Well before the sun burned through the fog blanketing Maryland's lower Eastern Shore, watermen navigated the Pocomoke River in force, running skiffs downstream to pluck wire crab pots from the bottom as sea gulls bobbed on the surface.

The regular course of life rolled on a day after Maryland authorities found what they said were as many as a half-million dead menhaden--small oily fish that school by the millions--in a tidal creek in Virginia, just across Maryland's portion of the Pocomoke.

"It's probably just a normal thing," said Fred W. Maddox, whose seafood company in this rustic, riverfront settlement was ground zero when the toxic microbe, Pfiesteria piscicida, bloomed here two summers ago, prompting the state to shut the river as a health hazard. "I don't see this as any major threat."

Pfiesteria has not been blamed in this week's kill, considered the largest near the Chesapeake Bay in a decade. Authorities have focused on low oxygen levels in the water, perhaps exacerbated by miserly rains and the relentless heat, which limits water's ability to hold oxygen.

The operative theory: Bluefish, now in the bay in abundance, have moved up into rivers, feasting on small fish such as menhaden and driving them into the upper portions of creeks. It is a false refuge; the creeks narrow, and algae grow thick, choking off oxygen.

The Pocomoke is merely the latest system to strain under hot temperatures. Since June, a dozen area waterways have yielded fish kills, according to the Maryland Department of Natural Resources. On the western shore of the Chesapeake, some 200,000 fish have died in tributaries of the Magothy and Patapsco rivers.

No major fish kills have been reported in the Potomac River this summer, and officials say oxygen levels in the river so far have not fallen dangerously low.

This morning, Maryland scientists returned to the Pocomoke and then traveled up Bullbegger Creek, on the Virginia side, where most of the dead fish were found. The scientists first found dead fish in Maryland's portion of the Pocomoke on Thursday, then followed the trail across the state line.

Charles Poukish, a fish kill expert with the Maryland Department of the Environment, dropped a measuring device in the water where Bullbegger Creek empties lazily into the Pocomoke. The oxygen level read 4 parts per million. "That, in itself, is not a lethal level," Poukish said, but it was low enough to kill fish schooled densely.

When Poukish and his partners ventured a mile upcreek, they found oxygen levels at 2.9 parts per million. "That would do it," he said. "We're talking about just a massive school of fish going up this creek and hitting this critically reduced level of oxygen. . . . They all died within seconds."

Another two miles upstream, a Virginia Department of Environmental Quality scientist got a lower reading still--about 2 parts per million, said spokesman Bill Hayden.

Virginia scientists concur that low oxygen is to blame. When they went onto Bullbegger Creek Thursday afternoon, they counted 10,000 dead menhaden, Hayden said. But that was several hours after Maryland's assessment, and many fish already may have dropped to the bottom or been devoured by waterfowl, he said.

Today, a healthier scene prevailed.

Brett Coakley, a fisheries biologist with the Maryland Department of Natural Resources, flung a net and pulled in a young croaker no bigger than the palm of his hand. "It's fine," he said. Back it went.

None of the fish the team caught showed signs of duress or lesions, considered an indicator of the presence of toxic pfiesteria.

Since the pfiesteria outbreak, attention has focused on the chicken houses that dominate the landscape. Chicken manure, which is spread as fertilizer, is a major source of pollution reaching Chesapeake tributaries on the lower Eastern Shore, according to state and federal authorities. Nitrogen and phosphorus from the manure feed algae.

Poukish stressed that this week's fish kills are a signal that too much pollution is present but that fish kills also are a natural occurrence.

"There is evidence that this thing goes all the way back to the 1680s, when Captain Smith documented fish kills," he said. "There's also no question that too many nutrients makes this worse. . . . It's a mixture of man-induced problems and natural events."

Don Jackson of the Chesapeake Bay Foundation, an advocacy group, emphasized the pollution. "We do see low dissolved oxygen in the summer naturally, but that's typically in deep water," he said. "This is four or five feet. Clearly, there are water quality problems."

Wade Walker, 40, a Crisfield waterman, wondered whether this week's kill was a portent or an aberration.

"When I was a little kid, you'd see menhaden die. They're a weak fish," he said. "But it seems like it's happening more nowadays."