CHICAGO - As Asian carp have stormed up the Illinois River in the past several decades, looming precariously close to Lake Michigan, scientists have been forced to ponder an alarming question: What if the invasive species actually breached the world's fifth-largest lake?
Many fishery managers have already resigned themselves to the fact that bighead and silver carp, the two most-feared species of Asian carp, may never be eradicated from Illinois waterways, as a single female can lay over 1 million eggs each year. These insatiable fish have proved capable of eating 120% of their body weight in a day, mostly microscopic plankton - the base of the aquatic food chain. In doing so, they deprive other fish while growing to as much as 100 pounds by adulthood, much too large for any native predator to feed on them.
However, since the 1990s, invasive zebra and quagga mussels have devoured so much of Lake Michigan's plankton that some experts have wondered whether Asian carp would be able to survive if they arrive here. Even if they could, some believe the carp would be limited to areas near shore where plankton is still plentiful instead of open water, which has essentially become a food desert.
A new University of Michigan-led study has turned that thinking on its head.
The research, published Monday in Freshwater Science, suggests that bighead carp, in particular, could fare a lot better than expected in Lake Michigan because scientists have underestimated the flexibility of their diet. In addition to plankton, the opportunistic eaters can sustain themselves by feeding on the feces and mucous-coated regurgitation of invasive mussels.
By factoring in the luxuriant layer of mussel excrement that has accumulated on the floor of Lake Michigan, the new computer models show the entire lake would provide suitable habitat for bighead carp at certain times of the year.
"The significance of (open water) being suitable, but not necessarily high-quality habitat, is that it could be a migration corridor these fish could travel across while seeking out more productive areas," said study lead author Peter Alsip, who conducted the research for his master's thesis at U-M's School for Environment and Sustainability. "So, they might be able to manage or mitigate their weight loss, which could facilitate their spread until they get to areas where they can be fat and happy, like Green Bay."
The new findings not only challenge previous thinking, but raise fresh concerns about the potential establishment and spread of bighead carp while plans to construct new defenses at Brandon Road Lock and Dam continue to plod along.
"This study shows that Lake Michigan would not be the end of the road for Asian carp," said Molly Flanagan, vice president of policy at Chicago-based nonprofit Alliance for the Great Lakes. "Rather, the lake would be a way for them to spread to tributaries and surrounding states to access other Great Lakes."
Because Asian carp haven't invaded a body of freshwater the size of Lake Michigan, it's difficult for scientists to say how they might disperse across its 22,000 square miles, roughly the size of West Virginia. While much of it would be considered poor habitat, because its plankton supplies have been drained by quagga mussels, the small amount of fertile waters could ironically buoy reproduction.
If multiple carp were able to invade, it's unlikely they would encounter one another if the lake was rife with plankton. However, since their favorite food is most abundant near the mouths of rivers - which could also serve as spawning habitat - that could act as a beacon to those areas.
"The vast amount of poor habitat might actually concentrate them in these areas," Alsip said. "And so, rather than deterring them, the poor offshore habitat might actually facilitate localized establishment."
Past research already suggests the prolific migrators could reach nutrient-rich areas within the first year of invading Lake Michigan through the Chicago River.
According to Monday's study, the outlets of the Fox River in Green Bay, the Milwaukee River and St. Joseph River in Michigan would be capable of supporting Asian carp most of the year.
In the computer simulations, silver carp were mainly confined to Green Bay. But the species could find suitable habitat across 11% of Lake Michigan during the summer months, including near Chicago, Milwaukee and parts of southwestern Michigan's shoreline.
Bighead carp, likewise, would thrive in Green Bay, but could sustain themselves across at least 97% of Lake Michigan from June through November, and the entirety of the lake in September and October.
Neither bighead nor silver carp have a true stomach, requiring them to feed almost constantly, according to experts. With spongelike gill rakers, the silver carp specialize in filtering microscopic algae, even outcompeting accompanying schools of bighead carp.
