Recent water samples collected from the Kalamazoo River, about 20 miles upstream from where the river flows into Lake Michigan, tested positive for genetic material from Asian carp. It's the closest to Lake Michigan a positive sample has been found.

Asian carp are an invasive species first introduced several decades ago in the South and Midwest. They're aggressive eaters and prolific reproducers, and have greatly expanded their range in recent years, threatening native species.

Fishermen who ply their trade in the waters of the Great Lakes have long been worried over what the economic and environmental implications of a silver carp invasion might entail for their livelihoods.

"Although not conclusive, this finding heightens our vigilance and sets into motion a specific response," said Keith Creagh, director of the Michigan Department of Natural Resources, in a statement. "We will work with our partner organizations and anglers on next steps to protect the Great Lakes and its tributaries against this significant threat."

Though the positive test is alarming, officials say it's not proof that Asian carp have established any sort of self-sustaining population in Lake Michigan's waters.