A sonic and bubble barrier may be only a partial solution to keeping the invasive Asian carp away from Minnesota waters, according to University of Minnesota experts.
Earlier this month the state Department of Natural Resources announced that researchers found the DNA of Asian carp in the St. Croix River. Experts are scrambling to deal with a likely infestation of the big fish.
Researchers testing the sonic and bubble barrier in the Engineering and Fisheries Lab on the university's St. Paul campus say it cannot stop all carp and at best may only help the state manage the problem.
In a round tub that looks like a primitive hot tub, scientists are experimenting on common carp, which came to North America more than 100 years before Asian carp arrived. They say what they're learning about the chubby, six-inch-long, common carp in the tank likely can be applied to the Asian carp that's swimming and jumping its way into Minnesota waters.
The barrier works to control common carp by using bubbles to create sound and turbulence, said Dan Zielinski, a PhD student in civil engineering. In his experimental tank, an arrangement of perforated plastic tubes occupies a small section of the bottom.
"As we push the air through there, bubbles will form, and when a bubble detaches from the plastic, it gives off a distinct sound field," Zielinski said. "The bubbles rise; that creates a lot of turbulence."
To track their movements, fish are fitted with radio tags that set off a signal when they pass under four antennae stretched across the tank.
DNR officials are considering installing a bubble barrier at the mouth of the St. Croix River. That might divert the carp into the Mississippi and the Minnesota Rivers.
Carp are greedy and destructive, and have evolved an amazing connection between their swim bladder and their brain, Zielinski said. That makes them much more sensitive to sound than a walleye, which is tone-deaf by comparison.
"They've got specialized rib bones that allow them to hear a more sensitive decibel range as well as frequency range," Zielinski said of carp. "The hope is that you can generate a sound that's especially annoying to them, or as a deterrent that they don't want to pass over it."
Zielinski has experimented with various bubbler layouts and bubble sizes, including a barrier that emits about 125 decibels - as much sound as a jackhammer delivers from a meter away. Two arrangements consistently cause the carp to turn away from the barrier 75 percent of the time.
Such barriers could never stop all the fish, but they could make a big difference if they were strategically placed to prevent the carp from spawning, said Zielinski's advisor, Peter Sorensen, who has spent most of his professional life learning about carp.
"That is actually the key to controlling any invasive species," said Sorenson, a fisheries biologist who has helped design ways to control the invasive sea lamprey. "If you can stop them from reproducing, or the young from surviving, you've got them."
Sorenson said each adult female carp can lay as many as a million eggs a year, but the eggs are probably vulnerable to other fish. He says if the habitat were improved for native fish, like paddlefish, they could do a lot to keep the carp in check.
"If you could get the paddlefish back, and the system back into balance, maybe paddlefish might be able to eat many millions of eggs, and take care of this whole thing in a flash," he said.
Sorensen and his crew worked for two years to design a barrier for common carp. He said they could move faster by using Asian carp, but the barrier would still need to be tested in the lab, and in the field.
The scientists plan to try their common carp barrier in a real-world stream later this year.
A barrier to control Asian carp could cost more than $75 million. The big question is where the state would find the money to pay for it.