How much grass could a grass carp eat?

It has a B-movie-monster’s name and a reputation to match. Hydrilla, a fast-growing aquatic weed native to Asia, has been rampant in many North Carolina waterways since its discovery, near Raleigh, in 1983. During the past few decades it has spread steadily westward; by 1999, it had made its way into Lake James, near Marion.

The submerged plant’s furious rate of growth — more than an inch a day — and total lack of competition are the sorts of things that keep water-resource managers up at night. Unchecked, hydrilla can fill lakes and ponds with dense, green curtains that choke out native species and snare boats and swimmers. But in Lake James, at least, hydrilla appears to have met its match in another equally exotic species.

“They’re eating machines,” says Win Taylor, a fisheries biologist with the North Carolina Wildlife Resources Commission. He’s talking about the grass carp. In the summer of 2002, 21,000 of the vegetarian fish were turned loose in Lake James. By that time, hydrilla had claimed 500 acres of the lake; but since the carp’s arrival, there’s been little sign of the territorial weed. In certain circles, the grass carp’s savage eating habits have earned it the nickname “H-Bomb.”

“It seems like that initial stocking has taken care of it,” says Taylor. Given hydrilla’s ability to grow in water up to 20 feet deep, it would eventually have blanketed 1,400 acres of Lake James — nearly a quarter of the lake’s surface — he predicts.

Historically, herbicides have been the weapon of choice against aquatic weeds. But these potent chemicals are expensive, touchy to use and bring their own set of water-quality concerns. Accordingly, biological controls are gaining favor in the fight against weeds, and none is more cost-effective than the lowly grass carp.

Native to eastern Asia, the grass carp was introduced to North America in Arkansas in the early 1960s and has since spread to 35 states. A distant kin to goldfish, grass carp grow to 40 pounds or more and can eat their weight in weeds each day. Indeed, their habit of denuding ponds and lakes of vegetation makes them a nuisance in much of their adopted range.

But if the idea of introducing a troublesome, exotic species to control another troublesome, exotic species sounds somewhat dicey, if not downright dumb (particularly here in the land of kudzu), consider this: The grass carp introduced at Lake James are “triploid” fish, rendered sterile by a pressure treatment while still in the egg. Try as they might, they simply can’t reproduce. Triploid grass carp also tend to die sooner than their fertile counterparts, living only about 10 years. That’s good, because with hydrilla (their preferred food) gone, these insatiable consumers inevitably turn their attention to native species.

Down but not out

Perched at the foot of the Blue Ridge, the 6,812-acre Lake James is the first big reservoir along the Catawba River; there are 10 others, all managed by Duke Power. The largest of these is 32,475-acre Lake Norman, near Charlotte.

“When invasive plants move in, their growth is not limited in any way,” Ken Manuel, an aquatic biologist with the utility, explains. “They have no pests, no predators; they’re very aggressive. They can out-compete the natives.” Slow-growing native plant species (such as elodea and spatterdock in North Carolina) are simply strangled by these interlopers.

Being a business, Duke Power is most concerned with hydrilla’s habit of clogging up water intakes used to produce electricity. But the weed is also subject to periodic die-offs, leaving thick mats of vegetation that rot and strip oxygen from the water, souring water quality. And if that weren’t enough, hydrilla patches have also proven to be favored breeding spots for mosquitoes.

Hydrilla, however, is only one of a whole rogues’ gallery of invasive plants infesting North Carolina waters. The state’s 1991 Aquatic Weed Control Act identified 28 species as “noxious,” banning their importation, sale or cultivation. Among those of particular interest are alligator weed, creeping water primrose, Eurasian water milfoil and giant salvinia. But recently, concern has shifted to a gaily named but positively maniacal South American plant called parrot feather. (A patch of it detected at Lake Hickory several years ago exploded from 10 acres to 100 in the space of two years.) And despite its thuggish reputation among biologists, parrot feather remains a staple plant of the water-garden industry, sold legally at nurseries and garden centers across the state.

“Parrot feather isn’t regulated at all,” explains biologist Rob Emens of the N.C. Division of Water Resources in Raleigh. And in any case, much of the responsibility for stopping the spread of aquatic weeds lies with individual homeowners, he notes. But heedless water gardeners have been known to pitch overgrown plants from their ponds into neighboring lakes or streams, unwittingly inciting vegetative riots there. Floods can also flush invasive species from gardens into local waterways.

Hydrilla, meanwhile, has spread from lake to lake mainly via bits of stem and leaf that cling to boats and boat trailers, says Emens. Boaters can help slow the plant’s migration by carefully examining their rigs for “hitchhikers” before putting in.

