Sick stripers’ fate remains a mystery
Photography, tagging program will track mycobacteria’s impact on rockfish population
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Mug shots of fish captured from Virginia’s Rappahannock River may provide important clues for scientists trying to resolve one of the the Bay’s most perplexing mysteries: What happens to the plethora of sick striped bass swimming in the Chesapeake?
Last fall, researchers from the Virginia Institute of Marine Science tagged, extensively photographed, and then released 1,811 fish captured in the river. Some fish seemed healthy, others had the ugly skin lesions that have become common on many rockfish in recent years.
As fishermen catch the tagged fish and return them for a $20 reward, scientists can go to their mug shot gallery and determine whether individual stripers seem to be getting worse, or better. And, if sick fish are returned at a lower rate, it would be the most concrete evidence—after a decade of research—that mycobacteriosis is killing striped bass in the wild.
“These are really difficult questions,” said Wolfgang Vogelbein, one of the VIMS scientists overseeing the project. “The goal is to see what the disease does over time, and through the tagging program to see whether there is any mortality associated with this disease, which we suspect, but don’t know.”
Mycobacteriosis—and what it means for the Bay’s most valuable recreational species—has been a frustrating issue for scientists and fishery managers alike since it was first discovered in Chesapeake striped bass in 1997. It is a chronic wasting disease which, in aquaculture, usually results in death. But mortality has been rarely reported in the wild, and scientists are unclear about the disease’s impact on Bay fish.
Part of the dearth of information about mycobacteriosis in the wild likely stems from not looking for it, either in the Bay or elsewhere. Indeed, an examination of archived fish samples by John Jacobs, a research fisheries biologist with the National Oceanic and Atmospheric Administration, indicates that some type of mycobacteria infected some striped bass at least as far back as 1984. “We never really critically looked for it,” Vogelbein said. “It may have always been here.”
Complicating the picture is that bacteria in the genus Mycbacterium include a host of species and strains, many of which are poorly studied. The ones most common in Bay striped bass, Mycobacterium shottsii and M. psuedoshottsii, were unknown until discovered in Chesapeake rockfish. But various types of mycobacteria have different effects—the ones in many striped bass may progress more slowly than M. marinum, a species that more commonly plagues aquaculture and is the variety most often studied in fish.
Against that backdrop, more than three dozen scientists working on the issue, along with a handful of fisheries managers, gathered in May to assemble the information they’ve collected and set future research directions. They concluded, in part, that scientists themselves have complicated the picture. Various researchers have defined what constitutes a sick fish differently over the years and used different techniques to capture the fish, as well as taken them from different areas—all factors that make it difficult to compare results.
“Sometimes, it’s not clear what we are talking about,” acknowledged Chris Ottinger, a fisheries biologist with the U.S. Geological Survey, which co-sponsored the workshop with the National Oceanic and Atmospheric Administration. Scientists agree that it is likely that the severity of the disease varies from place to place—past studies show widely different infection rates in various tributaries. Rates seen in one river, they say, aren’t likely to represent the population as a whole. Also, disease rates are vary strongly by age.
Further, many fish found to carry mycabacteria when examined using sensitive laboratory culturing techniques are not truly diseased. Mycobacteria, which occur naturally in the wild, may be present for years—maybe indefinitely—without showing classic signs of disease, affecting internal organs or sometimes contributing to ugly red sores on the outside.
Such reporting and definition problems have resulted in the number of sick striped bass in the Bay to be put at nearly 80 percent in some reports—a higher percentage than is actually the case for the population as a whole. “Part of the confusion in the past,” Ottinger said, “was that people were not being real clear about whether they were talking about exposure, infections or disease”—all of which have different rates. As a result, a key conclusion of the meeting was that scientists need to standardize their information collection and reporting techniques and create a common database to hold their findings.
Yet it is almost certain that the situation has worsened in more recent years as scientists have documented a steady increase in the number of infected striped bass. A Maryland Department of Natural Resources survey, using consistent sampling techniques, has seen prevalence of disease—those showing actual external or internal symptoms—rise from 25 percent of fish sampled in 1998 to 62 percent last year in fish ages 3 and older.
Researchers are also sure the disease rate increases with age, which is clearly seen in large studies conducted independently by the DNR and VIMS. The VIMS data, for instance, shows that only 25–35 percent of fish aged 1–2 years are diseased, but that grows steadily to 80 percent for 7-year-old males.
Limited studies outside the Bay show that fish in other places also carry the disease, but in lower numbers and lower severity. Scientists caution that those studies have been small, and more work is needed.
