Michigan, Wisconsin, Ohio, Minnesota and Pennsylvania want a federal judge to close Illinois shipping locks to prevent the invasive fish from overrunning the Great Lakes.
The first witness called by the states Tuesday was a biologist who found traces of Asian carp DNA in Chicago-area waterways near Lake Michigan.
David Lodge says the DNA most likely came from live carp.
Others have suggested the carp DNA could have been transported in the ballast water of barges and so the positive test don't necessarily mean the fish is present.
But Lodge says that and other explanations are far less plausible.