Grass Carp for Controlling Aquatic Weeds

One of the most common problems fisheries consultants have to deal with in the summer is unwanted aquatic vegetation. In dealing with private pond owners for over 30 years, I have heard some interesting descriptions of these plants. Pond moss, slime, seaweed, green gunk, and occasionally someone will call and say their pond has an “allergy”(algae). Regardless of the name, unwanted aquatic plants present a major problem to the lake owner.

Aquatic plants fulfill many natural functions in ponds and lakes such as providing cover and food for aquatic and semi-aquatic animals. However, the plants become undesirable when they interfere with human needs such as fishing, swimming, boating, and irrigation.

Unwanted aquatic plants are usually controlled by spraying aquatic herbicides or biologically by introducing a plant-eating animal. Chemical control has its limitations. Chemicals are usually very expensive, require some expertise to apply, and they do not provide prolonged weed control. Even after the vegetation has been treated, the conditions conducive to aquatic weed growth may still exist and reoccurrence of another weed problem is likely. Also, the pond owner’s worst nightmare is the possibility of killing too much vegetation too quickly resulting in a major fish kill due to low oxygen from the decaying vegetation.

Biological control requires less work, is less expensive and provides long-term results.

The most common and effective biological control for aquatic weeds is the grass carp. The grass carp, also known as the white amur (Ctenopharyngodon idella) was first introduced into the United States from Taiwan in 1963 by Auburn University and the U.S. Fish and Wildlife Service for aquatic plant control research.

The fish is native to the rivers of eastern Russia and China that flow into the Pacific Ocean. It has been introduced into more than 50 countries throughout the world for weed control and for culture as a food fish.

The grass carp is a member of the Cyprinid Family, which includes goldfish, common carp, and many of our native minnow species. As Cyprinids, grass carp have no jaw teeth. Instead, they have highly specialized pharyngeal teeth and a horny pad in the roof of their throat, which together act as effective grinders allowing digestion of 60 to 70 per cent of the nutrients of the vegetation eaten.

Grass Carp have a very short gut, which allows them to process and eliminate plant material quickly. They are primarily “grazers” and tend to feed in shallow water and on the surface. At times they can be seen feeding with their back and tail extending above the surface.

Grass Carp prefer soft, low fiber, aquatic weed and underwater plants. In general, small grass carp select small, soft plants like duckweeds, filamentous algae, or softer pondweeds. As the fish grow in size, filamentous algae becomes less preferred; duckweed and pondweeds are still preferred, but fibrous plants are more readily eaten. If the more desired species of plants are not available, the fish will feed on terrestrial vegetation hanging over the water. In fact the name “grass carp” comes from its unique ability to consume terrestrial grasses.

The amount of vegetation they will consume depends upon several environmental factors such as water temperature, water chemistry, and the kinds of plants available. Consumption rates also vary with fish size. For example, until they weigh about 6 pounds, grass carp may eat 100 per cent of their body weight per day in vegetation. Larger grass carp consume less in relative terms but do consume larger absolute quantities. Fish weighing up to about 13 pounds will eat 75 per cent of their body weight each day and above 13 pounds they slow down to about 25 per cent of their body weight each day.

I very often hear pond owners say “Well, I think I’ll just try putting 3 grass carp in my pond and see how they do. I can always add more.” My response is, “Yes, and you can put 3 cows on 20 acres of waist high Johnson grass. They will graze all day and all night but you can’t tell where they have been.” My point is that you have to stock the fish at a high enough density so that their consumption exceeds the growth rate of the plants.

The number of grass carp required to control weed problems vary depending on the degree of weed infestation, kind of weed, size of the lake, depth of the lake, and region of the country. A number of different approaches have been used to determine the appropriate number of carp to stock. It is recommended that pond owners contact a fisheries professional and have them determine the type of plants present, the degree of infestation, and recommend a stocking rate.

It takes time for grass carp to bring a weed problem under control. Weeds may or may not be controlled at the end of the first growing season. The general rule of thumb for ponds is to stock enough grass carp to control the weeds in one or two seasons, but not so many that they completely eliminate all vegetation in a short period of time.

In most situations where weeds have already become a problem, 5 to 10 grass carp per surface acre will achieve weed control. In severely weed-choked cases, higher rates of 15 to 20 grass carp per acre may be necessary to attain control. In some cases, it is sometimes more effective to treat the pond with a herbicide first, and then stock moderate numbers of grass carp for weed maintenance. In ponds with largemouth bass present, grass carp for stocking should be 9 to 12 inches long to avoid being eaten by the bass. Approximately 5 to 6 years after initially stocking grass carp, restocking may be necessary to maintain control since some will die of natural causes or be lost to bird or predation by otters. Also, carp are less effective when they are older.

The older carp that remain in the lake can reach weights of 10 to 20 lbs and will consume fish feed if an automatic feeder is being used to feed bluegill. I have seen many methods used to remove these larger fish such as bow and arrow, 22 rifle or shotgun. However one of the best ways is by angling.

Russell Thornberry, editor of BUCKMASTER MAGAZINE, likes to catch large grass carp on a fly rod. Like any good fly fisherman Russell tries to “match the hatch.” To create a fly that looks like a fish pellet, he takes a piece of cork from a wine bottle and trims it to the size of a fish food pellet. He then burns a hole through the cork with a hot needle and forces a small hook through the cork. He also ties a fly the size of a pellet out of spun deer hair. I have also seen pond owners take a pellet and use a drop of superglue to attach a pellet to the hook.

Throw out a few malt cups of feed and wait for the amur to surface. As they swim at the surface and begin to suck in the feed, toss your pellet fly a couple of feet in front of the amur and hang on. Don’t try to horse them since these are strong swimmers. Take your time, enjoy the fight and wear them down.

Once you land the fish don’t discard them. Grass carp are excellent table fare. They have firm white-textured flesh and are delicious to eat. Dr. Tom Lovell, Professor Emeritus at Auburn University’s Department of Fisheries conducted a number of taste test comparing grass carp to largemouth bass, catfish and rainbow trout. All fish were prepared in the same manner and presented to a panel of evaluators. “In almost every test grass carp ranked in the top two. This was a big surprise to a lot of people, according to Lovell. Don’t throw those grass carp away; they may be the best fish you never ate.