Officials say a three-day sweep turned up no Asian carp in Chicago's Lake Calumet, where DNA from the invasive fish had been found repeatedly in recent months.
A federal and state plan calls for intensive searches in the Chicago waterway system whenever three consecutive rounds of sampling detect genetic material from bighead or silver carp.
Authorities are trying to prevent the plankton-hungry carp from reaching the Great Lakes, where scientists say they could destabilize the food chain.
From Wednesday through Friday of last week, crews deployed over six miles of nets in Lake Calumet and used electric currents to herd fish toward them. Some nets were designed specifically for the carp.
Officials reported Monday that more than 6,300 fish representing at least 30 species were caught, but no Asian carp.