CHICAGO  After months of worrying over hints and signs and DNA traces suggesting that Asian carp, a voracious, nonnative fish, might be moving perilously close to the Great Lakes, the authorities here have uncovered the proof they did not want. They caught a fish.

One bighead carp  a 19.6-pound, 34.6-inch male  became entangled Tuesday in a fishing net about six miles from Lake Michigan, in part of a waterway that connects the Mississippi River system to the Great Lakes.

The authorities have searched for nearly a half-year with nets, chemicals and electrofishing equipment, but the fish was the first actual Asian carp to be found beyond an elaborate electric fence system officials spent years devising to avoid this very outcome.

Image An Asian bighead carp like these in an aquarium made it past a fence designed to keep it out of the Great Lakes. Credit... M. Spencer Green/Associated Press

The question state and federal authorities here are now racing to answer is whether the fish was somehow traveling alone  a prospect some environmental advocates consider absurd.