Oh, carp!

In the worst-case scenario, all that's preventing Lake St. Clair from becoming a field of giant jumping carp is some poisonings, a few nets and an underwater Taser of sorts: electric "fences" in Illinois waterways designed to keep the invasive fish from reaching the Great Lakes.

Because if they do, according to some scientists' predictions: disaster.

Originating from the great rivers of China, the fish are adapted in the finest Darwinian traditions to their home waters. They feed on plankton. They swim upstream to spawn. They prefer warmer, fresh water.

But in the Great Lakes? They could add to the plague known as invasive species — think zebra mussels — that cost the region millions of dollars in controls and unmeasurable environmental damage.

The closest the carp have been spotted is about 40 miles inland from Lake Michigan, but some tests have shown they could already be in the Great Lakes. If they spawn, the lakes' other fish could virtually disappear. That's what's happened in the rivers the carp already have infested, including parts of the Mississippi River and many of the Midwest's other river systems.

The fishing industry could collapse. Boaters could be injured by leaping fish. Who knows how far the fish could spread through the world's largest freshwater basin?

Sure, they were recruited to America for a worthy mission: a non-chemical means to clean up vegetation, pollution, even animal waste. But now they are not a welcome population.

Four species of carp are in the United States, but it's the bighead and silver varieties that are the most troublesome and that have been detected just miles from Lake Michigan. With the U.S. Supreme Court refusing on Monday to hear Michigan's suit to close the Illinois waterways and protect the lakes, the future threats from the fish are worth some explanation.

Are they already in the Great Lakes?

If they are, there aren't very many, says Ashley Spratt, spokeswoman with the U.S. Fish and Wildlife Services, and that means they'd be having difficulty reproducing. "The good news is we currently have no evidence to suggest a sustainable population of Asian carp exists in the Great Lakes," she says. Yet.

Where did Asian carp come from?

The fish's native habitat is in southeast Asia's waterways, mainly in China.

How did they get to the United States?

In the 1970s, fish farmers at the southern end of the Mississippi River began importing the silver and bighead carp, which grow quickly and live for decades. Fish farmers and researchers started testing the carp's ability to clean up pollution and waste in fish ponds, thinking they had application to do the same for municipal water systems and agricultural sites. With help from the state and federal governments, some researchers cultivated the fish. A 2008 report from the National Aquaculture Research Center in Stuttgart, Ark., and the University of Arkansas found that universities, state and federal entities "contributed significantly" to the spawning and release into "natural waters" of the silver and bighead carp. Researchers with the Illinois Natural History Survey trucked carp to ponds on hog farms to see how well they cleaned up manure.

As the carp in Arkansas left ponds and reached the Mississippi River, they moved upstream to spawn and continued to head north.

Does this mean the federal government should have a bigger role in controlling the carp's spread?

It should definitely be considered, says Nick Schroeck, executive director of the Great Lakes Environmental Law Center in Detroit. "The story has been that it's these farmers who brought in the carp to help clean the catfish ponds, and really the full story is much more compelling. The government funded and supported the research and actually brought Asian carp into this country," he says. "We need to have the full picture of how the carp were introduced into the waters and who ultimately bears responsibility for stopping their spread and dealing with the fallout."

But others aren't so sure it matters. "When you get back into the history of this, there's enough blame to go around," says Mike Freeze, an Arkansas fish farmer and former state game commissioner who was involved with the early research. "The problem is there were so many agencies involved."

All agree the focus needs to be on controlling the fish.

How did they get from fish ponds to the doorstep of the Great Lakes?

Their natural migratory pattern takes them upstream, most scientists agree. Some Arkansas researchers question whether fish from ponds there are truly the source of the Illinois population. But fish biologists have tracked the fish as they have headed north.

Aren't the mississippi river and the great lakes separate systems?

They used to be. At the end of the 19th century, Chicago's leaders and the Illinois Legislature had the flow of the Chicago River reversed so that it didn't drain into Lake Michigan and bring pollution to the treasured lakefront. The plan included constructing a canal — now the Chicago Sanitary and Ship Canal — to connect the Chicago River to the Des Plaines River southwest of the city. That way Chicago's pollution would flow to the Illinois River and ultimately to the Mississippi and not into Lake Michigan where it was offensive to residents. Now that connection — and the locks in the system — allows fish, plants and pollution to travel between the once-separated systems.

