Mycobacteriosis infection rate in Bay’s striped bass increasing
Seven years after it was first observed in the Bay’s striped bass, scientists say the mycobacteriosis infection rate appears to be increasing, with well more than half of the fish suffering from the chronic disease.
Surveys conducted in four Bay tributaries last fall showed the infection rate appeared to climb in each river from the previous year, exceeding 80 percent in the Potomac. And a new survey of the Bay’s mainstem found widespread infections among fish from the mouth of the Chesapeake to the Susquehanna flats, with the disease rates getting progressively higher as the fish get older.
Mycobacteriosis is a chronic, long-term wasting disease, and scientists believe that many of the infected fish will die unless something else kills them first, although the disease could take many years to take its toll.
As a result, scientists and fishery managers remain uncertain about what the infections mean for the Chesapeake’s most popular recreational fish.
“We don’t know a great deal more about why this disease is in the striped bass population than we did several years ago,” said Phil Jones, a fisheries manager with the Maryland Department of Natural Resources. “It’s not clear why other fish species in the Chesapeake Bay are not being affected.”
Mycobacteriosis is caused by a group of naturally occurring bacteria, known as mycobacteria. In aquaculture fish, it usually leads to death. It had never been reported in a wild population on the East Coast until 1997, when scientists began seeing it in the Bay’s striped bass.
Infections may not be apparent to anglers because they are often confined to internal organs, although in some cases they are associated with external sores—but not all external sores are necessarily signs of mycobacteria infections.
The disease can infect humans if the fish sores come into contact with a cut or open wound—hence its name in humans, “fish handlers disease.” While it is not readily contracted by humans, it can be difficult to get rid of, particularly for people with suppressed immune systems.
The rockfish infections have not caused any mass die-offs in the Bay, but some scientists suggest it could be causing chronic, low-levels of mortality as infected fish die one by one.
Two separate analyses of striped bass tagging data do indicate that the fish’s mortality is increasing in the Chesapeake Bay, something the scientists who did the analysis say is likely related to the infections
Desmond Kahn, an environmental scientist with the Delaware Division of Fish and Wildlife, said his analysis, using long-accepted techniques, shows that striped bass survival in the Chesapeake has decreased from 70 percent in the early to mid-1990s to about 50 percent in recent years.
Because reported catch rates have not significantly increased, he said, the figures suggest that more fish are dying of other causes. “Mycobacteriosis is a leading hypothesis because it has very wide prevalence and the pathologists say it is, potentially at least, a fatal disease,” he said.
A similar analysis by Victor Crecco of Connecticut’s Marine Fisheries Division showed the same results. Both Kahn and Crecco are on committees that provide information to the Atlantic States Marine Fisheries Commission, which oversees the management of striped bass along the East Coast.
But not everyone agrees with the explanation.
Jones said that if so many fish were dying of natural causes, it should be reflected in the reduced catches. “A reality check is what we see taking place in the recreational and commercial fishery in the Chesapeake Bay,” Jones said. “Last year, the reported recreational harvest was the highest since the fishery reopened in 1990.”
Nor could the ASMFC make a call on what the tagging data means. Its stock assessment report from last year agreed that the figures suggest that natural mortality in the Bay was increasing, but it also said the change might be explained by other factors.
Or, it said, it could simply mean that the assumptions used in making the analysis over the years might be wrong. “More analyses will be required to resolve these issues,” the report said.
The bottom line is that seven years after mycobacteriosis was first identified in the Bay’s striped bass, scientists remain almost as baffled as ever about why it’s here, and the degree to which it poses a threat to striped bass—a fish normally considered to be one of the Bay’s success stories as populations rebounded after hitting record low levels in the 1980s.
Last fall, bacteria cultures taken during a multiagency survey of striped bass indicated that 81 percent of the fish in the Potomac were infected, 67 percent in the York, 61 percent in Rappahannock, and 54 percent in the Nanticoke.
That was a change from the previous year, when the Rappahannock fish were the sickest. Nonetheless, all four rivers showed apparent increases in infection rates over those seen in 2002 when 53 percent of the Rappahannock fish were infected, as well as 48 percent in the Potomac, 39 percent in the Nanticoke and 32 percent in the York.
Despite the apparent increase in the infection rate, the fish seemed to be in better overall condition than the previous year, said Chris Ottinger, an immunologist with the U.S. Geological Survey’s National Fish Health Laboratory in West Virginia, who coordinates the 2-year-old survey.
“The infections are not going away,” he said. “But it kind of makes me wonder what the implications of other stressors are in the overall disease scenario. The outcome of the disease may in part be determined by how severe the summer is, and things like that.”
Meanwhile, new figures from the Virginia Institute of Marine Science’s ChesMAPP survey—which uses a standardized method to collect fish from the mouth of the Bay to the Susquehanna flats five times a year— shows that the infection rate seems to get higher as fish get older.
Figures from more than 400 fish examined last year, which was the first time the survey looked for mycobacteriosis in striped bass, show that only 30 percent of 2-year-old striped bass were infected, but the rate of infections climbed each year, until 70 percent of all 5-year-old fish had the disease.
David Gauthier, a VIMS graduate student who analyzed the fish, said the survey collected too few striped bass older than 5 to make a statistically significant year-by-year analysis. But, he said, “the infection rate was at or above 50 percent for the rest of the ages that we had data for.”
