It's quiet on the South Fork of the Shenandoah River here, save for the steady sound of rain dimpling the surface of the water.

A canoe shop near the river's edge is stocked with a rainbow of recreational equipment -- red kayaks, orange life jackets, yellow plastic inner tubes -- waiting for the weekend or a sunnier day to bring crowds of people.

But business is down this summer, said Trace Noel, owner of Shenandoah River Trips.

Normally, regular customers would return week after week toting buckets of minnows or live shiners in their rented canoes to fish the nationally renowned waters for smallmouth bass.

This year, a fish kill wiped out as much as 80 percent of the adult smallmouth bass population, the third major strike in four years affecting rivers in the region. And though nobody has identified a clear cause, the trend raises new questions about the quality of water that feeds into the Chesapeake Bay.

"The entire upper mileage of the Potomac River watershed is sick," said Jeff Kelble, 33, a full-time fishing guide based in Boyce, near Winchester, who travels two hours to the lower Potomac and small rivers where he can still find popular game fish. Some guides who spend lots of time on the Shenandoah said the water quality is deteriorating, while others said it has always been poor.

Starting in April, while they were spawning and their immune systems were suppressed, smallmouth bass, as well as redbreast sunfish, began developing lesions. Locals described them as cigar burns or canker sores.

Before long, dead fish could be seen floating in the river. The same sequence occurred last spring on the North Fork of the Shenandoah, and two years before that on the South Branch of the Potomac.

If nothing else happens to the river, it still could take several years for the young fish, which were passed over in the kill, to grow large enough to interest anglers.

"This is not a typical fish kill," said Steve Reeser, a fish biologist with the Virginia Department of Game and Inland Fisheries. A more typical kill can be traced back to a source of pollution such as a chemical spill or a pipe from a sewage treatment plant.

"Those are things that are easy to manage or regulate," Reeser said.

The state Department of Environmental Quality, the lead agency investigating the case, sent live fish with lesions to pathologists and fish disease specialists, who could not find a particular virus or cause of death but confirmed that the fish had been subject to environmental stress.

"It's no secret that the Shenandoah River has water quality problems," Reeser said, though the investigation has not found data to show that the fish kill was tied to water quality.

Fishing on the South Fork has been mainly a catch-and-release sport for years. Red-and-white signs posted at river access points warn people not to eat the fish, and high levels of mercury and PCBs have resulted from industrial dumping.

The river is also known to have high nutrient levels. Nitrogen and phosphorus in high amounts cause excess plant or algae growth, which can reduce levels of dissolved oxygen. Fish struggle to breathe, and that can weaken their resistance to disease or bacteria.

The land along all three rivers affected by the fish kills is primarily agricultural. With more than 900 farms in the valley, the poultry industry dominates. High-nutrient waste from the farms is used as fertilizer and can wash into the river.

The Shenandoah Valley is also a growing population center. Reeser said it's difficult to test for runoff from the countless farms, golf courses, lawns and parking lots.

It's also hard because the Department of Environmental Quality has limited resources for river monitoring. Two fish biologists test in 14 counties, said Don Kaine, a water compliance manager with the department who is overseeing the fish kill investigation. The agency relies on citizen monitoring groups to help augment its data.

The lack of conclusive information has been fodder for rumors and finger pointing in the valley. Some blame newcomers and development. Many blame the poultry farms, but often with averted eyes or in lowered tones, hesitant to incriminate neighbors or friends.

A hundred miles away in Richmond, Jeff Corbin, senior scientist and deputy director of the Chesapeake Bay Foundation in Virginia, said he does not understand the hesitation to pin blame.

"It's infuriating that people are saying we don't know what's causing this," Corbin said.

The river is "choked with nitrogen and phosphorus pollution," which "makes these fish live under stressful conditions all year," he said.

Along Route 340, Stonewall Jackson Memorial Highway, roadside signs for chicken farms alternate with churches and country markets selling lures and spinners.

Just south of Luray, George Gochenour took a break from shoveling 10 months' worth of chicken droppings from his 28,000-chicken breeding farm. By week's end, he estimated, he would have 350 tons to give to a farmer down the street.

"I can't see how it would be enough waste to get to the river and harm the fish," he said.

Gochenour, who works for Texas-based Pilgrim's Pride, said poultry farmers used to be less careful about where they spread litter. But now, he said, regulations are in place to monitor the content of the litter and where it is spread.

Corbin said Virginia's poultry waste regulations, passed into law in 1999, are just a start.

Reeser said he hopes the fish kill will be a wakeup call to people in the Shenandoah Valley.

"There's been a lot of talk and effort to clean up the Chesapeake Bay. People who live here think, why should I worry about that, [when] it's a couple hundred miles away," he said.

"Now the impacts are closer to home."

Staff researcher Bobbye Pratt contributed to this report.

A fisherman catches a young smallmouth bass in the South Fork at Raymond R. "Andy" Guest Jr. Shenandoah River State Park.Farms are considered a major source of overabundant nutrients, which lead to lower oxygen levels in the river.