A handful of scientists around the Bay are trying to solve a baffling ecological mystery: They want to know why as many as half of the Chesapeake’s striped bass are infected with a disease rarely seen in the wild.

Before showing up in the Chesapeake in recent years, the disease — mycobacteriosis — had never before been reported in wild fish on the East Coast, according to researchers. Further, scientists in both Maryland and Virginia believe they have discovered new varieties of the disease.

The disease is responsible for many of the external lesions reported on the popular sport fish in recent years. In addition, it often causes large, ugly growths inside fish that otherwise appear healthy on the outside.

Concern has grown because the disease seems to have reached epidemic proportions since 1997, when it was first spotted in Bay rockfish. Although there is no conclusive proof of what happens to infected fish, scientists strongly suspect that large numbers will gradually die off, something that could put the population of the Bay’s most popular recreational species at risk.

Humans may also be at some risk. People with cuts or open sores on their hands could pick up persistent mycobacteria infections from the fish which, in some cases, can take years to shake.

Despite the high stakes, few people have heard of the disease. It was first reported in striped bass the same year that Pfiesteria was blamed for several fish kills in Bay tributaries. But unlike Pfiesteria, mycobacteria never caused any sudden, high-profile fish kills.

While millions of dollars poured into Pfiesteria research, far less attention was given to mycobacteria, even as studies indicated it had become widespread in Maryland and Virginia. As a result, the answers to many key questions about mycobacteriosis in striped bass remain as much a mystery today as they were five years ago.

“The need to fund the disease research on the striped bass problem is more critical than the Pfiesteria problem at this point.” said Wolfgang Vogelbein, a fish pathologist with the Virginia Institute of Marine Science’s Aquatic Animal Disease Diagnostic Laboratory. “That’s my own personal opinion.”

“From a fish health perspective, it appears to be a more serious issue than Pfiesteria,” agreed Steve Jordan, director of the Sarbanes Oxford Cooperative Laboratory, expressing a view shared by many other scientists studying the disease. “It affects more fish, and it has more implications.”

Among those implications, many scientists involved with the issue say, is that the Bay has become an unhealthy place for rockfish. They strongly suspect that poor water quality, and a lack of food, are stressing the fish and making them susceptible to infections. But no one can say for sure.

No one knows why the disease suddenly appeared. No one knows how the fish get the disease. In fact, scientists don’t even know if it is limited to striped bass.

There is no reason to think it is restricted to striped bass,”said Chris Ottinger, an immunologist with the U.S. Geological Survey’s National Fish Health Laboratory in West Virginia. VIMS scientists report finding mycobacteria in flounder, but have looked at only a few fish.

And the list of unknowns goes on: How easily is the disease transmitted to humans? No one knows.

It is possible that the disease might simply run its course, then leave the population. Then again, it’s possible that with little warning, the whole striped bass population — once considered a model for fisheries management after a turnaround from rock-bottom levels in the 1980s — might simply crash.

“We have more questions than we have answers,” Ottinger said. “It’s the uncertainty of this whole thing that make it scary, because we don’t know the direction this is going to go.”

The first sign that something might be wrong with the striped bass population came in 1994, when biologists from the Maryland Department of Natural Resources began reporting sores and ugly external lesions on striped bass during routine sampling.

Initially, laboratory tests revealed a variety of bacteria infecting those wounds — something not surprising as research has shown that the Bay produces more bacteria than any estuary in the country.

But in 1997, scientists working independently in both Maryland and Virginia were surprised when they found mycobacteria growing in striped bass. “You don’t typically see this as a significant disease problem in a wild population of fish,” Ottinger said. “And that is a big reason for the concern.”

There are dozens of types of mycobacteria, including including the one that causes tuberculosis in humans. Many other types, called “atypical” or “environmental” strains occur naturally in the environment — including the Bay. But they typically are not widespread in wild fish — the vast majority of mycobacteria reports in fish stem from aquaculture or aquariums.

Because it is unusual in the wild and hard to identify, scientists can’t say for certain whether the germ may have occurred in some striped bass before 1997 and been overlooked. “We don’t know how long mycobacteria infections have been in the Bay population,” Ottinger said.

In 1998 and 1999, surveys of hundreds of fish in Maryland and nearly 2,000 fish in Virginia found that more than half of the striped bass were infected with mycobacteria, which scientists say has become — by far — the dominant infection in the Bay’s rockfish population.

But signs of infections are not always obvious to the casual observer. Fish that appear healthy on the outside often have widespread infections on the inside. When biologists cut open the fish, they see lumpy, gray nodules — or granulomas — forming in the spleen and other internal organs. Those lumps of newly formed tissue represent the fish’s attempt to wall off the growing mycobacteria infections.