However, the hardy bighead carp appear to be more suited to brave a food-limited Lake Michigan, because they tend to grow faster and larger while expending less energy to maintain their weight.
"If silver carp truly require more food like our results suggest, then it is possible that they eat more food and leave less for the bighead and other species," Alsip said. "In an environment like Lake Michigan, this could backfire on them, seeing as there is far less food than the environments they exist in today, which might favor a fish that can survive on less."
And getting by on less could mean resorting to ingesting mussel feces, which provide a fraction of the energy in plankton.
The signs that Asian carp have an appetite for fecal matter have been there for some time. While Asian carp were originally imported to the U.S. to devour nuisance algae blooms in sewage treatment facilities and aquaculture ponds, the carp escaped confinement due to flooding and developed a mixed diet in the wild.
Though Asian carp are known to graze on plankton, scientists probing the guts of these fish in the Missouri River found that they primarily ate detritus.
In 2012, researchers deliberately fed Asian carp the waste from invasive mussels in a lab setting. The experiment showed that carp were not only capable of surviving by solely feeding on mussel waste, but some actually gained weight.
Though the majority of the carp trimmed down, researchers said they suspected the fish weren't fed as much mussel excrement as they desired, and the results might be different in Lake Michigan where much more waste is available.
Monday's study might be a considered a conservative estimate of potential food sources for Asian carp. Scientists say they didn't consider harmful algae blooms, plumes of cyanobacteria that even quagga mussels won't eat, which are a delicacy for carp. These photosynthetic bacteria can produce toxins harmful to humans and some animals, though Asian carp appear to be impervious.
"The mussels will eat everything and spit out the cyanobacteria," Alsip said. "And bighead and silver carp will eat the cyanobacteria. So, there's room for coexistence if there's enough cyanobacteria. "
Climate change could make conditions even more suitable, especially for bighead carp. Heavier rainfall is expected to flush more agricultural runoff into the Great Lakes, which triggers these massive blooms in shallower embayments like Green Bay. Lake Michigan's surface water temperature has warmed over the past several decades, and if that trend continues, it could expand suitable carp habitat and allow them to actively forage for more of the year.
As for now, the priority remains keeping Asian carp out.
In May, Gov. J.B. Pritzker wrote a letter to the Army Corps of Engineers authorizing the agency to move forward with preconstruction, engineering and design for a channel with additional Asian carp deterrents at the Brandon Road Lock and Dam near Joliet. A final version of the nearly $831 million project, approved by the Corps of Engineers' top general, includes the installation of an electric fence, a bubble barrier and underwater speakers.
If funded in a timely manner, construction could be finished in 2028.
In the Great Lakes region, the governors of eight states and premiers of two Canadian provinces have endorsed the plan. Many dispatched staff to Chicago last month to discuss the project and potential cost-sharing.
If the project is to advance, Pritzker will need to pen another letter guaranteeing funding for the pre-construction phase, for which the state of Illinois must assume responsibility for $10 million in costs over three years. Michigan, the state with the most Great Lakes shoreline, has offered to pay for $8 million toward this end.
Federal lawmakers must sign off on funding on their end, too.
A broad spending bill with additional funding for the Army Corps of Engineers has already passed in the Democratic-majority House of Representatives. Once the Republican-led Senate returns from its summer recess, it will be up to senators to pass their own spending bill.
In a show of bipartisanship, a congressional delegation traveled to Joliet last month to tour Brandon Road Lock and Dam and see firsthand where provisions could be deployed to halt Asian carp. For some, like Sen. Debbie Stabenow, co-chair of the Senate Great Lakes Task Force, the U-M study was another reminder of the risks.
"This study further underscores what's at stake for our Great Lakes and way of life if Asian carp reach Lake Michigan," Stabenow said in a statement. "I am laser focused on completing the Brandon Road Lock and Dam project as quickly as possible to stop these fish from ever becoming established in our lakes and waterways."
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