Tennessee Pond Management: Grass Carp – the Rest of the Story

Suppose I told you there was a fish that would dramatically simplify management of your pond; it would be a one-time stocking, with a fraction of the cost, for a typical-sized pond, of the other species you’re planning on stocking, and you wouldn’t need to stock this fish again for several years. The only catch? Said fish will do catastrophic damage to the quality of fishing in your pond for the next fifteen to twenty years.

That fish is the white amur, commonly known as the grass carp.

But, you say, don’t many or most of the other pond management companies recommend them for every new pond they stock? Don’t many state game and fish agencies recommend them as well for new ponds, across the board? They do and they do. A few pond consultants are beginning to recognize the consequences this fish brings with it to your pond; but grass carp are an easy way to eliminate most weed problems before they start, so many consultants recommend them because it’s easy.
White amur were first imported into the U.S. in 1963 to combat hydrilla. Use of the fish was limited until 1984 due to concerns regarding their potential impact on native ecosystems. Once triploids were developed, which were sterile and could not reproduce, the fish began to be stocked widely in private ponds and lakes for weed control.

So where’s the problem, you may ask? White amur are classified as an omnivore (Hora and Pillay 1962); omnivores eat not just plants, but also animal foods. In the case of white amur, they consume invertebrates – the same worms and insect larvae that are the principal food source of the bluegill in your pond. They will also consume fry of other fish species including bass and bluegill. The Southern Regional Aquaculture Center is a widely-respected agency that has hundreds of publications available on everything from designing RAS systems to crawfish production in ponds. Here is a quote from their publication, “Using Grass Carp in Aquaculture and Private Impoundments”: “Fingerling grass carp will consume insect larvae, other invertebrates, and even small numbers of fish fry, but only when desirable vegetation is unavailable.” Andrew Zetts of the Zetts Fish Hatchery in Drifting, Pennsylvania noted as far back as the late 1980’s that he had observed grass carp eating largemouth bass fry; he did a study with three ponds, one pond that was stocked with white amur and two control ponds that were not, and the control ponds produced much higher numbers of largemouth fry than the pond that had grass carp. Kilgen and Smitherman (1971) found that invertebrates comprised 9% of the total diet of grass carp stocked into a pond without macrophytes (aquatic plants large enough to see).

So they eat a few of the same worms and insect larvae that the bluegill eat – what’s the big deal, you might say? A two-pound grass carp can consume up to 300% of its body weight daily; a ten-pound amur can consume up to 30% of its body weight daily (Masser 2002). Nine percent of six pounds of food is over half a pound of insects per day. Grass carp can grow up to two pounds per month (Sutton et. al. 1986). If you stock twenty grass carp into your pond and they weigh roughly two-thirds of a pound when stocked, they can exceed two pounds within a month of stocking; at that point, if each carp only consumes half a pound of insects per day, that’s ten pounds of insects taken out of your pond’s ecosystem each day. That’s seventy pounds in a week, and 310 pounds in a month.  After six months or a year, of course, they’re all up to ten pounds, so they’re only consuming a little over a quarter-pound each of insects per day, five pounds in sum for all twenty fish. Or 150 pounds per month. If they level off in their insect consumption once they reach ten pounds, even though they’re still growing and still need food; if we estimate on the low side and allow for the carp only to consume .27 pounds per day per fish of insects from the time they reach ten pounds onward…

One year after stocking them, your lovely weed razers will have taken roughly 2,700 pounds of insects out of your pond.

How many insects does one bluegill consume in a day? A year? The general rule of thumb for food conversion ratio for a fish eating natural foods is that it takes ten pounds of natural forage to put one pound of weight on the fish; so for one of your bluegill to reach one pound in size, a size that takes the average bluegill in the wild most or all of its life to reach, it has to consume ten pounds of insects. What does it do for that bluegill’s food supply when 2,700 pounds of insects are removed in one year from the pond?

Insects, of course, are not the only food source for bluegill in a well-managed pond; many ponds have automatic feeders that throw high-protein pellets to the fish one or multiple times daily. Feeding bluegill with a premium, fishmeal-based food is far and away the most straightforward way to grow giant bluegill in a private pond; if all other management factors are optimized, it can make an extraordinary difference in the average and top-end size of the bluegill in the pond. There’s only one small problem.