In a hopeful sign, the DNR survey suggests that after several years of fairly rapid increases, disease rates may have begun to level off over the past three years. The increase from 2004 to 2005 was just 3 percent. “We may be in a period where it has leveled off,” said Mark Matsche, a DNR fisheries biologist. “This coming year, it will be interesting to see the numbers.”
Likewise, the VIMS ChesMAPP survey, which collects random samples throughout the mainstem of the Bay several times each year, has not shown a clear increasing trend over the three years it has collected disease data. “I don’t want to say this is definitive proof that the disease is in equilibrium because we may be dealing in long-term cycles here,” cautioned Dave Gauthier, the VIMS researcher who analyzes the disease data. “It may take more than three years to see an upward or downward trend.”
While the disease may have stopped getting worse, scientists say there is no evidence that the situation is getting any better.
In fact, recent studies at the University of Maryland Aquatic Pathology Center and the Maryland Department of Agriculture’s Fish Health Laboratory found mycobacteria infecting a host of other fish in the Bay, including white perch, blueback herring, winter flounder, Atlantic menhaden and others. Those preliminary findings, though, don’t indicate whether the infections turn into actual disease, as sometimes occurs in striped bass, or even whether the same species of mycobacteria are involved.
“The fact that there are multiple ‘myco’ species causing infections, and multiple fish species that are affected, indicates that the problem is bigger than just striped bass,” said Andy Kane, director of the Aquatic Pathology Center at the University of Maryland, who led the research team that found the infected fish. “This may be one of many biological variables that we should monitor to show what I would call potentially negative environmental changes in the Bay.”
That begs many important questions: Mycobacteria are considered to be ubiquitous in the environment, so why has it become so widespread among striped bass? Has an environmental change caused the species infecting rockfish to become more abundant in the wild? Or are there environmental changes that make striped bass more susceptible to disease? Are mycobacteria originating in the water, or washing off the land? Does their distribution change from place to place?
“We really have a vacuum of eclogical information except for the obvious fact that it’s in striped bass,” said Howard Kator, a VIMS researcher who has been working on diseased striped bass for years. “How did this thing get here in the first place? Is it a natural occurrence? Did it come here in ballast water?”
Scientists have no solid evidence to explain even the most basic question of how fish become infected. They could get it from the water, they could get it from the food they eat or they could pass it to one another when crowded together. Some could even be spawned with it: Recent surveys reported by Kane found mycobcteria in fluids from the sex organs of both male and female fish, though scientists say its unclear they actually pass the infection to their offspring.
“We can’t make that leap and say they were born with it, but we can say that they definitely have ‘myco’ very, very early on,” said Matsche, who has found infections in fish weighing as little as five grams.
Such evidence makes many scientists suspect many, if not most, fish acquire infections early in life, although they may not show obvious signs of disease for several years, perhaps meaning that the ability of the fish to keep the disease at bay is reduced as it battles other stresses. Scientists strongly believe that the severity of infections in fish is likely a sign that the striped bass themselves are stressed—like humans, fish under stress are less able to fight off an infection.
Scientists believe major sources of stress could include the lack of high quality food (Menhaden, an important source of food, are at near-record low populations in the Bay while striped bass populations are high.) and poor water quality (Lack of dissolved oxygen in cooler deep water forces striped bass into warmer surface water in the summer.).
Preliminary laboratory studies by Jacobs show that when infected fish are fed reduced rations, they die of disease rapidly, while those with an adequate diet can withstand the disease, at least for a while. Other studies, by USGS and VIMS researchers, have shown that infected striped bass also die rapidly when exposed to high temperatures, similar to those often seen in the Bay during summer months.
The stressors affecting the fish could include all, or none, of those factors, as well as others. “It’s not unlikely that it is the combination of multiple stressors that could be having an impact,” Jacobs said.
Yet, since mycobacteriosis is a chronic, slowly developing disease, it’s unclear whether infected fish can ever rid—or fully contain—the disease regardless of stress. It might just continue to progress at a different rate. “We don’t know the timeline for the disease,” Ottinger said. “We can’t answer how much stress is contributing versus how much is driven by disease dynamics that would be occurring in the absence of any stressors. But that is a primary concern.”
Given that level of uncertainty, it was no surprise that scientists could find no consensus on the most pressing research issue raised by fisheries managers: Does mycobacteriosis kill fish?
Models using tag return data from long-term studies of striped bass caught in recreational and commercial fisheries suggest that mortality has increased among striped bass in the Bay since the disease become widely noticed in 1997.
