off by DuPont). And yes, there are weed-eating fish, such as the Chinese grass carp, which can be used to control waterweeds without detrimental effects, provided they are used correctly.

It wouldn't be the first time in environmental management history that a species introduced to solve one problem has created another. With no natural predators, a grass carp population can potentially:

1. breed
2. move beyond the water body in which they are introduced, and
3. become a pest animal that interfers with natural ecosystem and indigenous species.

Luckily, we are not the first to have the problem. There are specialists in the field who breed triploid (sterile) grass carp and provide consultation services on their application to address aquatic weed problems. The De Rust Grass Carp Hatchery in Bonnievale, Western Cape, is one such organisation. The carp they provide come with a sterilisation certificate, and the company is registered with the relevant provincial authority.

Why all the fuss, you might ask? Can't you just throw a few in?

• The sterilisation process has an extremely high rate of success, but its not 100% guaranteed.
• If too many grass carp are stocked in a dam, all the weed can eventually removed by these fish as they increase in size. This has a detrimental effect, as at least some weed is necessary. 20% weed cover in a dam provides for balanced natural biological activity.
• When the carp remove all of the weeds, the water becomes muddy as the fish disturb the bottom while searching for food. Muddy water is not only detrimental to natural food production, which is greatly reduced when sunlight penetration is reduced.
• Grass carp are capable of changing their diet from plants to insects. They don't turn into Piranhas but they do not necessarily die after the plants are removed.
• They are notoriously difficult to catch once released.

In other words, there is some science involved. If you don't know what you are doing, you can end up with a bigger problem than what you started with.

This year, we adopted a mechanical control method on our 'test site'. It looks absolutely brilliant, and didn't take long for life to start returning to the dam.

Next year, we'll invest in some hungry fish! Unfortunately, by the time the experts had come out to assess the situation, it was getting a little cold for a successful release (for the FISH, not US).

We'd like to thank the international directorate of the Wildlife Land Trust, Humane Society International, for their support in helping us thoroughly investigate the problem. Klipkop was accepted as a member of the Wildlife Land Trust in July 2009. The Wildlife Land Trust is an international network of sanctuaries which seeks to 'Save Lives by Saving Land'.

For more information:

Download Econ@UJ, Final Dam Report prepared for Klipkop; December 2010 (400 Kb, PDF)