Plants are important components of the aquatic environment. They form the basis of the food chain, directly or indirectly feeding all of the animals in the system. Plants provide breeding and nesting sites and cover for fishes, waterfowl, and mammals. Plants also supply oxygen through photosynthesis, stabilize the pond or lake bottom, and prevent shoreline erosion. Most aquatic habitats that contain no vegetation are incapable of sustaining an animal population. Consequently, some vegetation is desirable in most streams, lakes, and ponds. Excessive plant growth, however, can cause problems, making a weed management program necessary.
The amount of nutrients in water and sediment and the availability of sunlight for photosynthesis regulate the growth of vegetation in any aquatic habitat. High input of nutrients, particularly phosphorus and nitrogen, combined with large areas of clear, shallow water inevitably lead to excessive growth of algae or other aquatic plants. Once weedy growths become established, they can invade areas of deeper water and may entirely overrun a pond or shallow lake.
Algae and other aquatic weeds:
- clog water intakes and distribution systems used for irrigation, public water supplies, and hydroelectric generating plants;
- increase sedimentation in flood control reservoirs;
- harbor the vectors of human and animal diseases;
- interfere with aquaculture;
- impede boating access and navigation;
- make recreational activities such as swimming, boating, skiing, and sport fishing difficult and dangerous, if not entirely impossible.
Heavy weed infestations, especially of highly invasive, exotic species such as hydrilla (Hydrilla verticillata), Brazilian elodea (Egeria densa), and Eurasian watermilfoil (Myriophyllum spicatum), cause severe habitat reduction, stunt sport fish growth, and reduce the total number of harvestable fish. Several drownings also have been attributed to swimmers becoming entangled in dense growths of aquatic weeds. These examples show the need for vegetation management.
Physical, mechanical, chemical, and biological techniques are available for management of aquatic vegetation. Habitat manipulation such as drawdown, the use of barriers made of plastic or fabric that cover the bottom and prevent plant establishment and growth, and the use of pond dyes to shade out plants are ways to control the environment physically. Mechanical techniques involve the use of either hand labor (for example, raking and seining) or machinery (such as draglines, backhoes, or specialized aquatic harvesters) to remove vegetation from the water. Herbicides can be applied either over the surface or into the water column to kill unwanted vegetation. With biological control, natural enemies such as insects or fish are released into aquatic areas to feed on the weedy vegetation and maintain its density at an environmentally or economically acceptable level. Each of these techniques has its advantages and disadvantages. Several of the techniques are frequently integrated into a weed management program.
Fish as a Biological Control Agents for Aquatic Weeds
For submersed aquatic weeds, herbivorous (plant-eating) fish are an attractive alternative to herbicides and other temporary methods of weed management. This type of biological control often requires less labor, fewer treatments, and less expense than other methods, and it has good potential for long-term management of aquatic vegetation.
Occasionally, the blue tilapia (Tilapia aurea) has been used to control algae, and the redbelly tilapia (Tilapia zillii) has been used to control aquatic macrophytes (coarser vegetation) in North Carolina. Because tilapia are tropical species and cannot tolerate water temperatures below about 50deg.F, they are unable to survive normal winter temperatures and have limited use for weed control in North Carolina. Common carp (Cyprinus carpio) and several variants of this species (for example, Israeli carp) have also been used for weed control. In most situations, these fishes have not been very effective, as they are omnivorous (eat a variety of foods) rather than strictly herbivorous. In cases where they have been effective, most of the control has resulted from their habit of muddying the water while rooting in the pond bottom for food, rather than from the actual consumption of vegetation. The triploid sterile grass carp (Ctenopharyngodon idella) is the primary fish used for the biological control of aquatic vegetation in the United States.
