I often receive calls from pond owners who are faced with an aquatic weed problem, but do not wish to use chemicals to control them. In these cases, the pond owner is left with two options, manual/mechanical removal of the problem vegetation or perhaps a biological control. The process of removing aquatic weeds by hand can be labor intensive and time consuming. In some instances, the weed problem can be so extensive that the removal efforts cannot keep up with the weed growth. More importantly, the pond is there for enjoyment, and pulling weeds is not typically an enjoyable way to spend a morning on your pond. Therefore, if it is a viable option, the pond owners generally turn to the biological option for weed control, grass carp.
Grass carp, or white amur, can be a very effective aquatic weed control addition to a pond. The grass carp is native to the rivers of Russia and China. Being river fish by nature, grass carp require moving water to spawn, and therefore do not spawn in ponds. These fish are voracious consumers of aquatic vegetation when they are young. Some fish have been known to grow up to ten pounds in a single year. The effectiveness of a grass carp for weed control averages five years at which point their appetite tends to decline, and aquatic weeds can once again become a problem. After grass carp are initially stocked, it can take up to a year for them to control the problematic vegetation. Therefore, grass carp should not be viewed as a quick fix for weed problems, but rather as a long term maintenance device. Pond owners who choose to use grass carp for weed control should restock fish when they notice aquatic weeds beginning to return to their ponds. At this point, the grass carp do not necessarily die, and can be caught on hook and line with some luck. Grass carp are tremendous fighters, and are good eating.
Grass carp are not however, the silver bullet for aquatic weeds. They do not eat all types of aquatic vegetation, and proper weed identification is necessary to gauge their potential effectiveness. Further, they swim with flowing water, so ponds that loose water on a regular basis over a spillway should keep this in mind when considering weed control options. Sterile grass carp (triploids) are available, and cannot successfully reproduce if they escape. Pond owners who wish to employ grass carp, but are concerned by their potential escape to natural water bodies (creeks, streams, rivers, etc.) should strongly consider triploid grass carp. This potential for escape makes non-sterile grass carp illegal in some states. Alabama is not currently one of those states.
For stocking rates and procedures, pond owners should contact their county extension office. Ponds that contain bass should only use grass carp that are 8” and bigger to avoid predation by the bass.
For additional questions or information, contact P.J. Waters, Auburn University Marine Extension and Research Center.