U.S. authorities hunting for Asian carp in waterways near Chicago say they didn't find any during four days of intensive monitoring.
Crews used electric jolts to stun fish and set huge nets during stepped-up surveillance of Lake Calumet and the Calumet River this week.
The efforts came after tests found DNA from the carp in the waterways beyond electric barriers meant to keep them out of the Great Lakes.
The barriers repel the fish by giving them a jolt, and are in canals connecting Lake Michigan to the Mississippi River watershed.
Scientists fear the voracious carp could starve out prized Great Lakes sport fish if they get into the lakes.
One Asian carp was caught last year in Lake Calumet but officials say they're not sure how it got there.