Surveys conducted late last year suggest that zebra mussels continued to expand their foothold in the lower Susquehanna River, highlighting the need for boaters to be careful not to spread the invaders.

The bivalves were spotted at a marina above Conowingo Dam in 2008, and have since slowly expanded their range.

They turned up below the dam in 2010, and in recent years small numbers have been found attached to buoys near Havre de Grace at the river’s mouth. In 2011, one turned up in the nearby Sassafras River.

Last year, veligers — larval zebra mussels — were found in the Harford County Water Treatment Plant in Havre de Grace, indicating that a breeding population has been established below Conowingo.

Biologists believe a reproducing population also exists someplace in the Susquehanna above the Pennsylvania state line. Weekly monitoring near the water intake of the Peach Bottom Atomic Power Station, just above the border, has generally seen a growing number of veligers since 2009, though observed numbers declined somewhat in 2013.

Ron Klauda, a biologist with the Maryland Department of Natural Resources, said it’s hard to predict whether the population will grow to damaging levels.

He said he doubts the lower Susquehanna is prime zebra mussel habitat — they typically do better in lakes and reservoirs than rivers, though there are exceptions. “But they probably can hang on,” Klauda said. “We just hope that they don’t ever start to really flourish and expand to nuisance levels.”

He and other biologists hope the mussels are not accidentally spread to places where they could thrive. Because their larvae latch onto boats and can be carried on vegetation clinging to boat trailers, they are easily transported from place to place.

Zebra mussels reached Susquehanna headwaters in New York in 2000, and populations in parts of Otsego Lake and Goodyear Lake have mushroomed in recent years, especially in places where they can attach to solid surfaces, such as rocks.

“Five or six years ago, you would barely see one or two mussels,” said David Wong, of the State University of New York College at Oneonta. “Now they have 5,000 per square meter.”

Biologists don’t believe the zebra mussels found in the lower Susquehanna are from New York. Although monitoring is sparse, there has been little evidence of the bivalves in most of the 400 miles that separate the two populations.

Nonetheless, Wong said he would like to see stepped-up monitoring throughout the basin. While much of the river probably is not suitable for the mussels, pockets of habitat likely exist throughout its length. Over time, he expects the mussels to hopscotch their way down the river as they colonize areas, build up populations and produce veligers that move downstream to the next suitable site.

Named for the striped pattern of their shells, zebra mussels are thought to have arrived in the Great Lakes in 1988 in the ballast water holds of ships from Europe and have since spread through much of the country.

They have been a major nuisance for industrial and municipal water users as their dense colonies can block water intake pipes. They can also coat boat bottoms, cables and any other hard surface, and have been blamed for billions of dollars in damages nationwide.

Zebra mussels are powerful filter feeders. In some places with large populations, they have reduced the food supply for fish, causing populations to fall. Competition with the zebra mussels has nearly wiped out native mussels in some areas.

Help to stem the zebra mussel invasion!

Boaters, anglers and other recreational water users can help stop the spread of harmful zebra mussels by taking these simple precautions before launching and leaving the water:

  • Remove all aquatic plants and mud from boats, motors and trailers, and put the debris in the trash.
  • Dispose of unused live bait on shore, far from any water body, or in the trash.
  • Drain river water from boat motors, bilges, live wells, bait buckets and coolers before leaving to prevent aquatic hitchhikers from riding along.
  • Rinse boats, motors, trailers, live wells, bait buckets, coolers and scuba gear with high-pressure hoses or hot water between trips to different water bodies.
  • Dry everything at least two days (preferably five days) between outings.
  • Limit boating from place to place, particularly from the Susquehanna and Sassafras rivers to other water bodies where zebra mussels haven’t invaded.