The epidemic of potentially lethal mycobacteria infections which have swept through the Chesapeake’s striped bass population shows no sign of abating, according to preliminary results of the first coordinated Baywide survey conducted last fall.
An ongoing Maryland survey also showed continued high rates of infections in striped bass, and a separate study off the North Carolina coast indicated the infections are also in striped bass outside the Chesapeake.
“It’s not going away,” said Chris Ottinger, an immunologist with the U.S. Geological Survey’s National Fish Health Laboratory in West Virginia who coordinated the Baywide survey. “We know that in some of these rivers, the current epizootic has been going on for at least five years. It’s one thing for a disease to be in a river one year and gone the next. But when it’s there year after year after year, that means something else.”
Still, he and other scientists say they are far away from determining what the ongoing presence of the disease means for the Bay’s striped bass. Scientists believe, based on the severity of infections in some striped bass, that the disease is killing at least some of the infected fish. But so far, they don’t know how long the disease takes to kill fish in the wild, and whether the mortality rate will become high enough to threaten the population.
Mycobacteria is a chronic wasting disease, much like tuberculosis in humans. In aquaculture fish, it usually leads to death. It is rarely seen in the wild, and 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 as well.
Some types of mycobacteria can also infect humans who handle diseased fish if the infection comes into contact with an open wound. Usually, the slow-developing infections are not severe, but in some cases they can cause problems, especially in people with compromised immune systems.
Last fall, scientists from state and federal agencies and universities in Virginia and Maryland teamed up to study four rivers and develop standardized monitoring techniques. Scientists examined 50 fish captured in pound nets from each river.
Preliminary results showed mycobacteria infection rates of 53 percent in the Rappahannock River, 48 percent in the Potomac, 39 percent in the Nanticoke, and 32 percent in the York. The infection rates are consistent with earlier studies suggesting that up to half of the Bay’s rockfish could be infected with the disease.
The survey also showed that the condition of the fish was related to the infection rate. Fish in the York and Nanticoke rivers were in the best condition, followed by the Potomac, with Rappahannock fish in the poorest health. Scientists say it remains unclear whether the fish got infected because they were in poor health, or whether their health deteriorated after becoming infected.
Further complicating the picture, the survey showed that the majority of sick fish in the Rappahannock had multiple infections—either more than one kind of mycobacteria, or mycobacteria and one or more other infections. Some fish in the other rivers also had multiple infections, but the rate was not nearly so high.
Scientists plan to continue the coordinated sampling effort on the four rivers this fall to track trends. After that, Ottinger said he hoped to expand the study to look at 10 to 15 rivers the next year.
In addition, he said scientists from Delaware and New Jersey will begin a similar study in Delaware Bay this fall, while researchers from Cornell University plan to begin working in New York waters.
Meanwhile, preliminary results from the Maryland Department of Natural Resources’ annual fall fish health survey show that about 40 percent of the striped bass were infected, a slight increase. One concern, said Mark Matsche, a DNR biologist, is that the infections observed appear more severe than in the past. “We have seen over the past four years that the average severity code has crept up slightly,” he said. “That is an indication to us that the severity of the disease is increasing in the population.”
In January, the National Oceanic and Atmospheric Administration supported a limited examination of fish taken from the North Carolina coast which showed mycobacteria “is apparent in the coastal stock,” said John Jacobs, a NOAA scientist with the Oxford Cooperative Laboratory in Maryland.
Jacobs examined 20 fish taken during routine coastal surveys by the U.S. Fish and Wildlife Service and the DNR. A complete analysis of the fish has not been completed, but Jacobs said infections were clearly visible in several of the striped bass. Before, some had speculated the infections may go away once fish left the Chesapeake.
In addition to cooperating on the four-river study, scientists in Maryland and Virginia plan stepped-up fish surveys this year.
Virginia Institute of Marine Science researchers are examining striped bass caught during a fish survey conducted each season from the mouth of the Bay to the mouth of the Susquehanna River. During the spring survey, 150 fish were collected, but have not been analyzed.
“Hopefully, by the end of this year we will have a huge new data set that looks at the fish, not only in the tributaries like we have been doing, but throughout the mainstem of the Bay,” said Wolfgang Vogelbein, a VIMS fish pathologist. “It will be a first real attempt to look at the infections spatially throughout the Bay.”
Maryland DNR biologists this year hope to examine multiple age classes of striped bass, from just-hatched young-of-year to 4-year-old fish taken from several different areas. In all, they plan to look inside more than 500 striped bass—far more than they’ve examined in any other year.
“We want to get enough that we can actually say something about each year class,” Matsche said. Before, most of the fish examined were age 2 or 3. The expanded effort should yield clues about when the fish are becoming infected, he said.
No one knows why the disease showed up in the Bay, or seems confined to striped bass. In the big picture, scientists say, the fact that so many fish are infected signals that something is wrong in the ecosystem.
It’s not unusual for a fish population under stress to become vulnerable to disease, but it’s unclear exactly what is stressing the population even as the disease shows no sign of going away. Some believe the record-high striped bass population is suffering from malnutrition. Others believe water quality or other environmental conditions could be stressing the fish.