Grass Carp and Lake Management - Dispelling the Myths

By David Beasley and Aaron Cushing, Fisheries Biologists

 

As pond and fisheries managers, we are often tasked with manip­ulating submersed vegetation in lakes and ponds. For various reasons, there may be limitations on the use of herbicides as a management tool. As an alternative, triploid grass carp (also know as white amur), are often used to help control unwanted aquatic vegeta­tion due to their ferocious appetite for submersed vas­cular plants and some types of branched algae. These fish are regulated by most states, and a permit is typically required in order to stock them in your lake or pond. In some states however, they are not even allowed.

Although grass carp are commonly stocked across the country, there are many myths surrounding them and how they are used in aquatic vegetation management. Many pond owners who have stocked them in the past will say they didn’t work. Others have a lot of questions concerning the impacts, once stocked: How fast will they reproduce? Will they eat everything? Won’t they get out? The truth is, when stocked properly, all of these concerns can be avoided.

In 1983, US grass carp producers began production of a sterile (triploid) form of grass carp, meaning they cannot reproduce. The fact they are sterilized is a step in the right direction, but other measures need to be considered. Many people have learned through experience, or are told, that grass carp cannot control their submersed vegetation. Unfortunately, in many cases, this is a misunderstanding and human error played a role in the failure. Prior to stocking grass carp, it is very important to have accurate expectations surrounding what control you can achieve. Fish stocking rates, sizes, time of year, the species of vegetation present, how quickly you want results and even possible predators all play an important role in the successful introduction of grass carp. Since their mortality rate averages 20% annually, grass carp need to be restocked over time to maintain a sufficient population.

If the vegetation species is within the diet of the grass carp, then the higher the carp’s population is, the better control you will achieve. However, these carp can consume up to their body weight in vegetation each day while actively growing, and up to 80% of their weight daily once they become adults.

Often people want to eliminate all the unsightly vegetation in their pond as fast as possible and stock too many fish, only to find the carp quickly grow and consume all plants. This will leave the available nutrients that were once tied up in vegetation to fuel algae growth, leading to undesired blue-green algae blooms which produce toxins. Excessive grass carp also lead to the loss of habitat for smaller fish and wildlife and overstocking has many negative effects and should be avoided.

If using grass carp to control unwanted vegetation, you should only stock enough to eat a particular quantity of plants per day so that not all vegetation is wiped out. Please consult with a fisheries biologist prior to stocking grass carp.

David Beasley and Aaron Cushing are Fisheries Biologists with SOLitude Lake Management. SOLitude is a lake, pond and fisheries managment company serving VA, NC, SC, MD, DE, PA, WV, NJ & NY. Aquatic products and fisheries consulting available nationwide. www.solitudelakemanagement.com.