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Biological Treatment

What is Biological Treatment?

Biological Treatment is the reduction or removal of the plant using grass carp or other species.

Triploid Grass Carp

Grass carp (Ctenopharyngodon idella) is a species of fish which eats aquatic vegetation and is native to large river systems of eastern Asia (They are sometimes called white amur, because they are found in the Amur River). They were legalized in the sterile (triploid) form in Kentucky in 1986. Sterile fish cannot reproduce which makes them an excellent biological control agent. When the term “grass carp” is used it is referring only to the triploid form of the grass carp, as the non-triploid form is illegal to use in Kentucky.

PROS:

Grass carp are relatively inexpensive and don't require the use of chemicals. They also provide continuous control of vegetation and have little effect on other fish in the pond. Triploid grass carp do not reproduce so they can be easily eliminated from the pond if needed.

CONS:

Grass carp become less effective at plant control as they grow larger, requiring re-stocking from time to time. They also have a tendency to escape ponds through overflows, so barriers must be placed to prevent this. Grass carp prefer some aquatic plant species over others and my not effectively control some species. Over-stocking may require removal of extra carp after the vegetation is controlled.

Grass Carp versus Other Carp

Grass carp are one of five different carp species that can be found in Kentucky and the only species that eat aquatic vegetation. When stocking your pond be sure to get grass carp and not one of the other species; the other species will not work and can cause harm to the other fish in your pond. We don’t recommend moving fish from other ponds to yours.

Aquatic Vegetation Preferred by Grass Carp

Grass carp will feed on many species of aquatic vegetation found in Kentucky and tend to consume less fibrous vegetation first. The table below illustrates the vegetation preferences of grass carp. These ratings are based on tank studies of juvenile grass carp because they are most effective at aquatic vegetation control for the first 5-7 years.

Plant Name	Order of Preference
Hydrilla	1
Chara	2
Submerged Pondweeds	3
Naiads	4
Elodea	5
Watermeal	6
Duckweeds	7
Mosquito Fern	7
Coontail	8
Cattail¹	9
Salvinia	9
Eurasian Watermilfoil	10
Eel Grass	11
Water Hyacinth	12
Bulrush¹	13
Water Lillies	14
¹ Young underwater shoots are preferred

Preferences for different vegetation by grass carp. Adapted from: Masser, M.P. 2002. Using grass carp in aquaculture and private impoundments. Southern Regional Aquaculture Center, SRAC Publication No. 3600.

Grass carp will consume vegetation most of the year but are more active during warmer months when the pond temperature is between 70⁰ F and 86⁰ F (21⁰ C and 30⁰ C). During this time the fish can grow quickly. Grass carp prefer different types of plants over others. The first step is to determine what plant you have. Please refer to the Plant Identification portion of this website to identify the plant and see what control measures will work best on your plant. Then use the chart below to see how effective grass carp will be on the aquatic vegetation you have. It should be noted that there are some species of plants that the grass carp will prefer to eat, but they do not control successfully. The best option, on these plants, is to use either a mechanical or chemical control method.

Biological Control quick reference
Plants respond differently to various control measures. The first step is determining which plant you have. Please refer to the Plant Identification portion of this website to identify the plant and see whether biological control measures will work on it, Use the chart below for a quick guide. Each plant type is rated on its effectiveness (E = Excellent, G = Good, F = Fair, P = Poor, Blank = Ineffective).

	Biological Control
	Grass Carp Preference	Grass Carp Success
Algaes
Chara	E	E
Filamentous Algae	F	P
Planktonic Algae
Non-Rooted Floating Vegetation
Bladderwort	E	E
Duckweed	E	P
Mosquito Fern	E	P
Water Hyacinth
Watermeal
Rooted Floating Vegetation
American Lotus
Spatterdock
Watershield
White Lilly
Submerged Vegetation
Coontail	E	E
Elodea	E	E
Floating Pondweeds	E	E
Hydrilla	E	E
Mud Plantain	E	E
Naiad	E	E
Submerged Pondweeds	E	E
Vallisneria	F	F
Watermilfoil	G	G
Emergent Vegetation
Alligator Weed
Arrowhead	F	P
Bulrush
Cattail
Common Bulrush
Creeping Water Primrose
Giant Reed
Horsetail
Lizard’s Tail
Sedges
Smartweeds
Spikerush	F	P
Waterwillow
Willow

Stocking Rates

The goal of stocking grass carp is to control the aquatic vegetation, not eliminate it (please see Ecological benefits of aquatic vegetation). You should consider stocking grass carp when the aquatic vegetation is covering 20-40% of the pond; they should not be stocked in a new pond.

When stocking grass carp, it is best to begin your stocking program with lower numbers within the stocking range. It is easier to stock more fish than to remove them. Do not expect to see results overnight as it may take a year or two to control the problem; don’t consider re-stocking for least three years. If they have not controlled the problem by then, add additional fish to the pond but still stay within the range of the percentage of plant coverage in the pond.

Grass carp should be between 10-12 inches long (or larger) at stocking in order to reduce predation by largemouth bass. You may get quicker results if you stock fish late in the fall or early in the spring. Use the chart below for suggested stocking rates.

Percentage of plant coverage in lake	Number of carp per acre of water
10-20*	0
20-40	2-5
40-60	5-10
over 60	10-20
* At this vegetation density, use mechanical or chemical control methods

(adapted from the Missouri Department of Conservation Aquaguide)

Spillway Barriers

Before stocking grass carp it is important to install spillway barriers because these fish move downstream with flowing water and will escape the pond through overflow structures. Escaping fish can cause problems downstream, or outside your pond.
Barriers made of welded wire, chain-link, re-bar or other long lasting rust-resistant material can be placed over the drain pipe or spillway to prevent loss of these fish. Care should be taken to keep these barriers free of debris. In order to reduce the chances of the barrier clogging up with sticks and leaves use horizontal bars spaced 2 inches apart. Because grass carp are very adept at jumping, spillway barriers should be constructed at least a foot above the highest anticipated water level. If you have a large dam, a large lake or a high water flow-through rate, a barrier may not be practical as high water may damage your dam if the barrier is clogged. Stocking grass carp in these situations should be limited and other methods should be considered first.

Removal of Triploid Grass Carp

A variety of things may happen if the grass carp remove all of the aquatic vegetation in your pond. These long lived fish (in some cases up to 23 years) will continue to forage on overhanging terrestrial plants around the pond bank. They could also begin to feed upon the detritus (organic matter found in the pond bottom) in the pond which could change the water color. When the higher plants (like pondweeds and naiads) are eliminated they will be replaced with lower plants (like phytoplankton and algae). This can change the water color. Elimination of the weeds will open up predation by the largemouth bass on the sunfish population possibly eliminating or greatly reducing the quality of this fishery. At any of these points the pond owner may want to remove some of the grass carp.

There are many different methods one can used to remove the fish. Possibly the best method is bow-fishing. Hook and line fishing is also an option, but can be difficult (some tips for hook and line fishing: use invertebrates, worms, cherry tomatoes, mulberries, and corn bread dough balls for bait; try chumming the area by tossing corn 10-15 feet off shore in shallow areas; use 6-8 pound test monofilament line, size 1 hook with split shot attached 12-18 inches above and loosen the drag).
When you catch grass carp out of the pond, keep this in mind: Grass carp are considered excellent to eat as their flesh is white and firm but may be somewhat difficult to fillet due to the presence of “Y” bones.

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