In addition, the older fish had significantly worse infections, he said. “There was a big spike in severity in 6– and 7-year-old fish—probably a three– or fourfold increase in terms of severity.”
Infection rates were much greater in males than females, he said. Males tend to live much, or all, or their lives in the Bay, while females leave—sometimes as early as age 2—to migrate along the coast before they return to the Bay to spawn when they are about 5 or 6 years old.
Those findings, at least in part, mirror data collected by the Maryland DNR over the years. Mark Matsche, a DNR biologist working on the issue, said Maryland surveys show an increasing infection rate until the fish get to be about 4 years old. “After that, we can’t say because we don’t have many individuals that are older,” he said.
By the time striped bass get to be 5 years old, most have migrated out the Bay, returning only to spawn.
The Bay, though, is not the only place where striped bass have mycobacteriosis, although anecdotally, the Chesapeake fish appear to be more sick than those elsewhere.
Jed Brown, Delaware Bay coordinator for the U.S. Fish and Wildlife Service, said a survey along the west side of the Bay last fall found only 11 percent of the striped bass had mycobacteriosis. Further, he said, they looked healthier overall than those from the Chesapeake. “The fish were a little bit fatter than Chesapeake fish,” he said.
In North Carolina, Anthony Overton, an associate professor at East Carolina State University, last June surveyed fish from the Roanoke River. Preliminary results show that 50 to 60 percent of those fish had mycobacteria-type infections, but a final analysis was not complete.
“We’re seeing something a little bit different here,” he added. “We are not seeing the external lesions that they see in the Chesapeake Bay, at least not during the spring.”
In other parts of Pamlico Sound, there have been reports of striped bass with lesions during the summer, he added, but those reports have not been investigated for mycobacteria.
But Overton—who conducted some of the early mycobacteria studies in the Chesapeake—said it is important to not only look not at the disease, but why the striped bass are infected to begin with. Through the 1990s, he noted, a variety of bacteria were found in striped bass—often infecting large parts of the population—until mycobacteria infections became the dominant problem.
“We concentrate a lot on the bug itself, but I kind of think there should be a shift toward looking at the ecosystem as a whole,” he said. “Why is this happening? There was always some bacteria popping up.”
A popular theory is that the huge number of striped bass in the Bay are stressed, and therefore susceptible to disease, because they don’t have enough to eat. Their most important prey item, menhaden, is in short supply.
And because the today’s striped bass can’t be caught until they reach 18 inches—the legal size used to be 12 inches—there are more big fish in the Bay than ever before, requiring even more food. Several recently published studies suggest a growing imbalance between menhaden and striped bass populations.
But other factors may be stressing striped bass as well. The fish are particularly sensitive to warm temperatures, and fish caught during the summer and fall tend to look worse than those in the spring. Poor water quality conditions, such as low levels of summertime dissolved oxygen, may also be affecting the fish.
And in the broad picture, striped bass may not be unique. A recent analysis by scientists at Cornell University in New York and the California-based National Center for Ecological Analysis and Synthesis published a study this spring saying that the rate of illness was increasing in marine life worldwide.
“The bottom line is we don’t know what environmental factors madulate this disease,” said Wolfgang Vogelbein, a fish pathologist at VIMS. “We can speculate until the cows come home.”
He and his colleagues are working on developing high-tech molecular diagnostic probes that would allow scientists to more quickly and specifically identify infections in fish, and to determine which of several mycobacteria species are to blame.
Such a tool would allow scientists to more quickly detect the disease—right now it takes months to analyze collected fish. It would also allow them to analyze archived fish samples to determine whether the infections are truly new, or whether mycobacteria infections have been present in fish for decades and merely got worse in the mid-1990s because of outside factors or because the types of mycobacteria causing the infections changed.
“Without these diagnostics, it is much harder to ask those critical questions,” Vogelbein said. But the development of such molecular probes has proven elusive, he said.
Confounding the picture further, is scientists have had a hard time producing severe infections in laboratory experiments with Mycobacterium shotsii, the most common type of mycobacteria found in Virginia’s portion of the Bay. “It’s really hard to understand how you isolate M. shotsii as the only organism in the fish and see a lot of disease, but you can take that same bug into a controlled setting and see nothing,” Ottinger said.
But he, along with Vogelbein, who have cooperated on the tests, cautioned that the lab studies do a poor job of recreating Baylike conditions. “Our lab studies are done in fresh water,” Ottinger said. “Fish are on formulated diets specifically for striped bass. The only thing there is the bug.”
Most studies in an aquaculture setting show that mycobacteria infections usually result in death, while studies of fish in the wild are less clear.
A paper by Overton and other Bay region scientists published last year in the journal Fisheries Management and Ecology noted that their analysis of infected striped bass suggested that the severity of infections gradually increases over time. “It is likely that mycobacteria infections of striped bass begin in Chesapeake Bay estuary and from the moment of infection progressed until death occurs.”
But even if the fish die, it may have less of an impact if most fish live long enough to reproduce. Further, because most of the infected fish are males, the impact on reproduction would likely be smaller than if widespread disease was found in egg-bearing females.
“It may not affect the stocks,” Gauthier said. “We don’t know that yet.” But after seeing many trawls filled with “disgusting” looking fish during the ChesMAPP surveys, he said there are other reasons to be concerned.
“It is a quality issue as well,” Gauthier said. “If you get large numbers of fish throughout the Bay that fishermen don’t want to touch, then that is a serious problem whether it is causing a decrease in numbers or not.”
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