A resident spotted a single adult zebra mussel attached to a dock in the upper Eastern Shore river near Turner Creek, according to DNR.
"One mussel does not necessarily constitute a population, DNR biologist Ron Klauda said in a news release issued by the department. "but it is extremely unlikely that this is the only one out there in the Sassafras."
DNR biologists think the unusually low salt levels of the upper bay this sumemr may have helped zebra mussels spread. The Sassafras is not far from the Susquehanna River, where the mussels were first seen in the lower reaches in Maryland in 2008.
Zebra mussels have caused more than $5 billion in damage and economic losses across North America since they got into the Great Lakes in the 1980s, officials says. The mussels attach themselves to any hard surface, and as they grow can clog water systems and power plant cooling intakes. They're also outcompeting native mussels, some of them already endangered, for food and habitat.
Recreational boaters are often unwitting transporters of the mussels, DNR reports. State officials urge boaters, anglers and others who use the waters of the lower Susquehanna to take precautions, such as draining river water from bilges, live wells and other containers limiting boating from place to place.