Invasive silver carp have been moving north toward the Great Lakes since their accidental release in the 1970s. The large filter-feeding fish, which are known to jump from the water and wallop anglers, threaten aquatic food webs as well as the $7 billion Great Lakes fishery. But, for the past decade, the invading front hasn't moved past Kankakee. A new study, led by scientists at the University of Illinois, suggests that Chicago's water pollution may be a contributing to this lack of upstream movement.