"These fish prefer quiet waters in large river systems. You'll find them in the back waters and side channels, and Lake Calumet really fits that to a t. What we're trying to determine now is might that fish represent an individual fish on the lake or might it be part of a larger population," Rogner said.

Gillibrand and Slaughter want to temporarily shut locks to prevent the fish from getting into Lake Michigan. Industry groups say the waterways need to stay open for vital shipping traffic.