A specter of disease is haunting rockfish

Threat: Some scientists feel that many Chesapeake Bay striped bass are infected with the slowly progressing and potentially deadly mycobacteriosis.

May 09, 2002|By Heather Dewar | Heather Dewar,SUN STAFF

OFF HOWELL POINT - The big striped bass caught in the scientist's net is so strong that even a pair of veterans, a waterman and a biologist, struggle to hold it.

Nearly 3 1/2 feet long, at least 10 years old and bulging with eggs, this oceangoing female has come to the upper Chesapeake Bay to spawn. Thwarted, the fish fights back, casting off a glistening arc of water as its body flexes powerfully and its yellow-and-black eyes roll.

After measuring and tagging the giant rockfish for a scientific survey, state fisheries biologist Lisa Warner tosses it overboard, where it vanishes. "Go, baby, go!" says an admiring Warner.

Is this fish, its future offspring and its kin in trouble, threatened by a widespread and potentially deadly disease?

Some respected scientists suspect so, pointing to signs that many, if not most of Chesapeake Bay's striped bass are infected with a disease called mycobacteriosis that is almost always lethal in fish farms.

The slowly progressing disease's only visible symptoms are skin sores, found on about 8 percent to 13 percent of bay rockfish in one study. Researchers in Maryland and Virginia also have found internal infections, sometimes accompanied by severe organ damage, in as few as 38 percent and as many as 69 percent of bay-caught fish.

But five years after mycobacteriosis was detected here, no proof exists that it has killed any wild striped bass. The first rounds of laboratory tests were inconclusive, with some wild-caught fish getting sick from the disease, some recovering and a few dying - of other causes, researchers believe.

Fishery managers say the Chesapeake's rockfish population is booming. They point to record-setting batches of striped bass offspring in 1993 and 1996, and another large group last year. The managers say that if the disease were killing off an extra 10 percent or 15 percent of the population, surveys would show that - and they don't.

But everyone agrees that important questions about mycobacteriosis need to be answered, because the stakes are high. With blue crab and oyster harvests at record lows the past two years, rockfish are the last resort for many bay watermen.

Recreational fishermen love to catch them, too.

More than two-thirds of the East Coast striped bass population spawn in the Chesapeake's freshwater reaches each April and May. The hatchlings spend two to five years in bay waters before most of them move into the ocean, where they roam from Canada to North Carolina.

"Striped bass are a symbolic species for Chesapeake Bay," said Eric Schwaab, director of fisheries at the Maryland Department of Natural Resources. "And the whole Atlantic coastal fishery is based largely on fish that are born in Chesapeake Bay. That's an industry worth hundreds of millions of dollars."

The bay's young adult males, about 3 to 5 years old and 18 to 22 inches long, seem to be most seriously affected, scientists say.

Scientists at Maryland's Cooperative Oxford Laboratory noticed high numbers of rockfish with sores in the early 1990s, said laboratory director Steven Jordan. Concern increased after the Pfiesteria outbreak of 1997, which mainly affected another variety of fish but drew attention to the rockfish sores.

The next year, Wolfgang Vogelbein, director of the aquatic animal disease laboratory at the Virginia Institute of Marine Sciences (VIMS), reported that the culprit was an aquatic member of a large family of organisms called mycobacteria.

Some kinds of mycobacteria are harmless; others cause tuberculosis and leprosy in humans. One variety, mycobacterium marinum, can cause a rash known as "fish handler's disease" in people who handle infected fish. It can usually be cured by antibiotics, but in rare cases the infection can cause more serious problems.

Mycobacteria were detected in Pacific salmon in the 1950s and in Pacific striped bass in 1983. The 1998 discovery of the disease in Chesapeake rockfish was the first known outbreak in any wild Atlantic Coast fish.

Mycobacteriosis is a slow, wasting disease, like tuberculosis in humans, Vogelbein said.

Its infections "look like they are probably lethal in the long run," said Jordan, who is helping coordinate research on the disease. That is an assumption based on progressive, severe damage to organs and the high death rate among infected stripers on fish farms.

"We really don't know the effect on [wild] fish," Jordan said. "Some have said, `They're all going to die,' and others have said there may be circumstances under which they can heal."

"I suspect there's probably a chronic low level of mortality," Vogelbein said. "The most heavily infected fish are continually being lost and we never, never see those fish."

No one knows how long the disease has been here, how widespread it is, or whether the number of infected fish is increasing. Maryland scientists examined frozen tissue taken from rockfish in the 1980s and early 1990s and found suspected mycobacteria in a handful.

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