Despite the steady stream of newcomers arriving in Asheville, however, this voracious weed has yet to put in an appearance here. Western lakes such as Fontana and Hiwassee are heavily used for hydropower, and hydrilla can’t stand up to the extreme fluctuations in water level that this entails. Fontana Reservoir, for instance, is subject to drawdowns of as much as 40 feet — enough to expose and kill even the most determined plant invaders. “Aquatic weeds just can’t make it here,” proclaims Powell Wheeler, a fisheries biologist with the Wildlife Resources Commission’s District 9, which covers all the counties west of Buncombe.

Elsewhere in North Carolina, though, alien weeds remain a growing threat. Hoping to enhance their arsenal of biological weapons, scientists are testing various aquatic insects and fungi for use against invasive plants such as hydrilla. But until better, cheaper controls become available, it looks as though we’re living in a carp-eat-weed world. And because even grass carp, gluttonous as they are, tend to leave hydrilla’s roots alone — and those roots can go more or less dormant on a lake bottom and survive as much as 10 years — the feisty plant may spring back as energetically as ever once the neutered carp pass on. Thus, restocking the sterile fish may be needed for long-term weed control.

“You don’t really get rid of these plants,” asserts Manuel of Duke Power. “It’s deceptive to talk in terms of eradication. This is a management issue that has to be addressed over time — maybe forever, as far as I know.”

[Freelance writer Kent Priestley is based in Asheville.]


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22 thoughts on “How much grass could a grass carp eat?

  1. Steve Cherry

    A small concern: what happens to the white amur (grass eating carp) once the weeds/naturally occuring vegetation is void in the pond? We’ve had 4-5 white amur in our privately owned pond for almost 4 years. Now, it seems, that nothing is growing in the pond…and the fish actually swim upon the bank to eat grass and other “weeds” that are within their reach. What are your thoughts on this siteuation?

  2. karen seagraves

    I can’t remember the exact web site, when I was researching the grass eating carp for my pond,it
    said you could put fresh grass clippings (mowed)
    in the pond when there isn’t enough weeds for the fish to feed on.

  3. robert B. Shaw

    we have been using grass carp up in the adirondacks for milfoil control since 1998 with very good luck. We use stirle carp and stock no more than 15 fish per weed acre. My question is what is maximum carp life? upon death how do you dispose of them? Do reduce water quality? Have others have good success with carp as a means of milfoil control, who?

  4. wade runnels

    Will the carp eat dead vegatation that has sunk just below the surface of the water?

  5. Danny C Owens

    What are the answers to above questions such as[where can I buy these fish, how fast do they grow, how big do they get, are they good to eat?????

  6. wade runnels

    I live on lake livingston, tx and my cove was full of water lilies. We have a lot of carp in the lake and they seemed to have no affect. The TRA came and sprayed the them with round up and they died off and created sludge just below the water line; hence the reason I asked the question above. It seems they will not eat the sludge also because it is still here. If liles are the only thing to eat in the lake I would assume they would eat them but don’t know for sure.

  7. Rob Emens

    Purchase only grass carp that are certified as triploid. These are the sterile ones, and many states have banned the sale of diploid grass carp. Vendors that sell sport fish (bass, crappie, bluegill, etc.) tend to also deal in grass carp. These fish grow fast, especially in a warm climate and a weedy pond. Expect them to double their weight in a season, or at least pack on a few pounds a year if they are older. They can get big, the largest weigh in around 35lbs. Lifespan is approximately 10 years for the longest lived fish. We find that about 20% are lost annually, so don’t expect them all to live many years.

  8. David kennedy

    I live i south Carolina and looking on putting these carp in a pond , do you know of someone that sells theses carp near Charleston SC

  9. amy bach

    Does any one know where I can purchase a couple of grass eating carp for my 1/4 acre pond in New
    York State?

  10. Frank Kucharski

    I live in CT and would like to get some carp for our 3 acre pond. Any sellers in my area? Thanks.

  11. Peggy White

    I need one or two grass carp for a one acre pond on my land. I live in Greenville, Ms. and the weather gets hot. The heat makes the weeds grow on the water and I’ve been told that carp will solve the problem. I need to know where to buy them.

  12. Sue Rabbit

    I would like to purchase a couple grass carp in Columbus Indiana. THANKS!

  13. Chris

    is it illegal to put carp in ponds in ct state? and all the different hearsay is confusing. are they good, bad or trouble once put into the ponds ???? Is it safer to just let the weeds grow for the season and rake out in summer.????

  14. mike butler

    My pond is spring fed and about 30x50ft.I normally stock with trout.The veg is trying to take over.How can I get the carp.Oh,its about 5ft deep.Thanks Mike Butler Jonesborough Tn

  15. James H. Miller

    From April to November I have a lake, one to two acres in size. This is the third year Hydrilla has taken over. TVA lowers the lake level in mid-October and I have mud during the winter. In mid-April, the level is raised and the Hydrilla starts all-over. Will Grass Carp help me?

  16. Lee Koon

    I have an 1/2 acre pond in Louisiana. It is overgrown with hydrilla and other grasses. Do carp eat all grasses or just hydrilla? How many would I need for a half acre pond.

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