Water samples taken from Sandusky Bay and Sandusky River have tested positive for the DNA of Asian carp.
Officials say Tuesday that a about a dozen of the 150 samples taken had traces of DNA. The samples were collected at the end of July as part of sampling begun earlier this summer for the invasive fish in western Lake Erie.
No live Asian carp were found and according to researchers the DNA could come from a variety of sources.
"The breadth of positive samples from the Sandusky Bay area was not expected," Michigan Department of Natural Resources Fisheries Chief Jim Dexter said in a news release. "We need to understand the source of the DNA in order to address it and keep silver and bighead carp from establishing a viable population in the Great Lakes."
Ohio Natural Resources spokesman Rich Carter says testing for live Asian carp or other sources of environmental DNA will be increased.
Scientists fear bighead and silver carp could kill native fish and unravel food webs in the great lakes' ecological systems.