A 1998–99 study by researcher Anthony Overton for the Maryland Cooperative Fish and Wildlife Research Unit of the University of Maryland Eastern Shore, found that 53 percent of the striped bass checked had internal mycobacteria infections.

Results of sampling in Virginia, reported in a masters thesis by VIMS student Jennifer Cardinal, revealed that internal infections were found at a rate three or more times higher than that of external lesions.

Overall, she reported that 9.2 percent of more than 1,000 fish examined from the Rappahannock River had external lesions, and 45.6 percent had internal infections. Of more than 500 Potomac fish, 16.3 percent had external lesions and 47.6 percent internal infections, and in the York, 25.5 percent of 220 had external lesions and 57.7 percent internal infections.

But that may undercount the number of infected fish. Those Maryland and Virginia studies examined fish histologically — in which tissues were examined under the microscope.

VIMS scientists have recently begun using a more sensitive technique to culture tissue from fish, which reveals the beginnings of infections that go unnoticed under the microscope. In ongoing sampling using that more sensitive technique through last fall, researchers found that 69 percent of the striped bass examined had mycobacteria infections, according to VIMS scientist Martha Rhodes.

If mycobacteria are a natural part of the marine environment, why are so many striped bass suddenly infected with the disease?

The answer, scientists say, may be stress. Just like humans, who are more likely to get ill if chilled or exhausted, fish weakened by other problems are more likely to get sick.

“Most bacterial infections in fish have a strong environmental component,” Vogelbein said. “When environmental conditions are suboptimal, the fish are stressed and prone to disease. That is dogma in the fish health world.”

Striped bass have plenty to be stressed about.

One suspect in the stressed striped bass scenario is water quality. Research has shown that adult striped bass in the Bay suffer from what’s called the “temperature – dissolved oxygen squeeze.”

Adult striped bass try to avoid warm summer temperatures by seeking refuge in deeper, cooler water. But in the summer, deep water in the Bay usually suffers from hypoxia — or low-oxygen conditions — which makes it off-limits to rockfish. Those two factors combine to squeeze a growing striped bass population into a smaller area of suitable habitat.

And the squeeze has been on in the 1990s. Monitoring data show water temperatures have been warmer than average during much of the decade. At the same time, the Bay in the 1990s had some of the most extensive periods of hypoxia on record, which would have severely limited striped bass habitat. A 1999 EPA report proclaimed the Chesapeake to be the most hypoxic estuary in the mid-Atlantic.

In Virginia, scientists report the greatest rate of infections in the fall, which they said could stem from prolonged stress during the summer months. Those fish may be more vulnerable to infections by a mycobacterium species recently discovered by VIMS scientists. The new species, tentatively named Mycobacteria shottsii, is the most prevalent mycobacterium seen in striped bass in Virginia, and it prefers cooler, fall-like temperatures. Fish weakened from summer heat, some researchers say, could be prone to infections from the cool-temperature bacteria.

Striped bass could also be stressed because of nutrition. As striped bass stocks bounced back in the past decade from record low levels in the 1980s, the population of menhaden — the most important food for adults — began declining.

Opinions vary as to the cause of the menhaden decline: It could be fishing pressure; it could be that too many were eaten by the growing striped bass population; it could be that water quality has reduced reproduction; or it could be factors in the ocean — where menhaden spawn — that reduce the survival of young fish. Whatever the reason, the bottom line is that the menhaden stock has fallen to near-record lows in recent years.

Overton’s report for the Maryland Cooperative Fish and Wildlife Research Unit found that menhaden consisted of just 12–27 percent of the striped bass diet in 1998–99, depending largely on the age of the fish. Studies by researchers Kyle Hartman and Steve Brandt in the early 1990s had found that menhaden accounted for 37–66 percent of the striped bass diet.

With fewer menhaden, Overton’s field work showed that striped bass were turning to things such as blue crabs, which have a lower food value for rockfish.

“It is well-known that nutritional shifts have deleterious effects on fish, and in many situations affect the immune system,” Overton’s report said. “This is not to say that this is the only possible cause for the condition, but it is one of many possible origins of ulcerative dermatitis [fish lesions] in striped bass.”

Menhaden, a small, oily fish, is the preferred food of striped bass which — especially in the fall — need to build fat reserves. Both Overton’s report and VIMS scientists described many striped bass — both with, and without mycobacteria infections — as looking thin and emaciated, and having low fat content.

A compounding factor is that mycobacteriosis in fish is usually caused by Mycobacterium marinum. Although that is the culprit in many striped bass infections, VIMS scientists have identified multiple types of mycobacteria in the fish they’ve examined, with M. shottsii being the most common.

