OSHKOSH, Wis. - It's another gorgeous morning for fishing on Lake Winnebago. Clear skies. Sunshine. Fishing boats cruising across the horizon.

But under these sparkling waters lurks an invader that anglers fear could devastate Wisconsin's billion dollar-plus fishing industry, and if left unchecked, threaten waters to the west.

Wildlife officials detected the viral hemorrhagic septicemia virus in the Lake Winnebago chain in May, alarming the state's fishing community. The virus poses no threat to humans, but can cause a wide range of trophy fish, including musky, trout and bass, to bleed to death.

It already has spread through much of the Great Lakes, causing massive fish kills. Ohio wildlife officials estimate the virus wiped out hundreds of thousands of freshwater drum in Lake Erie last year.

The virus hasn't been detected in Lake Superior or the Mississippi River - yet - but it has appeared in inland waters in Michigan, New York and, now, Wisconsin.

No one knows for sure how much damage the virus could cause in smaller, inland waters. But the Lake Winnebago system is a gateway to hundreds of miles of trout streams and lakes, and some anglers fear Wisconsin's prized game fish - and the state's reputation as a national fishing destination - could be in jeopardy.

"Absolutely serious stuff," said Steve Severing, assistant tournament director with Angler's Edge Premier Bass Series in Loves Park, Ill. His organization held a tournament in May on the Mississippi River in La Crosse. "It could escalate all over the country eventually if left unchecked."

Wisconsin wildlife officials are downplaying the virus' impact, stressing that it remains isolated in the Lake Winnebago system - at least for now.

"Is it going to wipe out fishing? No," said Mike Staggs, Wisconsin DNR's fisheries management director. "But this is a threat. A significant threat."

The virus, better known as VHS, first turned up in European fish in the 1930s.

It appeared on the U.S. Pacific coast in the 1980s. Scientists believe a mutant strain of the virus spread into the Great Lakes through oceangoing ships' ballast water or imported bait in the last few years.

The Great Lakes host nearly 200 invasive species, but VHS is insidious, Staggs said. It can infect more than 50 fish species and is easily transmitted through fish fluids, bait fish and water.

The U.S. Department of Agriculture last year banned movement of VHS-susceptible fish out of the Great Lake states, as well as Ontario and Quebec. The agency has since modified the rules to allow movement of fish certified as healthy.

Michigan, New York, Ohio, Pennsylvania and Wisconsin have adopted rules that generally prohibit moving live fish. Illinois, Indiana and Minnesota are considering VHS rules, too.

The Wisconsin DNR posted signs at boat landings warning anglers not to move fish, clean their boats and drain live wells. The agency also launched statewide TV and radio ads featuring former Wisconsin lineman Joe Thomas, a first-round pick of the Cleveland Browns who spent draft day fishing with his father, urging anglers to follow the rules.

But life in the new VHS world isn't easy.

Ohio DNR Fish Chief Ray Petering said his agency usually collects eggs and brood for state hatcheries from Lake Erie. New rules adopted in May prevented the movement of live fish off the lake, forcing state workers to collect specimens from inland waters, a far more time-consuming task because they're not available in as large volumes as in the lake.

Wisconsin wildlife officials recently killed more than 20,000 sturgeon at a state hatchery that took in northern pike eggs from the Lake Winnebago system, best known as the home to North America's largest population of sturgeon, a prehistoric fish that can grow to more than 180 pounds.

Sample tests came back negative, but officials feared stocking them was too risky.

Dan Hilger, co-owner of R.J. Hilger and Sons, an Antigo bait dealership, said new rules requiring dealers to certify their product VHS-free is costing him about $800 a month. He's passed that on to retail bait shops, he said.

The virus eventually will reach an equilibrium with its hosts, said Geoffrey Groocock, a researcher at Cornell University, which tests fish for New York state. But it could kill a lot of fish until then, he said.

"It's sort of like when the little kid comes to school with chicken pox. Since they haven't got any defense against it, everyone comes down with chicken pox," Groocock said.

Bill Pielsticker, legislative chair of Wisconsin Trout Unlimited, doubts amateur fisherman will abide by the regulations, allowing the virus to spread.

"This goes right to the source of recreation for thousands of people," Pielsticker said.

Craig Hoopman's family has run Bay Fisheries, a commercial Lake Superior fishing company in Bayfield, Wis., for four generations. Hoopman, 38, fishes primarily for whitefish, which travel in schools of thousands, making it easy for the virus to cause massive kills.

"If it does get into Lake Superior … it would put us right out of business," Hoopman said. "It could just devastate the entire lake."

So far this summer, though, fishing in America's Dairyland remains as popular as Brett Favre, bratwurst and beer. Dick Sickinger, owner of Fox River Bait & Tackle in Oshkosh, said anglers worry more about the high price of gas for their boats than VHS.

"The way he's fighting, he ain't got a disease," said Frank Paulus, 82, of Oshkosh, as he battled a white bass on the shores of Lake Winnebago.