Fish Facts: Grass carp (Ctenopharyngodon idella)

Chris Fowler, who works at Orvis Austin, landed this beautiful grassy in Texas last summer.
Photo by Chris Fowler

Imported from eastern Asia, the invasive grass carp has established itself from coast to coast and is recognized as one of the toughest species to catch on a fly.

While the last decade has witnessed a transformation in the reputation of carp as worthy fly-fishing quarry—what were once “trash fish” are now “golden bones”—the common carp (Cyprinus carpio) has been the beneficiary of most angling attention. Perhaps that is because its more enigmatic cousin, the grass carp (Ctenopharyngodon idella), can be a seriously difficult nut to crack. Just getting within casting distance of an ultra wary “grassy” can be a feat unto itself, and because they are predominantly herbivorous, these fish are less likely to eat a fly. To a growing number of fly fishers looking to test their skills, grass carp offer an exciting challenge. As common carp are to bonefish, grass carp are to permit—stalking, hooking, and landing a double-digit grassy is a feat that not many anglers have accomplished.

Range and Species History
Grass carp (Ctenopharyngodon idella) are native to eastern Asia, from northern Vietnam to the Amur River, on the border between the Russian Far East and Northeastern China. They are sometimes called white amur and are the only member of the genus Ctenopharyngodon. In the United States, they can be found in all but five states: Alaska, Maine, Montana, Rhode Island, and Vermont. Among the more widely cultivated species in the world, grass carp are raised in China for food and in the rest of the world for aquatic weed control. The vast majority of grass carp released in the U.S. are sterile triploids, unable to reproduce, although such introductions are illegal in many states.

Although they prefer shallow, stillwater habitat, such as lakes, ponds, and backwaters of large rivers, grass carp need access to a river system with fast water in which to spawn. Thus, they often travel long distances to spawn. Spawning occurs when water temperature reaches 68 degrees, and females lay hundreds of thousands of eggs, which do not settle in the streambed, but float downstream, suspended by turbulence. Established populations—those that feature self-sustaining reproduction—have been identified in tributaries of the Mississippi and in the Trinity River drainage in Texas. Like most carp, they can tolerate high water temperatures, murkiness, and even brackish water.

Young Oliver caught this nice grass carp on Black Woolly Bugger with an orange bead.
Photo by John Carpenter

Grass carp grow very quickly—an 8-inch fish stocked in spring might be 18 inches by fall—which is one of the reasons they are favored by aquaculturists. Their normal life span is five to nine years, with some populations surviving beyond 15 years. Adults can reach immense sizes, up to 4.5 feet long and upwards of 90 pounds, although most are in the 10- to 20-pound range in good habitat.

What makes grass carp exceptionally useful for weed and algae control is that they can eat up to three times their own weight daily. They are primarily herbivores, but they will eat aquatic and terrestrial insects on occasion, as well as other invertebrates and detritus (grass, cottonwood fluff, etc.). Grass crap feeding on the surface present fly fishers with a real opportunity to catch these elusive fish in a dry fly.

Invader from the East
The first grass carp arrived in the U.S. from Taiwan and Malaysia in 1963, sent to aquaculture facilities in Alabama and Arkansas, where the plan was to use the species to control unwanted aquatic vegetation. Of course, it didn’t take long for them to escape, during a breach at the US Fish and Wildlife Service’s Fish Farming Experimental Station in Stuttgart, Arkansas, in 1966. This was followed by widespread authorized, illegal, and accidental introductions, and by the turn of the century, the species could be found in 45 states. They are useful for weed control, but problems arise when they feed on the wrong vegetation or “overgraze” a lake, altering the food web and competing for food with native fish or prey species. Stocked grass carp are often sterile, to avoid overpopulation.

Great Lakes Peril
All carp are invasive species in North America, and controlling their spread is a big job. Although bighead and silver carp are considered the most damaging members of the family Cyprinidae, grass carp can eat so much vegetation that they destroy vital habitat and spawning grounds. This year, biologists announced that non-sterile grass carp have been found in three of the Great Lakes—Erie, Huron, and Michigan—and that it is “very likely” that the species will become established in the next decade unless methods to stop their spread can be put in place. One theory is that likely grass-carp spawning grounds could be closed off with nets to keep the species from reproducing. We can also expect to see tougher laws to control importing grass carp for any reason into the region.

Flies & Tactics
Your first challenge in catching a grass carp is getting close enough to the fish to make a cast, as they can be extremely spooky. Use a 6- to 8-weight rod (depending on the size of the fish you expect to catch) and a floating fly line that will land on the water without much commotion. Use the strongest leader you can get away with, as these are strong, powerful fish. The world record was an 87-pounce, 10-ounce fish from Bulgaria.

Although some anglers swear by patterns that imitate grass or cottonwood fluff, the flies in your trout box are good enough. If the fish are not feeding on top, try a weighted Hare’s Ear Nymph or a small Woolly Bugger, which will land softly and get to the bottom quickly. Good surface patterns include terrestrials—beetles and hoppers—and anything fluffy, such as a CDC Caddis or emerger. Catching a grass carp is less about the specific imitation than about a good presentation.