Placing dam-like structures in Chicago waterways would be an almost foolproof method of preventing Asian carp from reaching Lake Michigan, while a less pricey electric barrier system also has solid prospects for shielding the Great Lakes from the invasive fish, according to a scientific analysis released Wednesday.
While other studies have weighed the pros and cons of different proposals for stopping the carp, this one went further by rating their likelihood of success based on "how sure experts are about each strategy," said Marion Wittmann, a University of Notre Dame post-doctoral researcher and the report's lead author.
The report was designed to help policymakers choose an effective plan that wouldn't take too long to carry out. It determined that other methods under consideration, such as using strobe lights and water cannons to frighten the carp away, might also be helpful but would be less likely to succeed.
The study was conducted by scientists with Notre Dame, the U.S. Forest Service and Resources for the Future, an independent research institution. Their conclusions were based on a survey of experts in fisheries management, aquatic nuisance species and other relevant topics.
They were asked to rate the likelihood for success of a variety of strategies for shutting down what's considered the most direct route to the Great Lakes for Asian carp: a network of rivers and canals around Chicago that link Lake Michigan to the Mississippi River watershed.
"An important finding of this study is that knowledgeable experts identified clear differences in the likely effectiveness of some Asian carp prevention technologies as opposed to others," said John Rothlisberger, a Forest Service aquatic ecologists and one of the report's writers. "Physical separation stands out from the rest as having the least associated uncertainty and the highest probability of preventing the introduction of Asian carp into Lake Michigan."
Bighead and silver carp, which gobble huge amounts of plankton on which many native species also subsist, were imported from Asia in the 1970s and have infested the Mississippi and its tributaries. Scientists say if they overrun the Great Lakes, they could upend the ecosystem and damage a $7 billion fishing industry.
The U.S. Army Corps of Engineers this month presented eight options for fortifying the Chicago waterways to prevent Asian carp and other invaders from migrating between the two giant watersheds. The alternatives included walling them off with physical structures, which could cost more than $15 billon and take 25 years to accomplish, and other methods, including deploying electric barriers in addition to an existing one in a canal 37 miles from Lake Michigan.
But critics say the Corps' timetable is too slow and could allow the carp to reach the lake before the work is done.
Authors of the new report, which is being published in the journal Environmental Science and Technology, said it offers one way to speed things up. By assigning expected performance ratings to 17 possible strategies based on expert opinion, it offers resource managers a basis for moving forward without waiting for time-consuming field tests, Wittmann said.
"Asian carp are so close to Lake Michigan, we don't have much time right now," she said.
The study found that physical separation could prevent 95 to 100 percent of Asian carp introductions into Lake Michigan. The experts acknowledged the possibility that catastrophic flooding could overwhelm the barriers or that the carp could reach the lake through other means, such as an angler dumping a bait bucket containing young carp that were mistaken for other species, Wittmann said.
The experts concluded that the electric barrier could prevent 85 to 95 percent of carp introductions.
Other deterrent methods that were considered ranged from depleting oxygen or boosting carbon dioxide levels in parts of the waterways to spreading chemicals to blocking fish passages with nets. The report said some could have high failure rates, although it said a combination of noisemaking devices, bubbles and strobe lights could be 75 percent to 95 percent effective.
Duane Chapman, a U.S. Geological Survey fisheries biologist and one of the specialists consulted for the study, said there is no scientific consensus on how many Asian carp would be needed to establish a breeding population in the Great Lakes. So it isn't certain what level of barrier effectiveness is required to thwart a successful invasion. But he said there's a significant difference between the protection offered by physical and electric barriers, even though their ratings in the study might not seem very far apart.
Electric barriers emit an underwater current designed to repel fish and jolt those that don't turn back. The Army Corps says the presently operating barrier is working well, although Asian carp DNA has been found beyond it. And the Corps acknowledged in December that small fish might be able to slip through the barrier in water flows created by passing vessels.
"A complete barrier that the fish can't swim through is better than one they could swim through under certain conditions," Chapman said.