Scientists at the Virginia-Maryland Regional College of Veterinary Medicine also say they have identified a new species, tentatively named M. chesapeaki, which they report as being the most common strain in Maryland fish.

It’s impossible to say whether the species are truly new, or went undiscovered, because no one looked for them before. If they are new, though, it raises the possibility that they are more pathogenic to striped bass than other species.

Sorting out how different diseases may affect a stressed population could be years away. “We are just at the tip of the iceberg in trying to identify the factors involved, and trying to understand the disease process,” said Howard Kator, a VIMS scientist. “Our dilemma has been trying to continue this research. Nobody seems to be really interested in trying to understand what the effects of these infections are on the resource.”

And a key question, of course, is what happens to all those sick fish. The answer is, no one knows. “We don’t actually know if it kills the fish,” Kator said. But, he added, “because of the high densities of the pathogens present in the fish, it has got to be having an effect.”

He and others strongly suspect that most, of the infected fish ultimately die, but they can’t prove it — no obvious die-off has occurred in the Bay. And they don’t know how long it takes for the disease to take its toll.

Mycobacteriosis in fish was once called “fish tuberculosis.” Like tuberculosis in humans, it is a chronic “wasting disease.” Rather than quickly killing fish, the victim’s condition gradually declines, and it may become more emaciated.

The seriousness of the infection, and the exact course of the disease, will vary fish-by-fish. In lightly infected fish, some studies suggest it may take nearly two years for the disease to even be detected. In more highly infected fish, obvious signs may be seen in weeks or months. Some fish may die in a matter of months; others may take many years.

Because of the slow pace of the infection, scientists say, it’s unlikely anyone would see a dramatic fish kill from mycobacteriosis. Rather, striped bass would succumb, individually, in scattered places around the Bay, with their bodies being picked off by passing birds, or sinking to the bottom and being devoured by crabs and other scavengers.

“That’s the typical pattern with a chronic disease,” said Vicki Blazer, a fish health biologist with the U.S. Geological Survey’s National Fish Health Laboratory. “You can have a few thousand die every day, but in a place like the Chesapeake Bay, you would never see them.”

There is a reason to suspect that the disease ultimately culminates in death for many, if not most, fish. Experimental work has shown that when healthy goldfish were injected with mycobacteria, their condition gradually deteriorated and ended with death. The same result has been seen in cod, yellow perch and flounder.

Sampling in the Bay shows that the most serious infections are seen in fish 3 years and older, while younger fish typically have lesser infections — a finding that suggests the disease could be progressing over time, according to scientists.

In the mid-1980s, scientists studying salmon on the West Coast discovered mycobacteria infections in wild striped bass. They blamed the infections for a die-off among the fish, although the case did not appear to be closely studied.

On the other hand, laboratory experiments with sea bass suggest they may be able survive the disease. Some studies suggest that lightly infected fish may hold the disease in check. And, anecdotally, scientists in have reported seeing striped bass in which it appears external lesions have healed over, especially among females that migrate out of the Bay.

But even if external signs disappear, it doesn’t necessarily mean the fish has healed on the inside. If the disease remains, it may slowly continue to grow, eventually resulting in death, either directly from the infection, or indirectly by making it more vulnerable to predation or other stresses.

“I think environmental conditions may modulate the external signs to some degree, but I don’t know that it would eradicate the internal infection,” Vogelbein said. “This is a difficult question because we don’t have the experimental data to say one way or the other. But my gut feeling is that once a fish is infected, it remains infected. It’s the rare animal that can eliminate the infection through its immune system. I think probably once infected, these fish will ultimately die.”

The degree to which the disease affects the population levels hinges not only on whether the fish die, but when they die. Since infections may take years to prove fatal, many fish may live to spawn — something that starts at about age 3 in males and 5 in females.

“If the mortality takes a long time, the overall rate may not be that high on an annual basis,” said Jordan, of the Oxford lab. “But we just don’t know the overall mortality rate associated with this.”

Fisheries officials stress they have seen no evidence of a drop in striped bass population, based on monitoring programs aimed at fish migrating along the coast. “I don’t want to suggest this is not a major problem,” said Phil Jones, of the Maryland DNR. “But we’re not seeing any unusual decline in abundance in striped bass over time.”

He and others note that the coastal stock, consists largely of females that leave the Bay early. There are some indications that they are less affected by the disease than males that remain in the Bay, though scientists say that is not conclusive.

Nonetheless, Jones added, monitoring programs may not detect modest changes in the population. Further, monitoring alone would not prove the cause of the decline.

Gradually increasing mortality could create a management problem because fisheries managers use computer models to estimate the size of the population — and to set catch targets for fish.

