Whirling Disease Found in Stocked Maryland Trout


The Maryland Department of Natural Resources (DNR) has confirmed the presence of whirling disease in a delivery of 8,000 commercially produced rainbow trout stocked in several Western Maryland streams. These fish are safe to consume, as the disease does not harm humans.

"While we believe this event represents a low risk, DNR considers the health of Maryland's trout streams and populations a key priority," said Fisheries Service Director Tom O'Connell. "Our goal is to eventually raise all stocked trout within DNR hatcheries. We are currently developing several new opportunities to expand our hatchery resources, which will allow us to rigorously monitor and control fish health issues."

On May 11, 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. DNR is continuing further testing to investigate this outbreak and working with the vendor in question to determine why potentially diseased fish may have been delivered. DNR will accept no fish from this vendor pending the results of this assessment.

DNR established an ongoing disease monitoring program on trout streams in 2007. Although whirling disease was introduced to several watersheds in fall 2006, test results have shown that it has not become established in any areas except the North Branch of the Potomac River. Regular monitoring and Maryland's past experience and enhanced understanding of the disease life cycle suggest that the risk of disease to wild trout populations from this introduction may be low. However, as a precaution, DNR biologists will conduct additional disease sampling of adults and sentinel fingerlings over the next several years.

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. In order to meet spring stocking goals, DNR is reallocating rigorously tested DNR hatchery produced fish originally slated for fall stocking.

DNR annually stocks approximately 328,000 fish for the spring trout season. DNR reminds anglers to help prevent the spread of disease and invasive organisms by cleaning boots and equipment thoroughly after fishing. Most importantly, do not move fish from one stream to another or discard carcasses in streams or on stream banks.

A brochure on whirling disease is available at dnr.maryland.gov/fisheries/pdfs/wd_brochure.pdf.

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