Environmental Science

Their takeover was dramatic. In the first years of the 21st century, researchers estimated that bighead carp, one of four Asian carp species now in American waters, comprised 97 percent of the Mississippi River’s biomass. Havana, a hardscrabble Midwest town of 3,000 people in central Illinois, gained minor fame as ground zero for silver carp when their stretch of the Illinois River was found to contain more of the invasive fish per square mile than anywhere else on Earth. In rivers they occupy, Asian carp are often the only fish longer than 16 inches, suggesting many competing native fish fail to reach adulthood.

Within a decade of their introduction in 1963, grass carp spread to 32 states with the enthusiastic support of government agencies, private interests and academia. Silvers and bigheads, introduced in 1972 and sometimes lumped together under the moniker “bigheaded carps,” have moved effortlessly through the Mississippi watershed, following The Big Muddy and its tributary rivers like an interstate highway through the South and Midwest. By 1978, Asian carp had spread 2,800 miles from their port of call in Arkansas, becoming what some believe to be the fastest spreading exotic species in North American history. “Their population exploded,” said Matt O’Hara from the Illinois Department of Natural Resources: “We saw our first fish in the early 1990s. Within a few years, there were fish everywhere.” Steve Butler, a biologist with the Illinois Natural History Survey, tells me he’s witnessed “billions of little, tiny silver carp everywhere” on the Illinois. “As far as the eye can see it was solid spawning carp.” Researchers believe one spawning season can increase silver carp by a billion fish. Or more.

Bigheads strike a prehistoric pose as though forgotten by evolution. Large, wide-set eyes sit low on bulbous heads, their mouths hanging in a perpetual frown. In rare cases, bigheads reach 140 pounds and seven feet in length, though 40 pounds and a length of 28 inches is standard—still big by American freshwater fish standards. Silver carp also sport frowning mouths and scaly heads, heads that are, comparatively speaking, less bulging than the aptly-named bighead. They shade from silver and caramel-colored to olive green and have grown to 100 pounds, though 30 pounds is routine. Both silvers and bigheads share many physiological traits with common carp found in waterways across the continent. Common carp aren’t native per se, but they pre-date anyone currently living, and are often thought of as “naturalized”. This European cousin of Asian carp was first introduced to North America from Europe in the mid-19th century and spread by human hands with unthinkably reckless abandon (imagine tossing live fish from trains into rivers and streams that early transcontinental railways passed by).

The more I scrutinized bigheaded carps the more remarkable I found the functioning of their bodies to be. Both fish are filter feeders that consume throughout the water column. They eat while breathing: it’s a common trait for filter feeders, though in the plankton-rich waters of the Mississippi, it’s proven an especially successful physiological trait. Gill rakers, a crescent of sponge-like cartilage just inside their mouths, usher even the smallest phytoplankton and other organic matter into their gaping maws.

In conversation with Duane Chapman, one of the US Geological Survey’s leading Asian carp experts, he suggests that what sets Asian carp apart from other specialized feeders is their adaptability. “This is an unusual thing,” he says. Specially trained eaters tend to be the best at performing one task particularly well. Think of the sword-billed hummingbird. With its thin beak, longer than its entire body, this South American bird can access nectar stored in a passion flower’s narrow petals that other birds cannot reach. For grass carp, their unique trait is an ability to take water-logged aquatic plants, a low-value food source that few fish eat well and obtain all their nutrients from it. Silver and bighead’s specialized traits are far more dangerous to the health of the Mississippi and Great Lakes’ watersheds. Microscopic organisms are their primary food source, the same phytoplankton that also serve as the predominant nourishment for most of North America’s juvenile (and many of its mature) native fish. Yet when phytoplankton is scarce, native fish will starve while bigheads pivot to target zooplankton and detritus to survive. Silver carp can even live on algae and bacteria. And because Asian carp consume upwards of 20 percent of their weight each day, both species have fundamentally altered the structure of phyto- and zooplankton communities throughout the continent. This may have enormous consequences for species dependent on the resources these invasive fish consume with such voraciousness. Aquatic ecosystems may never be the same as native fishes, and the complex web of predators and prey they interact with, struggle to adapt to life in rivers stolen by Asian carp.

Breeding populations of both species now swim just 76 miles from Lake Michigan, while solitary bigheads have been captured in Chicago’s Lake Calumet, a stone’s throw from the Great Lakes.