The latest federal research on Asian carp suggests the invasive fish does not necessarily live everywhere its DNA turns up.
The U.S. Army Corps of Engineers says the DNA can be transported by storm sewers, fish-eating birds, barges or equipment used in fisheries.
The report has implications for the debate over how to limit the spread of silver and bighead carp, whose voracious appetites have led them to crowd out native fish once the Asian carp are established.
The St. Croix River is among the places where silver carp DNA -- but no fish -- have been found. Minnesota's Department of Natural Resources launched an extensive search for Asian carp on the river in the summer of 2011, after the DNA turned up.
The Corps is working with the Fish and Wildlife Service and the Geological Survey on a three-year study of Asian carp, as the government works to keep the fish from reaching the Great Lakes and threatening the $7 billion fishing industry there.
Wednesday's report was released as elected officials including Gov. Mark Dayton and U.S. Sen. Amy Klobuchar attended an Asian carp summit meeting in St. Paul. Klobuchar renewed a call for the Corps of Engineers to close the Mississippi River shipping lock at St. Anthony Falls in Minneapolis to block the carp's ascent.
Last month the DNR proposed a barrier in St. Paul. It involves a system of lights, sound and bubbles that biologists say would discourage fish from entering the locks alongside the Ford dam.