The Army Corps of Engineers has modified the operation of Lock and Dam Number Eight to keep the invasive Asian carp from spreading up the Mississippi River. The dam’s spillway gates have been tweaked south of La Crosse at Genoa to prevent adult carp from swimming northward. Also, the Corps has installed speakers below the lock gates with low frequency sounds that are designed to deter Asian carp but not native fish species. Peter Sorensen and a team of University of Minnesota researchers suggested the changes. Officials say they don’t cost anything, and northbound barge traffic on the river will not be affected.