There is much interest in the potential of the Grass Carp to control, or even in some instances completely eradicate, aquatic plant pest populations. Interested parties include Nature Conservation, local government councils, conservation groups and, of course, the sport angler.

Without any further ado, the author gratefully extends credit and thanks to the following for their informative publications:

-Aquaculturalist Craig Jamieson of New Zealand,
-the Mississippi State University, and
-the Washington State Department of Ecology.

The author with two of the Grass Carp caught on a trout farm near Dullstroom All the Grass Carp caught were caught on plain white mielie mallow floaties

This all started in January 2004 when I was invited to a trout farm in the Dullstroom area in order to determine means of catching Grass Carp without resorting to netting. The objective was to remove Grass Carp without depleting the Trout population. This particular farm had introduced large numbers of what I presume is Triploid Grass Carp into the seven pounds. This has resulted in some ponds becoming devoid of vegetation and one pond with approximately 250 Grass Carp becoming turbid (muddy).

A total of eight Grass Carp were caught in quick succession with masses ranging from 4.5kg up to 6.5kg. (The water was clear). The technique used was a short bottom weight with No. 2 Stevenson hooks, Stevenson’s Plain White Mielie Mallow Floaties on the top hook, a TCP bread on the bottom hook and plain dry feed on the weight (25gram). No dips were used! Centre line of 0.35mm and hook lines 0.20mm. They all took on the top hook.

The author managed to bag 3 carp (lost two) and the five fly fisherman were introduced to bank angling. They now know that bank angling is as much a science as fly fishing!

During the stay at the farm, it was realised that:

a) Grass Carp need to be managed in order to prevent any environmental damage.
b) No stocking of river systems should be allowed. All Grass Carp caught must be killed.
c) Only dams/ponds that have a restricted inlet/outlet should be stocked at a pre-determined level.
d) Grass Carp must be culled to maintain the correct balance between vegetation and carp level.

Washington State Stocking Regulations: In some situations, sterile Grass Carp may be permitted for introduction into Washington waters.

Permits are most readily obtained if the lake or pond is privately owned, has no inlet or outlet, and is fairly small. The objective of using Grass Carp to control aquatic plant growth is to end up with a lake that has about 20 to 40 percent plant cover, not a lake devoid of plants. In practice, Grass Carp often fail to control the plants or all the submersed plants are eliminated from the water body.

The Washington Department of Fish and Wildlife determines the appropriate stocking rate for each water body when they issue the Grass Carp stocking permit. Stocking rates for Washington lakes generally range from 9 to 25 eight- to eleven-inch fish per vegetated acre. This number will depend on the amount and type of plants in the lake as well as spring and summer water temperatures. To prevent stocking Grass Carp from migrating out of the lake and into streams and rivers, all inlets and outlets to the pond or lake must be screened. For this reason, residents on water bodies that support a salmon or steelhead run are rarely allowed to stock Grass Carp into these systems.

The debate about introducing Grass Carp typically divides the issue between district council and drainage engineers who are very keen to use the fish, and many conservation groups who are strongly opposed to it.

Arguments produced for the introduction of Grass Carp (largely to either heavily modified or artificial waterways) are that current methods of clearing these waterways (usually consisting of at least annual mechanical excavations topped op with herbicide applications) are dangerous, expensive and environmentally inadequate.

Arguments against the introductions are the possibility of them breeding in the wild (very unlikely), escaping/being transferred into other areas (more likely) and their unknown impact on native plants, water fowl, sport fishing and freshwater fish communities. (If they are triploid (sterile), then the breeding is unlikely; if they are diploid (non-sterile), then very likely).

It is also hard to do much more damage to the system than is already regularly done with the devastation of mechanical clearing and the possibly cumulative, persistent and more insidious effects of sprays. Another example of abuse is the Vaal River system that is used as a reticulation system for the water that we use.

However, whatever the method chosen to control aquatic plant pest infestations, it must be selected on a case-by-case basis (what is suitable for one water body does not mean this method is the best for another) and should ideally be targeted at the problem, not the symptoms. The use of Grass Carp, drain clearing and herbicides all target the symptoms (rampant choking by plant pest growth) when targeting the problem instead (why do we have these excessive weed growths?) is a more logical, and frequently more ecologically sensitive and sustainable approach. Illustrating this point, the problem with most of these aquatic weed infested areas is that they have very high nutrient levels, and also very high levels of incident sunlight causing high water temperatures and encouraging rampant plant growth. Whilst lowering nutrient levels to such a degree that plant growth becomes nutrient limited is largely a catchment initiative that is frequently not viable, simple strategies such as riparian planting to shade such rampant growths may be a simple yet highly effective option as most of the plant pest species concerned are shade intolerant.

Further repercussions such as a better habitat for in-stream aquatic life, such as native fishes and the invertebrates they feed on, and even enhanced terrestrial life, e.g. bird life, etc., from both fallen and living woody material are likely to be added benefits. Only where the identified causes of the problem cannot be removed in a viable way should symptomatic solutions be considered as a last resort. One of the dangers of highly effective solutions to any problem is that the problem often does not have to be solved. It can be continually masked by the removal of the symptoms so that all appears well on the surface, when in fact the problem remains or even develops further.

The real concern with the use of Grass Carp is that its use as a symptomatic solution may be given precedence over other more suitable (and problem-oriented) solutions such as riparian planting. If the ready solution that is at hand for the decision makers (who all too frequently already have a lack of sound ecological understanding of the processes concerned) is ignored, it will result in Grass Carp being unduly used. Whilst Grass Carp and such biological controls can make a significant contribution to water management, these are not the final be-all and end-all solutions to these human-induced dilemmas.

Aquatic Weed Control: Using grass Carp, Carp, or white amur, have been used for aquatic vegetation control in Mississippi for the past two decades. These fish, used as a “biological control” for aquatic vegetation, can be effective and cost efficient when stocked at appropriate rates and when the problem-weed species is a plant preferred by carp. Although these fish are voracious herbivores, they exhibit preferences for the softer, low-fibre, high-moisture plants when given a choice. Even when there is no alternative, grass carp will not always provide adequate control of certain plants that they do not prefer to eat. It is important to identify the problem plant(s) properly and to choose to stock Grass Carp only when the problem plants are those that Grass Carp are proven to control.