Why Are Asian Carp So Fearsome?

Karen Rowan LiveScience.com Yahoo News 24 Jun 10;

Invasive Asian carp are inching their way closer to the Great Lakes. Now, one has been found just 6 miles (10 kilometers) from Lake Michigan, according to news reports, and experts worry that they could devastate the ecosystem and fishing industry of the region.

Asian carp wreak havoc with their ravenous appetites, high reproduction rates and their ability to survive predators.

Two species of Asian carp, the bighead and the silver carp, eat plankton voraciously. A fish will eat 5 to10 percent of its body weight a day in plankton - and they can weigh up to 100 pounds (45 kg), said Michael Hoff, a fisheries biologist with the Fish and Wildlife Service (FWS). This is far more than what native fish species eat.

Another species, the grass carp, can eat up to 40 percent of its body weight a day, mostly eating aquatic plants.

Eating constantly

Asian carp have poorly developed stomachs, which are inefficient at breaking down food and obtaining nutrients - and thus the necessary gluttony.

"They have a really primitive digestive system, so they need to eat constantly, Hoff told Life's Little Mysteries.

Because they consume such massive amounts of plankton (a group of tiny free-floating plants and animals) and other organisms, which are the base of the food chain in lakes and rivers, the fish can disrupt the natural flow of nutrients through the ecosystem.

And they have few natural predators. So the fact that they consume so much plankton means much of the sun's energy that those plankton captured during photosynthesis never becomes available to the other organisms in the food chain, Hoff said.

"They are like energy sponges, they consume foods that other species would eat, but nothing eats them. They are energy dead-ends," Hoff said.

The Asian carp's high growth rate and large size keep most enemies away.

By the time any fish reaches a size of around 12 inches (30 cm), it is too big to be eaten by some of the important predator species in rivers and lakes. Because Asian carp reach this size faster than native fish, their young have a better chance of escaping predation than the young of other species, Hoff said.

And predators such as walleye and catfish spend their time near the river bottom, while Asian carp tend to stay near the surface, further reducing the opportunities for predators to feed on them, Hoff said.

The reproductive rate of the carp also sets them ahead of the native species. A single female bighead carp can lay up to 1.9 million eggs in a single year, according to one estimate, Hoff said. Although it is likely that only about 1 percent to 3 percent of these eggs will grow to become adult fish, the numbers are still higher than they are for native species.

How they got here

There are now four species of Asian carp - grass, bighead, silver and black - found in the United States, and they likely first entered the waterways by escaping from fish farms beginning in the early 1960s, said Duane Chapman, research biologist with the U.S. Geological Survey (USGS).

Asian grass carp were probably in the waters in the 1960s, and silver carp were caught from the wild throughout the 1970s. In 1981, the first catch of a bighead carp was reported, Chapman said.

The fish were used on fish farms because they are effective at removing algae and other suspended particles from fish ponds.

Large floods struck near the Mississippi in the early 1990s, and many of the catfish farm ponds overflowed, unleashing more Asian carp into the Mississippi. Although this flooding event is often reported as the entrance of carp into the Mississippi, the fish were there long before this, and it isn't likely that these floods contributed to the population signficantly, Chapman said.

By outcompeting other fish for food, the carp have become the most abundant species in some areas of the Mississippi, and have steadily made their way north, according to the EPA. Densities of Asian carp in parts of the Mississippi River are thought to be among the highest in the world, according to the FWS.

The fish are now found in the Mississippi River from Louisiana to Minnesota. They've spread from the Mississippi River into both the Missouri and Ohio Rivers, as far west as South Dakota and as far east as Ohio. They are also found throughout the Illinois River Basin.

The giant fish have the potential to injure boaters - silver carp are startled by motors and react by leaping out of the water, occasionally landing in boats. These leaps could cause human fatalities, according to the FWS.