Striped bass in the Bay may be stressed out, but scientists studying the fish have no clear explanation as to what factor - or combination of factors - is bothering the prized species.

Fall surveys by the Maryland De-partment of Natural Resources found about 11 percent - nearly 500 of about 4,300 adult rockfish caught - had lesions, some of which were unusually extensive.

"It's a concern," said Reginal Harrell, a scientist with the University of Maryland's Horn Point Laboratory. "It's enough to raise flags. Probably yellow flags. I don't think red flags."

About 700 people called the state's fish health hotline from mid-September to mid-October to report sores on fish, mostly striped bass. Striped bass with lesions have also been reported in Virginia and Delaware.

The sores do not appear to be related to this summer's pfiesteria outbreaks, officials say. But some of the reports, they say, probably arise from heightened public awareness after publicity over pfiesteria-related fish kills involving about 20,000 menhaden in lower Eastern Shore rivers.

Sores on striped bass in late summer and early fall are not unusual, though many say the number observed this year appears high. Several scientists have suggested that an ailment afflicting 1 in 10 striped bass throughout the Bay could be of more ecological concern than pfiesteria, outbreaks of which were geographically confined. The issue has raised enough concern among scientists to spur a late November meeting on the subject.

But exactly what afflicts the fish, and why, loom as major research questions. "It's way too early to say what it is or what it isn't," Harrell said. "There's a lot of conjecture."

The lesions themselves are caused by bacteria infections. But scientists say some type of stress was probably necessary to weaken the fish to make them susceptible to such invasions.

A host of factors, ranging from unusual environmental conditions, catch-and-release fishing, increasing numbers of rockfish, reduced availability of food or more virulent bacterial diseases could cause increased stress.

One area of particular concern to some is that the overall heath of striped bass may be declining because of a decline among the fish they "forage" on in the Bay. The two highest striped bass spawns on record were in 1993 and 1996. The 1993 fish are now becoming adults with voracious appetites.

"The fish are showing signs of starvation and malnutrition," said Jim Price, president of the research-oriented Chesapeake Bay Acid Rain Foun-dation, who has studied the Bay's striped bass for years. "The effects of the loss of the Bay's forage fish will impact the ecology of the entire Chesapeake Bay."

Price cited data from the Maryland Department of Natural Resources that shows the numbers of menhaden and bay anchovy - two of the most important food sources for striped bass - have declined in recent years while the rockfish stock has increased dramatically.

Price, who has been analyzing the stomach contents of the fish for years, said few of the adult striped bass that he examined this year contained bay anchovy or menhaden - the main food source for adult rockfish. Instead, he said, blue crab and white perch - which are not as desirable because they contain less fat - have become more prevalent in their stomachs.

"There is evidence that points to a lack of adequate nutrition," agreed Eric May, chief of the Aquatic Animal Health Program at the Cooperative Oxford Laboratory, a joint operation of the DNR and the National Marine Fisheries Service. "Whether it is strong and compelling, I don't know." May, who himself considers a lack of food a plausible explanation, said there is a need to study the forage base to see whether the striped bass are finding enough fish to prey upon.

It is a tricky question. When there are more fish in a population, they tend to grow slower and weigh less because of competition for food. That doesn't necessarily mean the fish are unhealthy. But May said there are some troubling indications that the physical condition of the striped bass - mainly as measured by fat reserves - is also declining. "We need to look at what is happening to the energy reserves of the striped bass on a Baywide scale," he said. The diet, though, is a chicken-and-egg situation. Are the fish not eating because there is a lack of food? Or do they stop eating because they are sick?

"With many infections in fish, one of the first things that happens is they stop eating," said Vicki Blazer, a fish pathologist with the National Biological Service, who has examined several of the fish taken from the Bay this year.

Wolfgang Vogelbein, a fish pathologist at the Virginia Institute of Marine Science, said the fish he inspected that were only mildly infected appeared to have fairly good overall body condition. Only those with the worst infections, and which appeared to have been infected the longest, showed evidence of reduced body condition, he said.

Both Vogelbein and Blazer have tentatively identified a naturally occurring pathogen, mycobacterium, as the disease agent that appears to have caused many - though not all - of the striped bass lesions seen during 1997. Both agreed the lesions were likely triggered from any of a wide variety of potential stresses that weakened the fishes' immune systems.

"Diseases in cold-blooded animals are generally associated with stress," Vogelbein said. The Chesapeake, like other bodies of water, is filled with many types of naturally occurring pathogens that can attack fish if their defenses are weakened "If the fish is healthy, feeding well and isn't stressed by other environmental variables, they pretty much don't come down with disease, even though these organisms are present in the water around them," Vogelbein said.

Vogelbein and Blazer said it was critical for future monitoring efforts to include detailed histopathological and microbiological exams of healthy looking fish to determine whether they are infected, and whether the percentage of infected fish changes over time. Mycobacterium, for example, can be be present in fish at low levels but not cause noticeable harm until the fish is subjected to increased stress. Therefore, the pathologists said that it is possible that even apparently healthy animals may be carriers of the infection which could, in turn, contribute to the reduced body condition observed by Maryland scientists.

Because such expensive exams have not been routinely conducted in the past, the pathologists said it's impossible to tell how much of the population is at risk, or whether the incidence of infection has been changing over time.

"I'm not hopeful that we'll get a lot out of the historical data," Blazer said. "What needs to happen now is to start collecting that data, and doing it right."

That's important not only for fish health, they said, but also to monitor potential risks to humans as some fish pathogens, including certain types of mycobacterium, have in some cases been transmitted to humans who handle sick fish.

But scientists do agree that sorting all this out and determining what is affecting the striped bass will be challenging.

A wide variety of natural factors may play contributing roles.

Three of the past five years in the Bay have been characterized by unusually high flows into the Bay - 1996 was the highest flow on record - which altered water salinities, temperatures, oxygen contents and other variables which could add stress to fish. In addition, the huge striped bass spawns of 1993 and 1996 could contribute to crowding - a major stress for fish - in certain habitats.

"We recognize that it's an issue," Harrell said. "How much of an issue remains to be seen. Is it just part of a natural cycle, or is it something that we should start being concerned about?"