Why so much focus on Illinois?

While the U.S. Geological Survey has documented the four types of Asian carp in 23 states, the silver and bighead species' presence in the northeast Illinois waterways causes the most concern. From there, they can get directly into Lake Michigan, the four other great lakes and Lake St. Clair. As the carp spawn by heading upstream, their spread could actually reach several miles inland into streams and smaller lakes.

What would happen if they got into Lake Michigan and the rest of the Great Lakes? 

Duane Chapman, a research fish biologist with the U.S. Geological Survey, predicts it will take about a decade before the effects of invasive carp would felt on the Great Lakes. Others are less sure the carp will thrive in the Great Lakes, but a significant number of experts believe the results will be catastrophic.

What exactly are the potential threats?

Based on what's happened on the rivers, the U.S. Fish and Wildlife Service predicts three major threats: a catastrophic decline in native fish populations, devastating economic impacts to sport and commercial fisheries and injuries to boaters who are struck by the fish.

Because the silver carp jump up to six feet out of the water when they are disturbed by motors, recreational boaters and others look at what's happened on the Illinois River, for example, with dread. Motor boating, waterskiing and sport fishing all entail the danger of being slammed and injured by jumping fish. Many such injuries have taken place. As the fish generally prefer shallows, many scientists believe they would congregate along Great Lakes shorelines where most recreational boating takes place.

How would they decimate other fish species?

Because they're gluttons. Asian carp consume as much as 40 percent of their body weight a day and have weighed in at 100 pounds. Carp would compete with cisco, bloater and yellow perch for finite supplies of plankton to eat. Those three smaller fish are eaten by predator species including lake trout and walleye. So eventually the carp could outnumber all native fish, says Spratt of the U.S. Fish and Wildlife Service.

Has that happened?

Yes, on parts of the Illinois, Mississippi and Missouri rivers, Asian carp are the most common fish.

What about the Great Lakes fishing industry?

Commercial fishing is estimated to be a $7 billion industry, so losing it would be quite an economic hit.

Why is the Great Lakes ecosystem so significant?

As the Great Lakes are about 20 percent of the world's fresh water supply, a major change in its ecostructure scares many scientists and environmentalists. The lakes are home to federally and state-listed threatened and endangered fish, mollusks, plants, mammals, insects and reptiles. Other invasive species have affected these populations. Scientists fear that Asian carp could cause irreparable harm to the Great Lakes.

How would the fish migrate in Lake Michigan and the rest of the lakes?

"They're big fish, so we know they're definitely going to move," says Kelley Smith, the Fisheries Division chief at the Michigan Department of Natural Resources and Environment. "A lot of people ask me how fast they will spread and we just don't know." Whether they would tend to travel up the Wisconsin or Michigan sides of Lake Michigan isn't known, but Smith says Michigan offers some "great habitats for them." The carp may prefer the relatively warmer waters of southwest Michigan's rivers — the St. Joseph, Black, Grand — for spawning. If they travel around the mitten and into Lake Huron, then Saginaw Bay and Lake St. Clair could be prime habitat for them.

What about Lake St. Clair?

Kevin Irons, an ecologist with the Illinois Natural History Survey, doesn't think Asian carp will spawn very well in the Great Lakes because of the lack of current, but Lake St. Clair's current makes it a different story. It's also shallow and warm to their liking. By the way, since they can swim as much as 30 miles a day, the southeast Michigan waters are within their range in the not-so-long run.

What is the basis for the opposition to separating the Illinois waterways and Lake Michigan?

Short answer: money.

Long answer: money.

The waterways are used for commercial transport and for recreation and tourism. Sightseeing boats, for example, dock on the Chicago River and traverse the locks into Lake Michigan for shoreline tours. Barges carry freight on the canals and rivers in a tradition that pre-dates the interstate highway system. There is debate about the cost of shutting down such traffic. John Taylor, a Wayne State University associate professor of marketing and supply chain management, says Illinois grossly overstates the costs of closing the locks. He says net jobs could actually be created if trucking and railroad shipping replaced water routes.

