DNR: Bacterial infection likely caused fish die off in Fox River, Bay of Green Bay

The Wisconsin Department of Natural Resources (DNR) today announced that fish collected during the recent die-off in the Fox River and lower Green Bay waters were diagnosed with severe cases of the bacterial disease columnaris.

Reports of dead fish began June 18, leading DNR staff to conduct a two-week investigation into the cause of death. While it initially appeared that species other than channel catfish, such as carp and sheepshead, were affected, an estimated 99% of dead fish found over the course of the investigation were adult channel catfish. Only four of the dead channel catfish reported were suitable for testing.

“Despite almost two weeks of responding to public reports and searching, it was difficult to find either sick or freshly dead fish suitable for testing. Most of the fish seen had been dead for at least a day,” said Dave Boyarski, DNR East District Fisheries Supervisor. “Once a fish dies, they start to decompose and can quickly become unsuitable for testing, especially if the water is warm.”

Most dead fish were found downstream of the De Pere Dam and in lower Green Bay, but some fish were reported as far north as Sturgeon Bay and upstream in the Fox River near Wrightstown. It’s unknown where the fish originated from and is possible that strong currents and wind were pushing the fish north into Green Bay.

Columnaris is a common and widespread bacterial disease of freshwater fish caused by the bacteria Flavobacterium columnare, which can cause yellow-brown or white lesions on the skin, fins and gills of fish. Columnaris outbreaks are a normal occurrence, especially in late spring and early summer when the water is warming up.

“It is unusual to see a large die-off of wild, adult channel catfish, but they are known to be susceptible to columnaris,” said Nicole Nietlisbach, DNR Aquatic Veterinarian. “It is possible that warm weather and high levels of organic matter in the water from recent rainstorm runoff created a very favorable environment for Flavobacterium columnare growth during the catfish spawning season, which is an already stressful time for the channel catfish.”

While this particular die-off event seems to have resolved, the DNR encourages members of the public to continue to report sick or dead fish to their local fisheries biologists.

The columnaris bacteria does not infect humans or other animals and water quality samples collected from the area were normal. However, since all wild fish can harbor other bacteria and parasites harmful to humans, anglers are always encouraged to thoroughly cook their catch, never consume fish you find dead or dying, and follow the Center for Disease Control’s food safety guidelines.