Biology of the Grass Carp
The grass carp is native to certain large rivers in China and Siberia. It spawns naturally only in relatively long, fast-flowing rivers with fluctuating water levels and is not capable of reproducing in ponds, lakes, or reservoirs. Grass carp are grown throughout Asia and parts of eastern Europe for food and for control of aquatic weeds. This species was first imported into the United States from Malaysia in 1963 by the U.S. Fish and Wildlife Service as a potential biological control agent for hydrilla and other aquatic vegetation. Its culture is restricted in the United States because of potential adverse impacts on native species.
Juvenile grass carp feed on plankton (microscopic plants and animals in the water) but change their diet to feed exclusively on vegetation when they reach a length of about 6 inches. These small fishes may feed effectively on filamentous algae, duckweed, and watermeal, but again their diets changes as they grow larger to feed almost exclusively on submersed aquatic macrophytes. Because they are totally herbivorous (unlike the common carp), they do not compete for food with desirable game fish and commercially valuable species. When water temperatures exceed 68deg.F, grass carp feed almost continuously. Under optimal conditions, they may eat several times their body weight in plant material each day. When food is abundant, grass carp grow rapidly, attain relatively large size (20- to 25- pound fish are normal; weights of over 40 pounds are not uncommon), and may live for 10 years.
The Triploid Sterile Grass Carp
The use of grass carp has been limited in many areas because of concern about possible adverse effects from their establishment in large river systems and associated estuaries. In response to this problem, researchers developed a technique to create sterile offspring from normal parents by altering the genetic material during fertilization. This technique causes the newly fertilized egg to form one extra set of chromosomes (triploid condition), resulting in a fish that is normal in all respects except that it cannot reproduce. Red blood cells of triploid fish contain an extra set of chromosomes in the nucleus and, consequently, are larger than those of normal, fertile fish. A blood sample from each fish is examined before it is shipped from the hatchery to ensure that only sterile, triploid fish are sent to the distributor for sale. In some states, this test may be repeated on a sample of fish upon delivery.
Use of the Triploid Grass Carp in North Carolina
Stocking of grass carp in North Carolina may require a permit. Policy changes instituted in 1991 in North Carolina allow stocking of a maximum of 150 triploid grass carp without a permit. A permit from the Wildlife Resources Commission (WRC) is necessary when more than 150 fish will be stocked in a single body of water. In these instances, a biologist from the WRC must visit the impoundment before issuing a permit to determine whether or not there is sufficient containment to prevent the fish from leaving the targeted body of water. In some cases, it may be necessary to install a containment structure, such as a spillway screen, to prevent the fish from escaping. This both protects your investment and keeps the fish out of areas in which they may cause damage. The WRC biologist determines whether or not a permit should be issued and what stocking rate is needed. Grass carp may be released in those bodies of water for which the permit was originally written and may not be moved to another area without permission of the WRC. A list of licensed distributors or an application for a permit to stock grass carp in North Carolina may be obtained from the Wildlife Resources Commission (telephone 919-733-3633).
Grass carp can control many kinds of aquatic vegetation effectively (Table 1 below). They prefer the softer-textured plants such as hydrilla and other submersed species and will consume these first. Grass carp occasionally feed on the newly-sprouted, tender shoots of some emergent and floating-leaved plants (for example, lotus, water lilies, reeds, rushes, and bulrushes), but rarely cause significant damage because these plants tend to become rather tough and woody soon after sprouting in the spring. Before stocking grass carp, make sure these fish will be effective in controlling the plants you with to eliminate. Your county Extension Service agent or a WRC district biologist can help you to identify the weed species in your pond.
Stocking rates may vary somewhat with the type and density of the vegetation to be controlled, but generally fall into the range of 10 to 15 fish per acre in small ponds and 10 to 20 per vegetated acre in larger impoundments. Large fish (8 to 10 inches long) are stocked to reduce losses from predation by largemouth bass and large wading birds. Grass carp should be stocked well before the end of the growing season to increase their chances of surviving over the winter. By the following spring, the grass carp should be large enough to prevent significant regrowth of many weeds. If first-year growth of the fish is slow, control may not be achieved until late in the second summer or the following spring. In cases where weed control is not attained, the cause usually is loss of fish (by escapes or predation), an inadequate stocking rate, or stocking for a weed that grass carp will not eat.
