DNR Lakes and Streams survey

In this Aug. 16, 2011 photo Al Stevens, center, a DNR Lakes and Streams survey coordinator, and other workers travel along the shoreline sending electric shocks deep in the water to stun fish in an attempt to find silver carp in the St. Croix River near Taylors Falls, Wis. The discovery of Asian carp DNA in the river comes as scientists and engineers are developing a strategy for preventing the invasive fish from reaching the Great Lakes through points other than Chicago's shipping canals.