Maryland Stocked Trout Infected With Whirling Disease

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Stocked trout carrying whirling disease were released by the State’s Department of Natural Resources (DNR) into Maryland streams during spring 2011 stocking efforts. Maryland DNR has confirmed the presence of whirling disease in a delivery of 8,000 commercially produced rainbow trout stocked in several Western Maryland streams.

On May 11, 2011, DNR staff observed suspicious behavior in fish that had been stocked in the North Branch Delayed Harvest Area, Evitts Creek, Jennings Run and Sidling Hill Creek. They immediately ceased stocking activities and took samples for testing. Results of this sampling confirmed the presence of whirling disease.

The whirling disease parasite was introduced into the eastern United States from Europe in the late 1950s and is currently known to exist in 24 states. It was first discovered in Maryland in 1995 in the North Branch Potomac River. Although harmless to humans, the parasite can be fatal to trout and is particularly harmful to rainbow trout.

MD DNR continues testing to investigate the outbreak and is working with the vendor in question to determine why potentially diseased fish may have been delivered.

According to MD DNR,  hatchery resources cannot meet all the demand for stocked trout, so commercially produced fish are used to supplement spring trout stocking. Vendors that supply fish to the State are required to be certified disease free for three years.  Maryland annually stocks approximately 328,000 fish for the spring trout season.

The agency recently reminded anglers to help prevent the spread of disease and invasive organisms by cleaning boots and equipment thoroughly after fishing. Officials also ask that anglers do not move fish from one stream to another or discard carcasses in streams or on stream banks.

This latest whirling disease outbreak fuels an ongoing national debate about environmental effects that are associated fish stocking programs. Opponents of trout stocking programs cite the spread of invasive diseases, and declines of native trout due to introductions of non-native species and strains as evidence that trout stocking program reforms are long overdue.

A brochure on whirling disease is available at