What about just separating the lakes from the Illinois rivers?

Smith, of Michigan's DNRE, says keeping creatures from Lake Michigan out of the Mississippi system is also a concern. "Both sides have the same goal: minimize the introduction of invasive species because of the problems they cause," he says. Ecologists and other scientists have long discussed separating the systems. "It's really accelerated as a result of Asian carp," Smith says.

The U.S. Army Corps of Engineers is considering the separation as part of its Great Lakes and Mississippi River Interbasin Study, says spokeswoman Lynne Whelan. Part of the study, to be done by a multi-agency team, will look at all invasive species and their impact on all of the Great Lakes.

What controls are in place to keep the Asian carp at bay?

Scientists and environmentalists have been watching Asian carp heading north for the last decade and have taken steps to prevent the fish and other invaders from entering Lake Michigan. In 2002, "electric fences" were installed to prevent carp and other invasive species from getting through locks and into Lake Michigan. Fish kills are common and truckloads of the carp have been taken out of the rivers.

Are they working?

It's unclear. No Asian carp have been found in Lake Michigan, but some scientists believe they are there, just not enough for us to find, or (hopefully) for them to find each other and reproduce. Earlier this year, a new test detected carp DNA beyond the barriers and closer to the Great Lakes.

What other measures have been proposed?

Some of them are physical: shutting the locks permanently or filling in the canals with dirt, for example. But other temporary measures have been proposed, like so-called bubble screens and lighting that deter the fish from moving through those areas. Other plans include targeted fishing and greater DNA testing, continued poisoning with rotenone, and studying of Asian carp spawning habits.

What is the significance of the U.S. Supreme Court's announcement Monday that the court would not consider a request from Michigan Attorney General Mike Cox to re-open a century-old case challenging Chicago's diversion of Lake Michigan waters?

It means states would need to file suit in federal court in Illinois to force the closure, which could take years. It means the Obama administration, which argued that the court should not take the case, has assumed some responsibility for the carp's future as Solicitor General Elena Kagan contended the Army Corps of Engineers and other federal agencies had the problem handled. It means that the controversial case — which dates back to when Chicago reversed the flow of its river and moved water from Lake Michigan to the Mississippi River system — is still an issue for the Midwest. Missouri originally challenged Chicago's diversion of the river at the time, in part, because of sending pollution down the river. Chicago's "control" of the waterway and refusal to do what its neighboring Great Lakes states are asking now is the latest round in this historic dispute.

What other political efforts are being made?

Governors of most of the Great Lakes states, including Michigan's Jennifer Granholm, have urged Illinois to close the Chicago canals and the locks. Bills are pending in both houses of Congress to direct the U.S. Army Corps of Engineers to close the locks and separate the Great Lakes from the Illinois rivers. Michigan legislators this year have begun holding public hearings and urging constituents to write to the White House, which so far has favored controls, not waterway closure.

Is President Obama favoring his adopted home state?

Many make that claim. He has sided with Illinois and publicly spoken against lock closures. He did hold a White House summit in February that was attended by several Great Lakes governors and has supported $78.5 million toward carp control.

If we can't beat 'em, can we at least eat 'em?

Yes, although Americans generally prefer firmer fish. Still, an article titled "Can China eat enough Asian carp to save the Great Lakes," recently published in OnEarth, the online magazine of the Natural Resources Defense Council, detailed some of possibilities, including gefilte fish, in which bones are ground up with the meat. In China, Asian carp are very popular, and as rivers there are overfished, large fish aren't available. The Big River Fish Co., in Pearl, Ill., processes millions of pounds of fish a year, shipping as much as 40,000-pound containers of frozen Asian carp. They've got a contract for 30 million pounds to go to China.

What do they taste like?

Kevin Irons, from the Illinois Natural History Survey, reports the bighead carp are "actually pretty good eating, if bony." While the prevalence of bones makes processing them a bit more expensive, there may be a market for the fish as menu items.

For a dinner party, Irons served them deep-fried in cornmeal, along with yellow perch and trout. "People wanted to try it," he says. They're a fattier fish. "If you trim off the fattier meat in the whole process of filleting, it is very good eating."