CHICAGO — This week’s oil spill in Michigan's Kalamazoo River is a bigger threat to Lake Michigan than invasive and voracious Asian carp, according to Chicago Mayor Richard Daley.

Some scientists and environmental groups, however, aren't so sure.

Mayor Daley is facing political pressure from environmental opponents and neighboring states, both of which say city and state barging interests are preventing the closure of two navigational locks that would prevent the Asian carp from reaching Lake Michigan through a shipping canal connecting the lake to the Mississippi River.

A pipeline operated by Enbridge, Inc., a Canadian company, ruptured Monday near Marshall, Mich., releasing 819,000 gallons of oil into the river. Only about 10 percent of the oil has been recovered. So far, no reason has been given for the break.

While the leak has been stopped, the oil is currently about 80 miles away from Lake Michigan. If the oil reaches Great Lakes waters, Michigan Gov. Jennifer Granholm warned, it would become a “tragedy of historic proportions.”

But is it really worse than Asian carp, which Great Lakes states worry could destroy their fisheries and wreck havoc on local economies?

Michigan and other Great Lakes states filed several lawsuits against Illinois and various federal agencies, charging that too little is being done to prevent the invasive species from crossing through a canal connecting the Mississippi to Lake Michigan.

The June discovery of a 19-pound Asian carp six miles from Lake Michigan became the most direct evidence that the fish may have found a way to move past the navigational locks meant to keep them from reaching the lake.

The latest lawsuit, filed by Michigan Attorney Mike Cox on July 19, seeks a temporary closure of those locks and the building of more protective measures. Mr. Cox said President Obama’s framework for addressing the program was wrought with “bureaucratic delays.”

Daley has dismissed those claims. “Asian carp are legal in rivers, but they want to make it illegal in a lake. ... If it’s legal in the Mississippi ... how do you say it's illegal in a lake?" he said.

Some scientists say the mayor’s argument that the oil poses a greater threat than Asian carp is just not true.

Stephen Hamilton, a zoology professor at Michigan State University in Lansing, called the two threats “incomparable.”

The oil is a temporary problem, Mr. Hamilton says. “If the Asian carp gets in there, it gets in forever and will spread to the rest of the Great Lakes and inland waters,” he says. “The far-reaching effects are forever, while the oil spill will be a blip on the screen.”

To be sure, the oil isn't a good thing for the Kalamazoo River, a thin waterway that is incapable of handling more than 30 miles of dense oil. Hamilton says the oil will not only harm the river’s ecosystem, it will also leave deposits in the vegetation and soil due to flooding this week.

David Jude, a fisheries biologist at the University of Michigan at Ann Arbor, says Asian carp and oil are “apples and oranges,” but he downplays immediacy of the carp threat. He says the Lake ecosystem has, in past years, been drained of the volume of nutrients the fish needs to survive in significant numbers. Increased sewage treatment plant regulations plus a recent zebra mussel infestation have both helped lower the amount of algae in the lakes - something the fish depends on to reproduce.

If Asian carp got into the lake, he says, the threat will be a slow build, between 10 to 15 years for the fish to grow in numbers that are devastating.

“Whereas with the oil spill, you’re still able to clean it up and respond so [the Lake] can rehabilitate itself,” he says.