Striped bass mycobacteriosis survey
Maryland DNR initiated a health survey of striped bass in 1998. This general health survey involved collections of striped bass from the lower, middle and upper portions of Maryland’s Chesapeake Bay, including numerous tributaries, and was designated to identify problems in the population. Complete examinations were performed on these fish and samples were processed for microbiology (pathogen identification), histopathology (visible signs of disease at the cellular level), nutritional status and parasite load. Results indicated that the disease of concern was mycobacteriosis and that infection rates were rising.
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Fish with severe Mycobacterosis may be emaciated and have multiple reddened or grayish ulcers.
| Comparison of spleens from healthy and diseased fish.
Diseased spleen, found along the intestine,may appear enlarged, have numerous gray-colorednodules(granulomas - yellow arrow), and occasionally have fluid-filled abscesses (blue arrow). |
Starting in 2003, DNR began a more detailed study of mycobacteriosis in striped bass, focusing on resident fish ages 0-5. Each year fish are collected from various locations within Maryland’s portion of Chesapeake Bay. Samples collected are processed for histopathology, microbiology and PCR to identify mycobacteriosis infections. Infection rates vary among different age classes with an increase as fish get older. Young of the year (age 0) have an infection rate of less than 1% whereas 5 year old striped bass are greater than 60%, infection rates do not vary between different portions of Chesapeake Bay or its tributaries. Since the beginning of this survey overall infection rates have plateaued around 60% with severity of the infection varying from a mild to a severe state.
Fish Kill
While the Maryland Department of the Environment (MDE) has the mandate for the investigations of fish kill events in Maryland, The Maryland Department of Natural Resources (DNR) aids in field investigations, training via sample collections and preparation, and performing fish health diagnostic services. Fish kills can be caused by various factors, including pollution, disease, and hypoxia/anoxia. Field investigations may reveal sufficient environmental information to determine probable cause, but more detailed examinations of fish are often necessary. The Aquatic Animal Health Program at the Cooperative Oxford Laboratory provides these diagnostic services to help determine probable cause when possible.
Swan Point, Potomac River fish kill investigation February 20, 2007 (Photo courtesy of MDE)
