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Invasive Asian carp may be present in the Twin Cities stretch of the Mississippi River after water samples tested positive for genetic material from silver carp, the Minnesota Department of Natural Resources said Thursday.

The environmental DNA, or eDNA, testing is a chemical indication that some silver carp are in the Mississippi downstream from the Ford Dam in Minneapolis, the DNR said. But the tests don't reveal the possible number of fish present, how big they are and whether they are breeding, the agency said.

The National Park Service and the DNR conducted the Mississippi River testing in September after similar DNA testing in June indicated the presence of silver carp in the St. Croix River between Minnesota and Wisconsin.

The DNR said it will immediately hire a commercial fisherman to begin netting and searching for Asian carp below the Ford Dam, also known as Lock and Dam No. 1. No Asian carp were discovered in the St. Croix after a nine-day search by DNR biologists and a commercial fisherman. But the DNR said that doesn't necessarily mean some fish are not present.

Mississippi River biologist Tim Schlagenhaft said in a DNR news release that while the eDNA tests are "very sensitive," they can only indicate the presence of DNA in the water.

"In other states where DNA testing has resulted in positive samples, the fish have proven very difficult to subsequently capture, and we expect this to be the case in the Mississippi River if the fish are ... present in low numbers."

Fourteen of 49 samples tested positive for silver carp in the most recent round of Mississippi River eDNA testing, the DNR said. The samples were among 275 collected in September from the Mississippi, Minnesota and St. Croix rivers. The rivers are being tested for bighead and silver carp, two species of Asian carp. Test results are pending from the other river locations.

The new results could mean that Asian carp are present in a stretch of the river that includes the Mississippi National River and Recreation Area, a unit of the National Park System that runs along 72 miles of the Mississippi in the Twin Cities, the National Park Service said.

"These eDNA results are like a smoke alarm blaring. Until we find the source, we have to assume there is a fire. We have to assume Asian carp are here," park Superintendent Paul Labovitz said in the news release.

So far, no silver carp have been caught in the St. Croix or Mississippi Rivers above Lake Pepin, although some have been caught downstream near Iowa, the DNR said. Only two bighead carp have been caught in the area _ one in 1996 and another on April 18, 2011, both in the St. Croix.

Voracious Asian carp devour plankton, threatening a river's native fish populations by disrupting the food chain. Silver carp also can leap 10 feet out of the water when startled, endangering boaters. Asian carp began spreading up the Mississippi River basin after escaping from southern sewage treatment plants and fish farms years ago.

Michigan, Minnesota, Ohio, Pennsylvania and Wisconsin have filed a federal lawsuit demanding quicker action to physically separate the Great Lakes and Mississippi systems to prevent Asian carp and other species from migrating between the two watersheds. The U.S. Army Corps of Engineers has promised to conclude by 2015 a long-range study of ways to cut off potential avenues for species to transfer between the two basins, but critics argue that's too slow.

An electric barrier about 25 miles from Chicago is designed to prevent aquatic species from moving between the Great Lakes and Mississippi River basins. While the Army Corps contends the barrier is doing a good job blocking the carp, critics want physical barriers put up in the Chicago area to cut ties between the two watersheds.

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