A new study provides the strongest indication to date that mycobacteriosis, a disease that has plagued Chesapeake Bay striped bass for more than a decade, likely results in the death of substantial numbers of rockfish.
Mycobacteriosis is a chronic wasting disease, distantly related to tuberculosis in humans, which was first discovered in the Bay's striped bass population in 1997. It can cause extensive tumor-like growths inside the fish, and is sometimes associated with ugly external lesions on the outside.
The disease infects the majority of rockfish by the time they are 3 years old, but scientists studying the infections have lacked any smoking gun evidence to answer one of their most basic questions: whether the infections are lethal to fish.
The new study, published in the October issue of Ecological Applications, used a computer model that drew on an extensive set of monitoring data to construct a strong circumstantial case that the disease leads to death, especially in older female fish.
"Based upon our best knowledge of the biology of this disease in fishes, the models are consistent with disease-associated mortality occurring," said David Gauthier, an assistant professor in the Department of Biological Sciences at Old Dominion University and lead author of the study. "This is really one of the first attempts to figure out what a chronic disease is doing in a wild, migratory finfish population."
It is difficult to track the fate of individual diseased fish in the wild, as the disease likely take months, if not years, to kill them, so the fish die one-by-one over time. As a result, the ability of scientists to prove the ultimate fate of the fish is problematic.
"It's not like a big fish kill where you wake up one day and there is a big pile of them washed up on the beach; these slowly die," said Rob Latour, associate professor of fisheries science at the Virginia Institute of Marine Science, and a co-author of the paper. "It is very difficult to observe these deaths."
The study overcame that problem by using information gathered from 1,420 striped bass collected from 2003 through 2005 by the Chesapeake Bay Multispecies Monitoring and Assessment Program, an ongoing VIMS program aimed at unraveling the Bay's complex food web.
Scientists examined each fish to determine whether it was infected. The results showed an increasing rate of infection for striped bass from age 1, when about one in four females and one in eight males was infected, to age 5, when about 80 percent of the males and 90 percent of the females had the disease.
After age 6, the prevalence rate stayed roughly the same for males, but dropped in half-to about 40 percent-for females.
That information was used in a model designed to help predict the impact of disease on aquatic populations that was recently developed by Dennis Heisey of the U.S. Geological Survey's National Wildlife Health Center in Wisconsin, a co-author of the paper.
After a series of analyses using the model, the scientists found that data best fit a scenario in which the odds of survival among infected fish was greatly reduced.
In any given year, a female striped bass infected with mycobacteriosis has a 49 percent chance of surviving compared with a non-diseased fish, according to the model. Males also appeared to have a lower survival rate, although the model was less definitive about that.
"The model is interpreting the low disease prevalence in older animals as being indicative of mortality," Latour said. "If prevalence is going down, that means that more diseased animals are dying. So when you take a sample, you get fewer [infected fish] because they are already dead."
The model hinges on the assumption that fish infected by the disease will ultimately die. The paper notes that there is no evidence that fish can heal themselves. Studies of mycobacteriosis in other fish have indicated the disease is progressive, and ultimately fatal. Lab studies have also shown that with striped bass.
Nonetheless, some have have suggested that striped bass may recover because of the appearance that external lesions have healed on some fish. But Gauthier said that when those fish are examined, active disease is still present inside.
"The fish is not cured, at least as far as we know," Gauthier said. "There really is no evidence for regression of the disease."
The suggestion that females are more likely to succumb than males is a bit of a surprise. Females typically leave the Bay when they are 3 or 4 years old to migrate along the coast until they return to spawn, usually starting at about age 5. Because the prevalence of disease appears to be focused in the Chesapeake, some biologists had thought the females would have fared better because they leave the Bay.
But if the disease is still progressing inside the fish, then the added stress females face when returning to the Chesapeake may result in their deaths, Latour said. "You could argue that the combined stress of migration with spawning makes their mortality rates different from that of males," he said.
The scientists said it's unclear whether the disease kills the fish directly, or makes them more susceptible to death by other causes, such as predation, or stress.
Another question that remains unanswered is whether the mortality rate is great enough to affect the overall striped bass population.
There is some evidence that that could be happening. An analysis of fish tagging data published last year by Hongua Jiang, of North Carolina State University, found evidence that natural (non-fishing) mortality has increased among striped bass in Maryland since the late 1990s. That study did not indicate why natural mortality might be increasing, but noted that the increase began shortly after scientists began detecting mycobacteriosis in striped bass.
Stock assessment models used by scientists to estimate fish abundance and in turn, provide the basis for fishery regulations, assume natural mortality remains constant over time. If a disease is changing that, it could have implications for the overall stock. On the other hand, if most of the mortality is taking place after many fish have reproduced, the effect may be minimal.
One of the next steps would be finding a way to incorporate disease impacts into stock assessment models, Gauthier said.
"We have demonstrated now that we do have this disease-associated component within the system," he said. "Now we need to build this into fisheries models in order to try to estimate what the effects on the population could be."
Although the paper states that it provides the most definitive evidence to date that mycobacteriosis is causing "significant disease-related mortality" in the Bay, it said that further research should be aimed at validating the assumption used in the model.
"We intend this to drive future research directions," Gauthier said. "We've demonstrated that it looks like we've got something very significant going on in older female fish. So this is a portion of the population we really need to take a careful look at."
Other authors of the paper include Chris Bonzak, Jim Gartland and Wolfgang Vogelbein at VIMS, and Erin Burge of Coastal Carolina University.