Disease and Chesapeake Bay Striped Bass

Scientists Report the Fin Fish are Dying from Mycobacteriosis

Jan 3, 2009

Analysis of three years of data prove that mycobacteriosis is causing a decline in Chesapeake Bay's striped bass population.

Models show that chronic bacterial disease, first detected in 1997, infects more than half of all striped bass in the Chesapeake Bay. However, it has been difficult to determine if the bacterial infection is causing the fish to die any earlier than they might die of old age or for other reasons.

Recently, researchers from the College of William and Mary’s Virginia Institute of Marine Science (VIMS) in Gloucester Point, Virginia, have conducted studies and their conclusion is that mycobacteriosis has contributed to increased mortality of the striped bass, commonly known as stripers or rockfish.

What is Mycobacteriosis?

Mycobacteriosis is a generic term used to describe diseases caused by a number of different single-celled bacteria called mycobacteria. It is widespread and common, with a few mycobacteria known to cause disease in other animals including humans. A newly described species, Mycobacterium shottsii, is most commonly associated with the current outbreak of mycobacteriosis in striped bass the Chesapeake Bay.

Diseased fish usually exhibit nodules on internal organs and the less frequent external signs are reddish ulcers. The unsightly ulcers have concerned anglers and others.

Can the Lesions Infect Humans?

Some types of mycobacteriosis are known to infect humans, however it is unclear that this particular species is problematic in that way. Fish handlers are urged to use caution (wear gloves, wash thoroughly) and it is not advisable to eat fish that exhibit mycobacteriosis.

It is especially important to use caution because the mycobacteriosis is similar to another species of mycobacterium that have infected humans, causing reddish nodules on the hands, knees and feet and swollen joints.

Importance of Understanding Mycobacteriosis Impact

The impact of mycobacteriosis on striped bass populations is important to understand since rockfish are the most important commercial and recreational fish species in the Chesapeake Bay. Fisheries managers need to know whether mycobacteriosis ultimately cause the fish to die sooner. The Chesapeake Bay is the primary breeding ground for striped bass on the Atlantic Coast of the United States.

The Study

Scientists had observed what they thought to be an increase in early death rates. To prove that the rate increase was due to mycobacteriosis, they collected data for three years for analysis. They also consulted data collected by other groups studying fish demographics and the interactions of fish in the food web. The analysis showed that an infected fish is about 70 percent as likely to survive another year as is a non-infected fish. Older females are more likely to die sooner than are males. Finally, death rates increase through the summer, illustrating the impact of additional stress from warm water temperatures and low levels of oxygen. The conclusions of the VIMS scientists were published in the Ecological Society of America journal.