TRAVERSE CITY, Mich. (AP) — Genetic material from Asian carp has been discovered in Lake Erie water samples collected nearly a year ago, officials said Friday.

Researchers with the University of Notre Dame, Central Michigan University and The Nature Conservancy detected DNA from the invasive fish this week when examining more than 400 samples taken in August 2011. It’s the first time DNA from bighead and silver carp has turned up in Lake Erie, although three bighead were caught there between 1995 and 2000.

Scientists are uncertain about whether carp DNA signals the presence of actual fish, but the findings are unsettling because experts have described Erie as the lake that could suffer the biggest harm from an Asian carp incursion. Some say the DNA could be from other sources, such as feces from fish-eating birds.

Chris Jerde, a Notre Dame biologist and member of the team that discovered the DNA, said the most likely explanation is that live Asian carp have reached the lake, although their numbers and how they got in remain unknown.

“The number of alternative explanations is dropping precipitously,” Jerde said. “It’s still not a game-over situation. We don’t know how many fish there may be at this point. But the alarm bell has been sounded.”

Four positive hits for bighead carp were found in samples from Ohio’s Sandusky Bay — less than three miles from where the live bigheads were caught years ago. Two hits for silver carp turned up in water from northern Maumee Bay in Michigan.

The samples that tested positive were among 2,000 that Jerde and his colleagues took from Lakes Michigan, Erie and Superior last year as part of a broader search for invasive species. They have been processing the samples since then and found no Asian carp DNA until this week, he said.

The only other Asian carp DNA found in the Great Lakes has been one sample taken from Lake Michigan’s Calumet Harbor at Chicago.

Officials with Michigan, Ohio and the U.S. Fish and Wildlife Service will plan more water sampling next week, said Jim Dexter, fisheries division chief with the Michigan Department of Natural Resources.

“We have to assume that at the time the sampling was done, there was the presence of live fish,” Dexter said. If another round gets positive readings, “our efforts would ratchet up considerably” and could include bringing in commercial fishermen to search for the unwanted carp, he said.

Although the smallest of the Great Lakes by volume, Erie has the most abundant fish population thanks to its relatively warm temperatures and plentiful food supply.

Asian carp have moved steadily northward in the Mississippi River and its tributaries since escaping from Southern fish farms and sewage lagoons in the 1970s. They gobble huge amounts of plankton, a crucial nutrient for many fish.

An electric fish barrier near Chicago is meant to keep Asian carp out of the Great Lakes. More than 130 samples taken beyond that barrier in the waterways south of Lake Michigan in the past three years have tested positive for Asian carp. But despite intensive searches — the latest of which took place this week — just one live bighead and no silver carp have been found there.

A U.S.-Canadian report released this week said if the carp become established in the region, they could spread to all five Great Lakes within two decades and cause some native species to decline.

Scientists with the U.S. Geological Survey reported in January that the Maumee River, which flows into Lake Erie at Toledo, Ohio, is a highly suitable area for Asian carp to spawn. The Sandusky River was described as moderately suitable.

In 2010, officials placed a chain-link fence in a wetland area near Ft. Wayne, Ind., to prevent the carp from migrating from the Wabash River — which they’ve already infested — to the Maumee River.

“This alarming discovery underscores the need for action now to stop Asian carp and other invasive species from devastating our Great Lakes and the hundreds of thousands of Michigan jobs that depend on them,” said U.S. Sen. Debbie Stabenow, a Michigan Democrat.

Congress recently gave the U.S. Army Corps of Engineers an 18-month deadline to complete a study of how to prevent species invasions into the Great Lakes.