When a body of water is totally covered with dense vegetation, most of the oxygen produced during the daytime either is lost to the atmosphere or is used by the plants during the night. In these circumstances, there will not be enough oxygen left in the water for the grass carp. Herbicides or mechanical removal should be used before stocking to open small areas where the grass carp can get sufficient oxygen for survival.
Because grass carp fingerlings must be individually checked for triploidy and also must be raised to a fairly large size to prevent predation losses when stocked, they are relatively expensive (about $7 per fish). Even though an initial stocking costs about $100 per acre, the long-term cost is quite low, averaging about $10 per acre of weeds per year over the 10-year life span of the fish. This is a small price to pay in comparison with that of aquatic herbicides, which can cost from $100 to well over $300 per acre for a single treatment. The cost per fish also decreases when larger numbers are purchased.
In North Carolina, stocking of grass carp is not allowed in rivers or in large lakes from which fish are likely to escape. This restriction results partly from concern that the grass carp will not stay in the targeted areas and feed on the weeds but instead may feed on desirable vegetation (such as muskgrass) and eliminate habitat that sustains commercial or sport fisheries.
Grass carp generally either eat all of the edible vegetation available in a pond or are entirely ineffective. No scientific basis has been found for determining precisely how many grass carp are required to provide effective control in a given body of water without total loss of vegetation. However, many desirable plants are not usually consumed by grass carp, minimizing their adverse impact on fisheries and wildlife habitats. Once grass carp have eliminated the edible vegetation, they may begin to starve. In these cases, the fish can be fed grass clippings or commercial fish food, or may be harvested.
Though very little is known about the effects of large-scale grass carp stocking, what we do know suggests that the actual adverse effects may be substantially less than has been predicted. Studies at Lake Baldwin, Florida, and Lake Conroe, Texas, revealed only minimal impacts on sport fisheries, even though essentially all of the vegetation was removed by grass carp stocked for hydrilla control. Current studies in Texas and at Lake Marion, South Carolina, may provide the information needed to evaluate whether using grass carp in large lake systems is an environmentally sound practice.
There has also been some concern that escaped fish migrating downstream might damage desirable vegetation, particularly the seagrass beds in the estuaries, and thereby have an adverse impact on shellfish and marine fisheries. To date, there is little evidence to support this hypothesis. Fertile (diploid) grass carp were released in the Arkansas River and have been recovered from the Mississippi River system and its tributaries. Similarly, fertile grass carp released in Lake Conroe also have been found downstream. No adverse impacts have been seen in these locations.
Advantages and Disadvantages
There are a number distinct advantages in using grass carp for aquatic weed control. A major advantage is that long-term control (over a period of 5 to 10 years) may be attained with a single stocking, whereas treatment with herbicides is often needed annually or more frequently. Avoiding the use of herbicides also prevents the possibility that chemical residues may harm desirable aquatic vegetation, crops, fish, livestock, or humans. Oxygen depletion caused by the decay of vegetation after herbicide treatments and resultant fish kills are also avoided when grass carp are used.
Several disadvantages are associated with the use of grass carp. The fish are expensive to buy, and it may take two or more seasons to attain acceptable control. Grass carp are preferential feeders on submersed vegetation and hence may not be effective on many of the weeds of concern – especially floating, floating-leaved, and emergent species (Table 1). Part of the vegetation eaten is not completely digested. This partially digested material will decompose and release nutrients into the water, occasionally causing algal blooms. The fish also tend either to consume all of the vegetation present or not to control the weeds at all.
For a list of licensed grass carp distributors and additional information on grass carp or other means of aquatic weed management, contact your county Extension Service Center or a WRC district biologist in your area.
Publication date: Nov. 19, 2016
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