Grass carp are fed pellets by every hatchery that raises them. It’s the quickest and most economical method to grow them to marketable size. By the time they’re stocked into your pond, they’re as pellet-trained as any bluegill or channel catfish you’ll ever meet. Many pond consultants have now recognized the problems channel cats cause in ponds where feeding programs are implemented: the catfish will hog the food at feeding time, to the point of physically bullying the bluegill away from the feeding area by rolling their bodies and slapping their tails on the surface, so they can have the food all to themselves; this is why many consultants now discourage their clients from stocking channel cats.

As it so happens, grass carp are even bigger feed hogs than channel cats. Not only will they learn what times the feeder goes off and be waiting a few feet away when the pellets start flying; they’ll physically bully bluegill away from the food just like catfish do. But grass carp grow much faster than channel cats; it can take a channel cat four or five years to reach ten pounds, whereas a grass carp can reach that size in six months, and they don’t stop growing once they reach that size. Bigger fish require more calories; and the bigger a fish is, the less likely other fish are going to want to get on its bad side. What’s an eight-inch bluegill going to say to a ten- or twenty- or forty-pound grass carp? Not much. The bluegill will hang on the outer edge of the feeding area, sometimes fifty feet or more away from where the majority of pellets are floating, waiting for the few pellets the carp don’t clean up as they swim along the surface with their mouths open, vacuuming up your expensive food that you bought to help your bluegill and bass.

But what if you don’t feed and don’t plan to? Macrophytes, or aquatic plants large enough to be visible to the naked eye as opposed to microscopic plants such as phytoplankton, are the preferred food of grass carp; bluegill and largemouth don’t eat macrophytes, so where’s the problem? One of the ways biologists gauge productivity of a pond or lake, i.e. how many pounds of fish per acre the lake can produce, is what percent coverage there is of macrophytes in the lake; lakes with significant aquatic vegetation have been found to produce between four and sixteen times as many fish per acre as lakes without plants (Richardson 1921, Needham 1929, Burdick 1940) . You don’t have to look far for examples of this principle: Kentucky Lake was not known as an exceptional bass or bluegill lake thirty years ago, but then water milfoil and other invasive plants got into the lake and began turning the turbid, low-productivity water into clearer, high-productivity water, and now anglers from all over the country flock to fish the lake for its largemouth, bluegill, and redear. Ask any ten anglers in the Volunteer state what the best public water in the state is for bluegill, and odds are that at least nine of them will name Reelfoot Lake as the hands-down winner; if you’ve ever fished Reelfoot, or even seen a TV show of someone fishing on the lake, you know that it’s the closest thing this state has to the weed-filled lakes of Florida that draw anglers from far and wide. Reelfoot is also known as a top destination in the state for largemouth and crappie. The fact that all three species get big in the lake is not in spite of the vegetation; it’s because of it.

One caveat: if you’re managing your pond for trophy bluegill, too many weeds can be a bad thing, for the same reason too much cover in general is bad. As I note in my post from a few years ago on trophy bluegill ponds, too much cover makes it harder for the bass to keep bluegill numbers low, and low bluegill numbers are essential for growing big bluegill. That said, sometimes it happens that a pond that is overrun with weeds produces giant bluegill; no one has figured out why this happens sometimes, when the most common result of excess weed growth is bluegill overpopulation.

If your pond goal is big bass, you’re simply cutting the potential of your pond in half, or more, if you stock grass carp.

Why then do most consultants manage their clients’ ponds to eliminate all macrophytes? Because it’s easy. It’s the lazy way out. They don’t have to explain to the customer that even though plants can make fishing more difficult at times, they’re still a valuable part of the ecosystem, even essential if you’re trying to grow big bass. They know that plants, depending on species, may reach levels at which they have to be knocked back by herbicides; it’s easier just to wipe them out from the start with carp.

We were the first to warn pond owners against the dangers of stocking grass carp in their ponds. When we started this blog post a couple hours ago, we knew of one other consultant in the southeast who had begun to alert his customers about this issue; now, evidently, more companies are following our lead, as one of our chief competitors in this state, who as recently as a year ago had nothing but good things to say about white amur on their website, now has a paragraph at the bottom of their grass carp page warning about what the carp can do to a feeding program.

If all you want from your pond is scenery and you can’t remember the last time you fished, grass carp will give you a weed-free pond. If, on the other hand, fishing is an important part of what you get from your pond, and you want it to be the best it can be, keep grass carp far away from your pond.