Ever since we first heard of the curse called Asian carp, the news has always been bad. These invasive fish have proven to be more prolific, more aggressive, and more destructive than we might have initially feared.
Now, it appears they are more resilient too.
A recent study by Purdue University indicates that Asian carp are able to spawn in Midwest rivers and streams that were originally believed to offer too little water or inadequate flow for reproduction.
That conclusion adds additional gray to a bleak outlook for slowing the relentless push of Asian carp toward the Great Lakes.
“It looks like the carp can probably become established in a wider range of environmental conditions than once thought,” said Purdue assistant professor of forestry and natural resources Reuben Goforth, who led the study.
The predominant theory held that these exotic imports were less of a threat to invade moderate-sized waterways because the carp required very specific conditions to spawn. Many Midwest streams were assumed to be too thin or lacking the volume of water Asian carp preferred.
Those conclusions were founded on the behavior of these fish in their native rivers in Asia. The report from the Purdue study paints a contradictory and increasingly grim picture.
Goforth contends there is adequate evidence to indicate that Asian carp are spawning in waterways and conditions thought to be unsuited for such activity.
His Purdue research team found confirmation of Asian carp in Indiana’s Wabash River spawning further upstream than expected, and carp eggs adrift in the waterway into September.
The “Asian carp” term refers to four species of fish native to China and other parts of Asia: bighead, silver, black and grass carp.
Black carp feed primarily on mollusks and were brought to the United States more than 40 years ago by southern fish farms to control a parasite that affected catfish.
Grass carp were introduced to U.S. waters from Taiwan and Malaysia in the 1970s, primarily to control vegetation in ponds and lakes.
Bighead and silver carp, often referred to collectively as the bigheaded carps, were imported about the same time for use in aquaculture and sewage treatment operations, because they are filter feeders that consume plankton, algae, and detritus.
Common carp are are native to Europe. These carp, first stocked in Ohio waters as a food fish more than 130 years ago, are present throughout the state. They eat crustaceans, insect larvae, mollusks, some aquatic plants, and small fish.
The Asian carps entered many waterways, including the Mississippi River system, during floods or through careless release. The bighead and silver carps present the most serious threat to native species, because they are so prolific and they consume huge amounts of plankton, and can grow to more than 60 pounds. Many native species also eat plankton at some stage of their life, but are not successful at competing with Asian carp.
The bighead and silver carps have spread throughout the Mississippi system, and are found from Louisiana to downtown Minneapolis, as well as South Dakota on the Missouri River. They are pushing towards Pittsburgh in the Ohio River. This conquering horde is now just a few miles from Lake Michigan in the Chicago Sanitary and Ship Canal, a manmade link between the Great Lakes and Mississippi River watersheds.
“The reason truly invasive species are so successful is because they overcome obstacles,” Goforth said. “When you base their limitations on what happens in their native ecosystems, it's a good start. But it may be a good idea to go back and take this new data to recalculate more precise limits based on these new understandings.”
The science on Asian carp had indicated that it would take significant spring rains to increase the depth and flow in certain rivers to allow for successful spawning, Goforth said.
But after the region experienced severe drought in 2012, Asian carp eggs were found in areas of the Wabash River that had been considered too small for spawning to take place.
“We didn't have the normal spring floods and they spawned anyway,” he said. “Those changes in water flow and depth might be a cue to spawn, but they're not something that's absolutely necessary.”
The Purdue team will continue to study the invasive fish and attempt to learn what adaptations have taken place that permit the carps to spawn in circumstances deemed unsuitable in their native habitat in Asia.
“What we don't know is whether they have the ability to acclimate to those conditions or whether there is a sort of micro-evolution going on that allows them to adapt to these conditions," Goforth said.
It is feared that if the bigheaded carps reach the Great Lakes and establish breeding populations, they could overwhelm native fish populations, such as walleye, smallmouth bass, and yellow perch in Lake Erie, and devastate the $4.5 billion sport and commercial fishery of the Great Lakes.
Contact Blade outdoors editor Matt Markey at: firstname.lastname@example.org or 419-724-6068.