Those models assume a fixed percentage of the population dies from all sorts of natural causes each year. If a disease causes greater mortality than is assumed in the model the population could be overfished, and no one would know it.

“It’s going to be real difficult to pick up an obvious signal in a hurry,” said Jim Uphoff, a biologist with Maryland DNR. “The question is, what change is big enough to prove this is happening?”

The signal that something has gone wrong would be an unexplained drop in the catch with no obvious explanation, he said. But conclusively proving — and convincing people — that something is wrong could be hard.

The problem is compounded as striped bass have proved to be a notoriously difficult species to manage. With each coastal state vying for a share of the catch, many might to be skeptical of claims that the population is in jeopardy without conclusive proof. “Without a Bay full of dead fish, it is going to be a lot harder to deal with this,” Uphoff said.

Even if mortality occurs at low rates over a number of years, the disease could create a problem. If infected fish spend too much energy trying to keep mycobacteria infections in check, the disease could reduce their ability to produce eggs and sperm.

“In one sense, they might as well be dead if they are not contributing anything to future years as far as spawning,” said Robert Beal, the striped bass coordinator with the Atlantic States Marine Fisheries Commission, an interstate agency that manages migratory fish, and sets quotas for striped bass.

But the disease issue has not been a major topic for ASMFC committees dealing with striped bass because there is little concrete information, he said. “We know there is a bunch of fish that carry the disease, but beyond that we’re not really sure what happens,” Beal said. “If we had pretty good scientific estimates of what mortality is caused by these diseases, we would roll it into the models. But it is theoretical right now. There are more questions than there are answers.”

Those answers may not come quickly. “We can’t predict the course of the disease unless we observe it, and that may take years,” Jordan said.

Scientists at the USGS National Fish Health Laboratory plan to launch new laboratory studies soon that will allow them to watch infected fish over time, and subject them to different types of stress, to see how they respond.

The results of those studies will help scientists better understand the course of the disease in fish and whether it ultimately results in death. If it results in death, the lab studies will offer clues as to how long it takes, and what types of environmental stress hasten that outcome.

Laboratory work can also show whether there is a difference in the way various types of mycobacteria affect fish.

Those studies may also provide clues about how the disease spreads among striped bass. Research with other fish, primarily in aquaculture, have shown that fish can acquire the disease through food. Other studies show they may acquire it from the water when it is present in large quantities, perhaps by excretions from nearby fish.

If that is the case, the USGS’ Ottinger said the disease has the potential for “exponential growth.” The more fish that become infected, the greater their potential to expel mycobacteria, allowing it to further spread through the population.

Research with other species has also suggested that the disease can be passed on to new generations through eggs. In the Bay, though, it’s not clear whether very young fish are infected. Last year, Maryland biologists found obvious infections in 1-year-old fish. But examinations of young-of-year fish revealed no evidence of disease, although scientists caution that does not mean it wasn’t present.

“We can’t say it’s not there, we can only say that we didn’t find it,” said John Jacobs, a biologist with the University of Maryland’s Center for Environmental Science. It could simply be a matter that the disease had not progressed enough to be detected, he said.

So far, field results tend to show minor infections in 1– and 2-year-old fish, with the most severe conditions appearing in fish that are 3 years and older.

Last year had a near-record striped bass spawn in the Bay, and Maryland biologists hope to track that population over the next several years to learn about the course of the disease. In addition, Maryland, Virginia and the USGS plan to better coordinate their efforts this year. They want to improve, and standardize, sampling methods to get a better picture of what is going on in the Baywide population.

Although scientists in both states have seen the disease in a large number of fish, it’s not totally clear how much the infection rate may vary in different places. Also, Virginia findings show a significantly lower rate of infection among females — which spend much of their lives outside the Bay migrating along the coast — which has led them to speculate that males remaining in the Bay are less healthy. Maryland sampling, though, did not find a significant difference between males and females.

Ottinger said populations near the shore seem to be more impacted than other striped bass. But it’s hard to draw Baywide conclusions because Maryland and Virginia have used different sampling techniques.

As the answers come in, they may provide insights about what management can help. Does the Bay have a bigger striped bass population than it can support in its present condition? Can stressful conditions be reduced? Or does the disease just have to run its course — whatever that may be?

The last time striped bass were in trouble was the early 1980s, when overfishing drove the population to record lows. In 1980 alone, it was estimated that the reduced striped bass population caused a loss of 7,000 jobs along the East Coast and $220 million in economic losses. The population crash ultimately resulted in emergency action by Congress that led to a coastwide closure of the fishery.

This time, scientists say they would like to solve the mysteries around the problem before there is a crisis. “We have an opportunity to try to sort this out right now,” Jordan said. “We’ve got to keep after this the best we can.”