Winter 2007

Muskie fishing, Lake St. Clair, Roseville, Michigan. Photo: Michigan Travel Bureau
UW Sea Grant Research

A Freshwater Debut: New Fish Virus Poses Serious Threat

By John Karl

A fish virus discovered in Lake Ontario in 2005 seriously threatens the sport and commercial fisheries of the Great Lakes region, experts say. The highly contagious virus can kill 80 percent of the fish it infects, and it has been found in walleye, smallmouth bass, muskellunge, and many more species, including fish harvested and raised in the baitfish industry. The virus has already caused large die-offs in lakes Ontario, Erie, and St. Clair. Affected species have included yellow perch, freshwater drum, northern pike, and others.

The virus is called viral hemorrhagic septicemia, or VHS. Analysis of frozen fish shows it was in the Great Lakes at least as early as 2003. Prior to appearing in the Great Lakes, VHS was known only in the marine environments of the Atlantic and Pacific, where it infects salmonids.

While Great Lakes scientists and resource managers can learn much about VHS from their colleagues on the Atlantic and Pacific coasts, no one knows exactly what to expect of the virus in a freshwater environment. One difference in the Great Lakes region is heavier lake-to-lake boat traffic, which makes it likely the virus will spread into the region’s small lakes.

Infected fish can show no external signs, or they may display bulging eyes, bloated abdomens, decreased or increased activity, hemorrhaging, or lesions. However, these signs are common to many diseases, and a reliable diagnosis of VHS can only be made in a laboratory.

Infectious diseases often claim most of their victims early in an epidemic, according to Phil Moy, Wisconsin Sea Grant fisheries specialist. The most vulnerable individuals fall quickly, but more resistant ones survive and pass their resistance to their offspring, Moy said. While that general pattern might be expected with VHS, it’s not clear how extensive the impacts might be, and many questions remain unanswered, Moy said. Could it infect Great Lakes trout or salmon? How fast might it move through the lakes on its own? How much could movements of ballast water and trailered boats accelerate that movement? Can the virus be effectively cleaned off of boats, live wells, and fishing gear by washing them with a hose, or are stronger measures needed? Could the virus be transported out of the Great Lakes basin via the Chicago River to infect the Mississippi River?

To help answer questions like these, Wisconsin Sea Grant has established a programmatic priority of supporting research into “improved methods to identify, detect and control diseases, parasites and other pathogens” such as VHS in its recent call for proposals. “This virus could be a big issue in many of our thematic research areas, including aquaculture, biotechnology, and, of course, fisheries and aquatic invasive species,” said James Hurley, UW Sea Grant assistant director for research and outreach. “We’re looking for first-rate scientific research into the topic.”

To slow the spread of VHS, the U.S. Animal and Plant Health Inspection Service (APHIS) issued a Federal Order in late October prohibiting interstate transport of 37 species of live fish among the eight states bordering the Great Lakes. The order also prohibited imports of those fish from Ontario and Quebec. However, the restrictions prevented many baitfish aquaculture operations from sending their product to established customers across state lines, Moy said. The order has since been modified, and such transfers are now permitted provided the fish are inspected. The virus poses no threat to people, according to APHIS.

If you see a fish kill on the Great Lakes, please contact the Wisconsin Department of Natural Resources at (608) 266-8782 or call your local DNR office. For more information, see

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