VIMS links bass mortality to bacteria

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November 14, 2008

1:25 AM

Researchers from the College of William and Mary’s Virginia Institute of Marine Science have found that the bacterial disease mycobacteriosis, currently found in over half the striped bass in the Chesapeake Bay, has increased the mortality rate of the fish.

“We have demonstrated that in the Chesapeake Bay, infection with this bacteria is statistically linked with an increase in mortality,” VIMS researcher Chris Bonzek said.

Bonzek, along with other VIMS researchers and scientists from Coastal Carolina University and the U.S. Geological Survey’s National Wildlife Center, studied the striped bass between 2003 and 2005.

The researchers found that older female striped bass are more likely to encounter the bacterial disease because they use more energy in reproduction and migration than males. The number of fish that actually died from mycobacteriosis increased during the summer months, which researchers hypothesize is due to lower oxygen levels in the bay at that time.

Mycobacteriosis is a problem for the Chesapeake fishing industry because the bay is the largest reproduction area for striped bass on the Atlantic Coast.

Data for these findings were culled from food-web interactions and fish demographic patterns in the bay. A new advanced technology that measures demographic patterns also gave aid to the striped bass research.

“Since death from such infections in a stock spread over a broad geographic area can’t be measured directly, estimating mortality associated with a chronic disease is a very difficult nut to crack,” Bonzek said. “Coast-wide stock assessments … of the popular and iconic striped bass have shown an unexplained increase in natural [non-fishing] mortality in recent years.”

This disease and the accompanying increase in natural deaths pose a problem for the Chesapeake fishing industry. Fisheries remove a certain number of striped bass from the bay based on the number of fish that die from natural causes to prevent a decrease in the overall stock.

“If the number dying from natural causes, such as disease, goes up, then the number taken by fishermen has to decrease,” Bonzek said. “Further, the lesions caused by this disease are extremely unsightly and anyone catching one of these fish is going to throw it right back. So, even though a recreational fisherman might still catch a fish, he or she won’t have had a fun experience, and a commercial fisherman wouldn’t be able to sell such a fish.”

According to Bonzek, this increase in natural mortality could mean restrictions on fishing.

“In the short term at least, fishery managers can’t directly control the environment, can’t control the predation, can’t control disease. The only tool they have is to control the harvest,” he said. “While management of striped bass has been a terrific success over the last 20 years, this is a potential dark cloud in an otherwise sunny picture.”

Bonzek said he is not sure if a cure for the spreading disease can be discovered.

“There’s not likely to be any method discovered by which we can help fish fight this disease, but the more we understand about it the better we can help fishermen, fishery managers and fish consumers,” he said.

This research, Bonzek said, has cost the state little because it is part of larger fisheries research projects, and recent state budget cuts have not significantly affected this project.

“As the funding for this work comes from federal sources and not the Commonwealth, the recent budget cuts announced by the governor don’t directly impact this work,” Bonzek said. “However, federal agencies are also likely to be cutting back and we just hope that our projects can continue.”

Bonzek added that this research, while important, is only a small part of VIMS’ mission.

“One of the important missions of the Virginia Institute of Marine Science is to advise state fishery managers on the status of the various fish populations,” he said. “This is just one of a thousand such contributions that researchers here at VIMS make every year in support of wise utilization of the Commonwealth’s fish resources.”

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