TRAVERSE CITY, Mich. (AP) — Scientists have begun searching two of western Lake Erie's bays and tributary rivers for signs of dreaded Asian carp, officials said Wednesday.

State and federal agencies are focusing on sections of the Sandusky River and Bay and the Maumee River and Bay, where samples taken a year ago turned up DNA from silver and bighead carp. Because of processing delays, the positive results were announced only last month.

Asian carp have infested the Mississippi River and its tributaries. Authorities have spent more than $100 million on efforts to keep them out of the Great Lakes, where scientists say they could unravel food webs by gobbling plankton on which other fish rely.

Water samples are being taken this week for further DNA analysis. Next week, crews will survey fish populations, using electric currents that temporarily stun fish and make it easier to land them in nets.

In addition to revisiting areas where positive samples were collected previously, scientists will collect water from places that may be "reproductively favorable" for Asian carp, said Tammy Newcomb, research program manager for the Michigan Department of Natural Resources. "These are the areas where we can be most effective in preventing expansion of these species should they be present."

Researchers with the University of Notre Dame, Central Michigan University and The Nature Conservancy spotted DNA from the invasive fish in July when examining more than 400 samples that had been taken in August 2011.

Four positive hits for bighead carp were detected in water from Sandusky Bay, while two samples from Maumee Bay tested positive for silver carp.

It was the first time DNA from bighead and silver carp has been found in Lake Erie, although three bighead were caught there between 1995 and 2000. Asian carp DNA has been found repeatedly in Chicago-area waterways near Lake Michigan.

Scientists are uncertain whether the DNA signals the presence of actual carp but said the findings were unsettling because of all the Great Lakes, Erie likely would suffer most from an invasion. Although the smallest by volume, Erie has the most abundant fish population thanks to its relatively warm temperatures and plentiful food supply.

The Ohio DNR and the U.S. Fish and Wildlife Service are teaming up with Michigan on the sampling project, which will "inform and guide future assessment and management actions," said Rich Carter, the Ohio agency's executive administrator for fish management and research.