A barrier of noise and bubbles at the Ford Dam near Fort Snelling, costing at least $12 million, is the best option to stop the spread of invasive Asian carp to Minnesota's northern lakes, state officials announced Thursday.

And even though there is no certainty that the barrier will work, the risk from the spread of the voracious fish to Minnesota's lakes is so great that officials from the Department of Natural Resources said they want to start construction next fall and will seek funding from the Legislature this year.

"The long and the short of it is that any type of technology would have be considered experimental," said Steve Hirsch, the DNR's director of water and ecological services.

The recommendations were based on an engineering analysis that ruled out an electric barrier like those used in other rivers and in Chicago's shipping canal. Though an electric barrier probably would work better than sound and bubbles, it's not practical at the Ford Dam, Hirsch said. It would be a safety risk for boaters who use the lock, and electricity could damage the lock's mechanism. For those reasons, the Army Corps of Engineers would not permit the construction of such a barrier, DNR officials said.

The decision to recommend the noise-and-bubbles barrier is a significant step. The state has been wrestling for years with strategies to stop the carp, which have devastated long stretches of the Mississippi and Missouri rivers in the south, and which are now threatening the Great Lakes. A few of the fish have been found in the Mississippi and the St. Croix in Minnesota, but not in sufficient numbers to represent a population large enough to breed.

The Legislature already has provided $7.5 million from the state's 2008 Legacy Amendment for research and planning for a barrier, and has funded a $16 million upgrade to the Coon Rapids Dam that would also be a barrier -- but which also would not be foolproof.

The best and least expensive solution would be to close the upper lock and dam at St. Anthony Falls in Minneapolis, officials said, but that is also the most politically difficult: Doing so would require an act of Congress.

"This only reinforces the need to close the upper St. Anthony lock and dam," said Trevor Russell, program director for the Friends of the Mississippi River, a state advocacy group. "We are running out of options and we are running out of time."

In the meantime, however, an acoustic and bubble barrier is the best option, Hirsch said. Based on the limited research available, that would stop 60 to 90 percent of the fish. In combination with the new Coon Rapids dam, it might protect Minnesota's northern waters until a better solution is found.

The barrier would be constructed across the 200-foot wide channel that funnels boats into the lock, which makes up one section of the dam. It's the only place where a barrier could be constructed because the river is much too wide, and the flow is too erratic, officials said.

The proposed barrier would consist of a curtain of bubbles that would function as a visual wall for the fish. Underwater speakers would emit sound at a pitch abhorred by the notoriously noise-sensitive fish. The plan might also include flashing strobe lights.

The barrier would cost $1 million to design, $12 million to $19 million to build, and $250,000 per year to operate, officials said.

If some fish do get through the barrier and then get past the lock and dam at St. Anthony Falls, their northern progress would be impeded by the Coon Rapids dam.

"It would be the last hope," Hirsch said. If they get past that, "they would have access to Mille Lacs."

Asian carp are capable of eating 5 to 20 percent of their body weight each day. They feed on algae and other microscopic organisms, often outcompeting other fish for food.

Scientists believe the fish could severely disrupt the aquatic ecosystems of Minnesota waters. In other states, Asian carp have become dominant species in rivers, displacing most other game and non-game fish.

Josephine Marcotty • 612-673-7394