MINNEAPOLIS — Tests have found signs of Asian carp in the Mississippi River north of a key physical barrier keeping the invasive species of fish from spreading into many of the state’s most popular lakes, officials said Thursday.

The sensitive tests detected DNA from silver carp in the water above the Coon Rapids Dam, which is upstream from Minneapolis, the Minnesota Department of Natural Resources said. Nineteen out of 48 water samples taken near the dam in September tested positive for silver carp DNA, and three of the positive results were from above the dam.

No live Asian carp have been caught there yet, and experts aren’t ruling out the possibility of false positives. But DNR officials said they were surprised and disappointed in the results because they thought the dam would be a good barrier.

“We believe that the risk is just too high to not assume that there are live fish upstream,” said Tim Schlagenhaft, Mississippi River manager for the DNR. “Consequently we need to move forward.”

Silver carp and another Asian species, bighead carp, have become well established farther south in the Mississippi and Illinois rivers since escaping from southern fish farms, probably around the late 1970s. They threaten native species by outcompeting those fish for the plankton that forms the base of the aquatic food chain. Scientists believe their numbers stayed low for several years until their populations grew large enough to sustain reproduction, then exploded.

Silver carp are known to jump as high as 10 feet into the air, at times striking startled boaters.

Minnesota lawmakers in July approved $16 million for improvements to the Coon Rapids Dam. The work isn’t scheduled to begin until next year, and DNR Commissioner Tom Landwehr said it’s is needed now more than ever.

“This is not the time to take our foot off the gas. It’s time to put it on the gas,” Landwehr said.

Above Coon Rapids, dams on the Mississippi at St. Cloud, Sartell, Royalton and Little Falls are “pretty high” and should be effective fish barriers, Schlagenhaft said. But he added that the invaders could still infiltrate up the Rum River, which connects the Mississippi with Mille Lacs Lake, one of the state’s premiere fishing lakes. They could also move up the Crow River system.

“So there are some of our significant fisheries in central Minnesota that they would have access to,” he said.

Testing for environmental DNA, or genetic material from a living organism found in the wider environment, had previously found indications of silver carp in the Mississippi downstream from the Ford Dam between Minneapolis and St. Paul, and in the St. Croix River between Minnesota and Wisconsin.

But those tests didn’t reveal the possible number of fish present, how big they were or whether they were breeding. Large-scale netting in response to the tests did not find any silver carp, which Schlagenhaft said indicates any populations are very small and there’s still time to keep them small. Similar netting is now planned around the Coon Rapids Dam.

Only a few Asian carp have been caught in Minnesota and its border waters, and it’s not clear if those catches were strays or a signal that the fish have established a permanent presence.

Similar environmental DNA testing has stoked fears that Asian carp may soon enter the Great Lakes via a canal that runs through Chicago, damaging the lakes’ $7 billion fishing industry. Electric barriers have been installed in the canal in hopes of keeping them out of Lake Michigan.

Few live Asian carp have been found in the canal, and Schlagenhaft said a study under way in the Chicago area may provide insights into other potential sources of the DNA. That could help determine whether the DNA found in the Minnesota waters is coming from live fish or something else.

Kelly Baerwaldt, an Asian carp program manager with the U.S. Army Corps of Engineers, said that study is looking into whether Asian carp DNA might be spread by fish-eating birds, or if it might come from dead jumping carp that get kicked off barges or from commercially caught fish that get turned into lawn fertilizer.

DNR fish geneticist Loren Miller said it’s “very unlikely” that the DNA is coming from other carp species.

Test results are still pending for water samples collected in the Minnesota River, which flows into the Mississippi south of Minneapolis, and above the St. Croix Falls Dam on the St. Croix River, which enters the Mississippi further south.

Landwehr said discussions are continuing on ways to give the Corps the authority to close the federally operated locks in Minneapolis to provide a barrier.

Still under discussion, Schlagenhaft said, is the possibility of installing sonic bubble barriers outside the Ford Dam lock, at the mouth of the St. Croix and perhaps on tributaries to the Minnesota.