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In the United States, the fish didn’t really reveal themselves to be a problem for several years after introduction. “It’s very, very common for invasive species to go through several generations where they aren’t really able to do too much with population expansion,” Chapman says. “Then something happens. Maybe it’s critical mass, or something changes in the environment, but you hit this exponential growth phase.”
And that’s what happened with silver and bighead carp in the late 90’s. First bighead, then silver about five years later. “So that lag phase lasted 20 to 25 years,” he says. Relative to the Great Lakes, Chapman worries that if the fish successfully invade but do not become visible, people will become complacent about the threat, possibly for decades, while the fish have time to reach that point of exponential growth. “It’s very difficult to keep the public focused on an issue for that long,” he says.
Once the population explosion occurred in the Mississippi system, the fish began to move inexorably north. Commercial
fishermen on the Mississippi complained that they would pull up nets with only Asian carp, and so many that the nets were
too heavy to haul, so the fishing sites were abandoned. One big wakeup call came in 1999, when a fish kill in a Mississippi
River backwater revealed 97 percent of the fish were Asian carp. Boaters complained that silver carp were leaping out of
the water, and it was becoming dangerous to water ski or even drive a boat—for protection, some people have built cages over their boat’s steering area. A key problem is that carp habitat is often similar to that preferred for fishing and water skiing.
In 2002, Jerry Rasmussen, a fish biologist with the United States Fish and Wildlife Service, wrote a report titled “The Cal-Sag and Chicago Sanitary and Ship Canal: A Perspective on the Spread and Control of Selected Aquatic Nuisance Fish Species.” Rasmussen foretold with disturbing accuracy the situation in Chicago today: “... it is easy to speculate that the
Asian carps will navigate the Cal-Sag and Chicago Sanitary and Ship Canal system with ease, and unless control measures
are taken soon, they should easily find their way into Lake Michigan within 2 to 3 years.”
Last fall and winter, researchers say they proved the accuracy of Rasmussen’s predictions. Through a novel process called environmental DNA (eDNA for short) testing, a team from the University of Notre Dame found evidence of at least one silver carp in Lake Michigan near the mouth of the Calumet River and evidence of additional carp, both bighead and silver, in other parts of the Chicago waterway system close to Lake Michigan. The eDNA process involves sampling water and looking for DNA markers unique to Asian carp—fish leave shreds of DNA in fish slime, feces, and urine.
Some people have challenged the certainty of the data, given the newness of the procedure, but Phil Moy, a Wisconsin
Sea Grant scientist and co-chair of the Great Lakes Aquatic Invasive Species Panel, says the “the chance of it being false
positives is almost nonexistent.” In an independent technical review of the study and test procedures, scientists said the
data should be considered “actionable”—meaning it’s solid enough to guide policy decisions. “If we didn’t have the eDNA
technique, and were still relying on nets and electro fishing as monitoring technique, we probably wouldn’t even know they
are at the barrier, let alone past it,” Moy says. The eDNA discoveries in late 2009 thrust the carp issue into the media like no other results have and convinced the government to put higher priority on stopping the invaders.
In a dramatic response in December 2009, when the electric fish barrier was partially shut down for maintenance, the
government convened more than 400 people downstream of the barrier to help poison a 5.7-mile stretch of the Sanitary
and Ship Canal with Rotenone, a toxin that prevents oxygen uptake by organisms with gills. Crews collected 30,000
to 40,000 dead or surfaced fish that day, but only one was an Asian carp, a bighead. But a caveat: Asian carp generally
don’t float to the top when they are poisoned, so researchers believe more than one carp was probably killed.
The eDNA findings also convinced Michigan and Wisconsin to ask the Supreme Court to force Chicago to immediately close the two locks on the waterway, the O’Brien lock and the Chicago River lock. The states first filed suit in the fall when eDNA showed evidence of carp near Lake Michigan and again this winter when a sampling showed evidence of a carp in Lake Michigan—but the court refused to close the locks.
Meanwhile, Michigan’s federal legislators Senator Debbie Stabenow and Representative Dave Camp have introduced companion legislation to force the locks’ closure, but the bills lie dormant. Even if the locks were to be closed, the fish could still migrate unimpeded up the Little Calumet River to Lake Michigan.
If the Notre Dame eDNA evidence is accurate, and the fish have made it to Lake Michigan, what does that mean? The question is obvious, but the answer is not. One of the biggest problems with invasion science is there are so many unknowns.
For starters, the carp would need adequate food. Some biologists speculate that zebra and quagga mussels have removed so much plankton from Lake Michigan that the Asian carp could not thrive. Chapman, however, feels the fish would adapt. He knows of a lake in Hungary with both zebra mussels and Asian carp, and the carp are doing well in the low-plankton environment. Researchers suspect the carp have changed their diet; one possibility is partially digested food called pseudofeces that’s produced by zebra mussels, which are very similar to quagga. Studies will be underway.
The fish also need the right water temperature. Lake Superior might be too cold, but the other Great Lakes would not present a problem—the carps’ homes are at the same latitude as the lakes. As for predators like lake trout and salmon, they would only have a shot at the carp during the earliest phases of the carps’ lives.
The carp also need the right habitat to reproduce—and this is where biologists see the greatest potential for keeping the population down if they do invade. The eggs from silver and bighead must drift, and in order to drift, they need to be laid in water that moves fast enough to keep them from sinking to the bottom for about 30 hours until they hatch. After the eggs hatch, the larvae continue to drift for about 70 to 80 hours until they can begin swimming. What this means is the carp need a river with an unimpeded flow of about 60 miles. If the river runs too short or the current too slack, the
eggs and or larvae sink to the bottom and die.
In Lake Michigan, a half dozen rivers appear to meet these criteria. To stymie reproduction, one strategy would target those rivers with things like acoustic bubble barriers (a barrier that uses sound and a curtain of bubbles to drive fish away,
which is safer than electrifying the water) in rivers where the fish could spawn. Chapman’s concern is, just as with the fish
adapting to a new food source, carp might also be adaptable enough to hatch eggs in less-than-ideal conditions.
But Chapman also sees hope: “One thing we know is that these fish have been introduced all over the world,” he says. “And in no place have they been able to establish a population without approximately 60 miles of flowing water.” For there to be a big impact on the lake, a big population of carp is required, so biologists will launch a number of studies in coming months to determine if carp can reproduce in Lake Michigan tributaries, and to devise methods for interrupting that process.
But all of this is conjecture about an invasion that biologists concede is too complex to predict the results. “Speculating is very difficult,” says Dr. Dan O’Keefe, a fish biologist with Michigan Sea Grant, based near Grand Haven. He poses the question: Would anybody have thought that when zebra and quagga mussels invaded, they’d clarify the water so much we’d have cladophora blooms, and those blooms would give rise to botulism, and that round gobies—another invasive—would become infected, and birds would eat the gobies and get botulism too, and we’d have thousands of birds dying from botulism because of a mussel invasion?
“Perhaps changes due to Asian carp would not be this large, but they could be even larger—nobody can know,” O’Keefe says. “It’s best we don’t find out.”