WILDLIFE MANAGEMENT- GRASS CARP IN WEED CONTROL
Using fish to control water weeds
T Pike
Weeds in dams often cause major problems to angling, boating, swimming or extraction of water for irrigation purposes.  
 
Although these weeds are regarded as major problems, they do serve a valuable purpose in dams, mainly by recycling nutrients, providing food or cover for a variety of different species of fish and by producing oxygen. Excess weeds can be controlled by various methods such as dragging out plants or with certain weed killers. Mechanical removal is a temporary measure as plants soon grow again, and most weed killers are ineffective on submerged plants. Weed killers can also be poisonous to farm animals or aquatic organisms, and are therefore not very satisfactory control measures.Weedeating fish such as the Chinese grass carp can be used to control waterweeds without detrimental effects, if the fish are used correctly. Redbreast tilapia are also weedeaters, but have not proved successful. They do not survive low temperatures, and are small and initially have little impact on the weeds, and breed prolifically in warm water to over-populate and completely denude such dams.

Chinese grass carp (Ctenopharyngodon idellus)
Grass carp are indigenous to south east Asia, living in large slow flowing rivers where special conditions occur to stimulate the fish to spawn and enable their eggs to hatch. As they are not known to spawn under natural conditions in KwaZulu-Natal, these fish have to be bred artificially.

Grass carp will eat most of the common soft-leafed waterweeds and new shoots of bulrushes, but they do not control the growth of some plants such as water hyacinth, Kariba weed or parrot’s feather. Large quantities of plant material are consumed while numerous portions of plants are broken off, but not eaten. The grass carp’s appetite increases as water temperatures rise and in very warm areas large fish can eat as much as their own body mass in plants per day.

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. It has been calculated that 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 angling but natural food production in the water is greatly reduced when sunlight penetration is reduced.

Grass carp are capable of changing their diet from plants to insects, so the fish do not necessarily die after the plants are removed.

Grass carp are extremely difficult to capture, therefore it is important not to overstock. Only sufficient fish should be introduced so as to maintain a balance between weeds and grass carp. Dams, which are heavily overgrown with weeds, obviously require more grass carp than dams with partial weed growth. The stocking density of grass carp per hectare should therefore be determined in relation to the extent of the weed growth and not in relation to the entire surface area of the dam. If recommended stocking rates are used, a reduction in the amount of weed growth only become noticeable after about twelve months or more, with large scale removal occurring at a later stage.

Grass carp can survive at water temperatures ranging from 0°C to 38°C, so they are suitable for stocking dams anywhere in KwaZulu-Natal. These fish can be used in combination with other fish species, but as bass or trout are capable of eating small grass carp, it is important to introduce grass carp of not less than 150mm in length to dams, which contain bass or trout. Such grass carp are too big to be preyed on. Grass carp have been know to live to ages of 50 years in the Far East, with maximum masses of 45kg being recorded.

Legal requirements
Recently, concerns have been raised that the possibility exists that grass carp could breed under certain conditions in parts of South Africa and impact negatively on natural water bodies. This has led to an agreement between all the provincial conservation authorities, that only triploid grass carp, which are sterile, can be introduced in future. Anyone wanting to obtain grass carp for introduction purposes should contact The KwaZulu-Natal Nature Conservation Service for information. Please note that a permit is required from the KwaZulu-Natal Nature Conservation Service to move the fish from one property to another. This is in the public’s interest. Control measures ensure that the correct stocking rates for grass carp are applied and that undesirable species are not stocked in dams. Benefits to productive sport angling and nature conservation will result.

KEY POINTS
Grass carp are suitable for dams throughout KwaZulu-Natal.
Overstocking leads to total weed removal and muddy water.
Some waterweed growth is necessary for a healthy dam.
Grass carp do not breed in dams
Redbreast tilapias have not been successful in controlling weeds.