Models developed by Desmond Kahn, of the Delaware Division of Fish and Wildlife, and Victor Crecco, of the Connecticut Division of Marine Fisheries, indicate that natural (nonfishing) mortality for fish of less than 18 inches—which don’t leave the Bay— increased in the Chesapeake from 1995 to 2001, then dropped slightly, but remain higher than 1997 levels.
More recently, the mortality rate for larger fish, which includes those that migrate along the coast and return to spawn, has begun to increase in the Bay, with the sharpest rise coming in Maryland. “If they are having this increased mortality rate, that could affect abundance down the road on the coast as well,” Kahn said.
Whether the model’s predictions are actually happening have been debated for two years by the Atlantic States Marine Fisheries Commission, a state-federal panel that manages migratory species including striped bass. No consensus has been reached.
Alexi Sharov, a DNR fisheries biologist, said that while Kahn and Crecco’s models are “suggestive” of an increase in mortality, that conclusion has not been backed up with actual observations. “With that drastic rate of natural mortality, in three to four years we should have seen a significant decline in the resident stock in the Bay,” he said. “That is not being seen in the harvest statistics from the commercial fishery and the recreational fishery.
“I’m not saying that I’m 100 percent sure that nothing is going to happen in the future, but so far, the observations that we have are in opposition to the findings of the natural mortality increase,” Sharov said. “Whether this leads to increased mortality or not has not been shown clearly yet.”
Tracking the disease in the wild has been a problem. Often, the disease is only seen when the fish is killed and cut open and biologists find infected organs on the inside. Because they have to kill the fish to find the disease, scientists can’t track what happens to infected fish in real-world conditions.
Many believe the VIMS research project may be closest to clarifying that picture. After examining thousands of fish over the years, the VIMS team believes tiny punctures on the outside of the fish—barely larger than a pin prick—are associated with internal disease in about 70 percent of the cases. Normally, the tiny marks would go unnoticed.
“Our goal is to use that as a predictor,” Vogelbein said. “Then we would have a nonlethal diagnostic that, in conjunction with the tagging program, would allow us to say something.”
By photographing healthy fish, fish with obvious external lesions and fish with the tiny punctures, the biologists believe they can track the fate of fish in real-world conditions.
Early indications from more than 100 returns this spring of fish tagged last fall show little changed over the winter. “The lesions look pretty much the way they did when those fish went out,” Vogelbein said. “There was no real strong indication of healing over the winter, or of progression.”
The real test, he said, will come this summer when those fish—along with another batch tagged this spring—begin encountering stress from high temperatures, low dissolved oxygen and other factors. “I would suspect that if there is mortality associated with the disease, that it would be associated with the stressful summer months,” Vogelbein said.
But no one expects the findings to be the last word on a disease, where scientists continue to have more questions than answers. “Mycobacteriosis,” Ottinger said, “no matter how you look at it, is not a straightforward disease.”
Mycobacteria Research Priorities
- Understand the impact of the disease on the striped bass population and determine whether that varies from place to place.
- Understand factors that influence mycobacteria growth in the environment and how it is infecting striped bass.
- Understand the influence of environmental stressors, such nutrition and water quality, on disease progression.
- Better understand the distribution of mycobacteriosis in striped bass from other areas along the Atlantic Coast.
Mycobacteriosis & Humans
A lingering question about widespread mycobacteriosis infections in striped bass is the degree to which they pose a threat to humans.
Scientists generally agree that it’s unlikely there is a risk from eating cooked fish.
But mycobacteria infections can be passed on to humans if the bacteria come into contact with an open wound. The rate of Mycobacterium marinum infections in the region has risen somewhat in recent years, although reported infections still account for only a few dozen cases each year in Maryland and Virginia.
While the disease may be difficult for most people to contract, scientists say people with compromised immune systems may be at higher risk, and have difficulty getting rid of the disease. In humans, it is often called “fish handlers disease” and is manifest by ulcerated bumps on the hands and forearms. Generally, it can be treated with antibiotics, but if it becomes established, treatment may include surgery.
It’s unclear whether the Mycobacterium shottsii or M. pseudoshottsii, which are the most common mycobacteria in Bay striped bass, causes human infections. Scientist note it is closely related to M. ulcerans, which is known to cause health effects in humans, including skin ulcers, although those conditions have not been reported in the Bay.
Health officials caution that people who have open wounds should not handle fish, and that gloves be used when handling any fish with obvious sores. People should wash their hands with warm soapy water after handling any fish.
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