Scientists may be moving closer toward understanding the conditions that triggered the microbe Pfiesteria piscicida to turn deadly in the Pocomoke River, killing thousands of fish this summer.

At the same time, biologists analyzing past fish lesion records suspect the presence of the microbe is nothing new in the Bay, as pfiesteria-like lesions have been found sporadically in routine fish samples dating back at least to 1984, according to researchers at the Virginia Institute of Marine Science.

The new information, pieced together as scientists scrutinize any available information about the mysterious species and presented at a recent Chesapeake Bay Commission meeting, could help to understand the circumstances most likely to produce a pfiesteria outbreak. That could help scientists target potential "hot spots" for additional monitoring and potentially, for preventive actions.

Pfiesteria had been found in some Bay tributaries as far back as 1992, but it had never been associated with fish kills until the deaths of 10,000 to 20,000 fish on the Pocomoke this summer, causing scientists to wonder what caused the organism to suddenly turn deadly.

A hypothesis developed by Rob Magnien, who is heading the Maryland Department of Natural Resources's team investigating the problem, suggests that a set of unique conditions came together in the Pocomoke to allow the two kills that took place near Shelltown in August.

Nutrient levels were elevated in the Pocomoke above Shelltown, but that part of the river is narrow, deep and contains "blackwater" from cypress swamps in the area which helped to keep sunlight from spurring algae growth that soaks up the nutrients, Magnien said.

That situation changes around Shelltown, where the river widens, the flow slows and the river becomes very shallow. "It's only a foot or two deep in a lot of places," Magnien said. "It's a favorable area" for algae growth. While pfiesteria is often associated with fish kills, much of its life is actually spent eating algae, so it may have found good habitat around Shelltown, he said.

Algae also attracts menhaden, a fish that travels in schools and grazes algae in the Bay. Menhaden feeding on the algae were restricted to the area because of unusually low levels of dissolved oxygen in the deeper portion of the Pocomoke upriver from Shelltown, Magnien said. That concentrated a large number of fish in the same place as the pfiesteria.

Fish kills linked to pfiesteria tend to take place when large numbers of fish-typically menhaden-are present. Some evidence suggests that something in fish excrement causes the microbe to release a toxin that paralyzes the fish. Lesions associated with pfiesteria are often found near the anus of the fish.

The conditions that allowed for the high level of nutrients and low levels of dissolved oxygen in the river appear to date back to last year's unusually high flows which were experienced in the Pocomoke, and throughout the Bay watershed, Magnien said.

"I'm not saying this can't happen anywhere else," Magnien said, "but this has been a real hot spot."

The hypothesis has received good reviews for other scientists. "I think he's on to something," said Donald Boesch, chair of the University of Maryland's Center for Environmental Science and the head of a special scientific advisory committee that is overseeing the state's pfiesteria response.

"I think the circumstances he's describing fit really well with what we've been seeing" in research, said JoAnn Burkholder, a pfiesteria expert at North Carolina State University.

The hypothesis could also explain a link between nutrients and pfiesteria. Because pfiesteria is heterotrophic-it eats other organisms-it would not likely be stimulated directly from nutrient runoff in the river, unlike other types of phytoplankton, which directly use nutrients and may form blooms in the presence of sunlight.

Meanwhile, scientists at the Virginia Institute of Marine Science say a review of fish taken in monthly surveys for decades has shown evidence of pfiesteria-like lesions in fish taken from tributaries dating to 1984. They have been most common in menhaden caught near Tappahannock in the Rappahannock River, but have turned up in other river systems as well. A survey in the Rappahannock this September showed that between 50 percent and 75 percent of the menhaden between 3-4 inches long had pfiesteria-like lesions.

Gene Burreson, VIMS Director for Research and Advisory Services, said the lesions were not found in larger fish, and were found almost exclusively in menhaden. "Are these lesions in the Rappahannock River caused by pfiesteria?" he asked. "Maybe. We don't know."

Burreson showed slides of menhaden with similar lesions taken from the Rappahannock during a fall survey in 1984. Fish with similar lesions were "very common" that year both in other parts of the Bay and along the mid-Atlantic Coast, he said. "These are exactly the kind of lesions on the Pocomoke," he said.

Despite the finding, Burreson said there have been no fish kills related to pfiesteria reported in Virginia. He said pfiesteria, in low concentrations, while causing lesions, do not appear to result in massive kills.

"We have never had a human health complaint in the Rappahannock, and we've never had a fish kill in the Rappahannock that we know of," Burreson said, even though teams collecting the fish samples routinely wade into the river wearing shorts and tennis shoes. "The only point I'm trying to make is that these sorts of lesions are not new, they're not necessarily even unusual," he said. "We've seen them every fall in the Rappahannock, as well as at lower prevalences in other rivers; the York and James in Virginia.

Burreson also described the Rappahannock as "probably one of the cleanest rivers in the Bay, as far as nutrients go. So what's to be made of this is not clear."

VIMS scientists have never been able to explain the cause of the lesions, nor why they began occurring in 1984. "In our trawl survey records, we've not seen anything like this prior to 1984," Burreson said. "1984 was the first year it was very obvious to the fall survey crew."

Whatever the cause of the outbreak, said Virginia Del. Tayloe Murphy, chairman of the Chesapeake Bay Commission, it should serve as a warning of just how much people don't know about the Bay. "When we think we are getting a handle on one thing, another thing seems to come up." The commission is an advisory panel that represents the legislatures of the three Bay states.