Chesapeake's Rockfish Overrun by Disease

Most rockfish begin their lives in the rivers feeding the bay. When they are 3 to 6 years old, they begin their journeys to the Atlantic Ocean, where they range as far north as Canada. At spawning time, most return to their birthplace.

This vast migration route confounds scientists' efforts to track the infection. In 1997, mycobacteriosis was discovered in adult fish, but the disease was already advanced. To find out when fish become infected, researchers such as Mark Matsche of the Maryland DNR visit rockfish spawning grounds in the upper bay and the Choptank and Potomac rivers, collecting eggs and young.

"The fish are exposed to the bacteria right from the start. . . . It's ubiquitous," he found. "It can survive in water or sediment or mucus."

An infected rockfish can appear outwardly healthy. But inside, the bacteria settle first in its spleen. The creature builds walls of scar tissue in fighting it, but the infection spreads to other organs. The rockfish loses weight, even as its insides swell, and it often develops sores. At some point -- researchers do not know exactly when -- it dies.

In the bay, "by age 1, 11 percent are infected. By age 2, it's 19 percent," Matsche said. But he cannot go beyond that -- by the third year, some fish have left the bay for open water. There is no way to see the infection's progress without dissecting the fish.

"We can't even say they die for sure," Matsche said. "The severely infected fish I catch . . . a lot of them die. Some moderately infected ones have some sign of healing going on. But I'm not able to see that same fish a year down the line."

About the same time the first diseased fish appeared, some researchers grew concerned about a possible link to fish handler's disease. In Maryland, 18 cases of the skin condition were reported in 2000. In 2004, there were 46.

The Mycobacteria strain that causes the skin disease has been found only in a small percentage of diseased fish.

Michele M. Monti, director of the Waterborne Hazards Control Program at the Virginia Department of Health, said the fish handler's bacterium can also lead to other problems, including swollen lymph glands or lung disease.

Tracking the potential effect on humans is more difficult because the states do not require that the disease be reported. So, Monti said, the low number of cases "could either be because there's not a lot of it out there . . . or they haven't gotten it diagnosed."

In the mid-1980s, rockfish numbers were so decimated by overfishing that Atlantic coastal states imposed a moratorium. Populations surged, and by 1995 the fishing ban ended. Wildlife officials call the restoration a rare triumph amid the pollution, overfishing and disease that threaten blue crabs, oysters and other species. But less than two years after victory was declared, the first diseased rockfish landed on bay shores.

James E. Price of the Chesapeake Bay Ecological Foundation said studies show that declines in the amount of menhaden in the rockfish diet coincide with the appearance of the disease. "It's logical," he said, "but nobody has any way to connect it."

Every day, as he has done for eight years, Wolfgang K. Vogelbein is surrounded by rockfish, some healthy, some dying -- he's not always sure which. Vogelbein, a fish pathologist at the Virginia Institute of Marine Science, was the first to diagnose mycobacteriosis in the bay's rockfish and determined that three-quarters of them carry it.

Last fall, Vogelbein, fish pathologist David Gauthier and mathematician John Hoenig affixed plastic tags to the bodies of 2,000 rockfish in the Rappahannock River, some outwardly diseased and some apparently healthy, with notes offering a reward for their return. They've gotten 120 from anglers. Using mathematical models, they hope to show whether the disease actually kills bay fish and estimate how long that takes.

So far, Vogelbein's team has found 10 strains of the bacteria in diseased rockfish, including two so new that their effect on humans is unknown.

"It's a difficult process trying to figure out the role of disease in a population of wild animals in a huge system like the bay," he said. "In this case, we still don't have the tools to efficiently answer the more compelling questions.

"That's just the nature of the beast."