If ever there was an environmental crisis that exploded like a slow motion bomb, it is the invasion of Asian carp during the past 30-some years throughout the waterways of mid-America and now to the shores of Lake Michigan. But for a long time the explosion was barely visible, hidden by the muddy river waters of the South.
Our nation’s Asian carp crisis began back in 1972 with bighead carp and 1973 with silver carp, when an Arkansas fish farmer imported the fish to clean catfish ponds. They were intended to swim through the water and filter out unwanted algae that affected the flavor of catfish, but there’s no evidence the strategy really worked. Adult fish consume up to 22 percent of their body weight in plankton each day. (Note to fact nerds: The widely reported 40 percent of body weight in plankton a day is wrong.) Silver carp commonly grow to about 20 pounds, bighead commonly to about 40 pounds, although world records are much larger. Catfish rearing is big business down south, so government even lent a hand, helping support research that spread the carp to farms within and beyond Arkansas.
Amid the loosely regulated environment and more carefree management practices of the 1970’s, the fish swam out of ponds into adjoining rivers. “You hear over and over that the fish escaped in the floods of the 90’s, but that’s not true. They got loose in the early 70’s, and by the floods of the 90’s, there were already tons of them out there,” says Duane Chapman, who knows more about Asian carp than perhaps anybody in the United States. As a researcher with the U.S. Geological Survey in Missouri, he has studied the carp for decades as they have invaded tributaries of the Mississippi River system, including the Missouri River, Ohio River and Illinois River.
With their natural predators, parasites and diseases left safely behind in Asia, the carp’s strengths were free to shine. First, they have a stunning ability to breed. A single mature female bighead can lay 200,000 or more eggs a year (one study recorded a female bighead that produced 750,000 eggs in one year), compared to say, a rainbow trout that might lay 3,000 eggs or a lake trout that might lay 10,000 eggs. And once hatched, Asian carp grow with astonishing speed, quickly becoming too large for predators to eat. A silver carp can grow to 12 inches in its first year. And the long rivers of the South presented fine nursery habitat.
When biologists talk about Asian carp, they use the term loosely to encompass four varieties of carp: the silver, bighead,grass and black. In China, where the fish originate, people also have an umbrella term for them—the fish are such a popular food and so integral to culture they are called “the four famous domestic fishes.”
And actually, the fish have become somewhat famous worldwide. The carp are so cheap to feed, grow so fast and have such a subtle taste that they have been exported to every continent except Australia and Antarctica—something that becomes important when evaluating their Great Lakes invasion possibilities (more on that later). In a 1995 study, bighead carp were ranked fourth in global commercial fish production, at 2.8 billion pounds. “They’re kind of the hamburger of food fish,” Chapman says.
Despite the umbrella terms, there are differences among the carps—the silver and bighead carps are filter feeders; grass carp eat vegetation; and black carp eat snails, clams and other creatures on the bottomlands. The biggest immediatethreats to the Mississippi and Great Lakes are the silver and bighead. The black is not yet known to be reproducing in the wild (but it might be somewhere, Chapman cautions), and the grass has not proliferated to the point where it is taking over waterways. But biologists fear that both the grass and black have enough in common with the silver and bighead, notably their hardiness and reproduction prowess, that they pose a serious threat. Ironically, while the United States has been unable to control the population explosion of silver and bighead, their populations in China are collapsing in some places because of two familiar culprits: overfishing and habitat destruction.