DNA from the invasive silver carp has been found at 22 sites in the St. Croix River, a development that has deepened despair about the imminent arrival of the notorious leaping fish and doubts that state and federal officials can do anything to stop it.

The Minnesota Department of Natural Resources announced Thursday that 22 of 50 water samples taken between St. Croix Falls and Franconia tested positive for silver carp DNA. The samples did not test positive for the other three species of Asian carp that are believed to moving upriver from Illinois, and another 50 samples from the Mississippi River were negative.

The results are not conclusive evidence that the fish are living and breeding in the St. Croix -- none has been found in the river -- or that they are absent from the Mississippi, DNR officials said. The DNA could have come from dead carp, live carp someone dumped in the river or fish pellets used in hatcheries.

Still, it ratchets up the fear considerably, they said.

"This is disappointing news," said DNR Commissioner Tom Landwehr.

The four species of Asian carp have caused enormous ecological damage in the Illinois and Missouri Rivers, where they are well established. The carp eat 40 percent of their body weight every day in plankton and bugs, squeezing out every other creature up the food chain, from sunnies to fish-eating birds.

"They become the most dominant organism in the system," said Byron Karns, a biologist with the National Park Service. "They topple the food chain."

Much national attention has focused on stopping the carp from moving into the Great Lakes through a shipping channel in Chicago, and five states -- Michigan, Minnesota, Ohio, Pennsylvania and Wisconsin -- have filed a federal lawsuit demanding the closure of the shipping channel.

The threat to Minnesota and Wisconsin waters has not attracted similar concern from federal agencies or Congress. That's partly because the Mississippi is a long river running through multiple states, managed by many federal agencies but with no one in charge, said Paul Labovitz, superintendent of the National Park Service's Mississippi National River and Recreation Area.

"It's like the invader of the body snatchers are at the door and we have no generals to fight back," said Mark Peterson, executive director of Audubon Minnesota.

Taken alone, the new DNA results are not likely to change that, officials said.

"We need a fish," Landwehr said Thursday.

So far, no silver carp have been caught in the St. Croix. Two bighead carp -- a cousin of the silver carp -- have been caught in the river, one in 1996 and another on April 18 of this year.

This month the DNR will contract with commercial fishermen to look for the carp up and down the St. Croix. As early as next week, DNR staff will be on the river with nets and boats outfitted with electric shocking capabilities to search for fish.

An Asian Carp Task Force brought together in January by National Park Service officials will also conduct more DNA testing in both rivers.

Meanwhile, the DNR will issue notices to anglers and boaters to keep an eye out for the silver carp, which can be hard to miss. The fish is known for its astonishing ability to leap 10 feet out of the water and even knock people out of their boats.

The far more difficult question is what to do if -- or when -- they find live carp. There is no surefire way to stop the fish.

In the short term, Landwehr said, the state will seek emergency authority to close the lock and dam at the Ford Bridge in Minneapolis.

Long-term, state and federal officials also said they will pursue a tool used elsewhere, an acoustic bubble barrier. The fish don't like the bubbles, and the acoustics can be changed to deter carp but not desirable fish.

But bubblers are not guaranteed to work, either. And just the material to construct one would cost $7 million, officials said. The only likely place on the Mississippi narrow enough for such a barrier would be at mouth of the St. Croix at Prescott, they said.

Another option is reconstruction of the Coon Rapids dam. The Legislature appropriated $16 million to carp-proof it this year. That could preserve the state's $4 billion recreational fishing industry north of the Twin Cities but would mean sacrificing the lower length of the Mississippi, including Lake Pepin, and the Minnesota River to carp invaders. And it could be only temporary. Some who have studied it say it would work for only 10 or 15 years and might not be adequate at times of high water.

The most effective solution is also the most politically difficult -- permanently closing the locks at the Ford Bridge or the St. Anthony Falls in downtown Minneapolis.

But that would require an act of Congress, and by the time that happens it might be too late.

"I don't think we need to wait to find the fish," said Peterson of the Audubon Society. "The fish are going to find us. It's just a matter of time."

Josephine Marcotty • 612-673-7394