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Mussel Survey in Roanoke River Yields Positive Results

Feb. 22, 2006

Raleigh, North Carolina — It’s tough being a mussel — particularly one living in the Roanoke River in Halifax County. Ever since a dam altered the river’s flow in 1963, water has bypassed a significant portion of the original river channel, leaving a river bed that, until two years ago, was virtually dry and devoid of all but the most tenacious aquatic wildlife.

But through a relicensing agreement, Dominion Virginia Power Co. began releasing water back into the original river channel, or bypass reach, in May 2004. Biologists with the N.C. Wildlife Resources Commission hope the water’s steady flow will prompt the return of greater numbers and varieties of mussels and other aquatic species to an area where they once flourished.

To assess what animals will eventually recolonize the reach, wildlife biologists had to know what species were present before the water release. In June 2004, they conducted a survey in a 1.5-mile stretch of river to determine the distribution and abundance of mussels, snails and crayfish in the upper and lower sections of the reach.

By comparing these data with information they’ll collect after completing the next survey in five to seven years, biologists will know what species, if any, recolonized the reach on their own.

The initial survey yielded encouraging results in the lowest section of the reach, which is basically the backwater of the river. There, biologists found nine mussel species, two crayfish and five snail species. Biologists had expected to find a diverse aquatic community given that the habitat in the lower reach was similar to the current river channel, with comparable substrate and water depth.

Conversely, they found little aquatic wildlife — only one or two mussel species, a few snails and a couple of crayfish — in the upper section. This discovery was not surprising since the area lacked the water flow necessary to sustain large numbers of wildlife.

Biologists hope that with the water flowing continuously, mussels and other aquatic species now present in the lower reach will recolonize the upper reach in greater numbers, if given time. How much time they don’t know for sure, although they expect the presence of fish in the reach will expedite mussel movement upstream.

“In order to complete their life cycle, larval mussels attach themselves to the gills of certain kinds of fishes and then are carried by the fish to different parts of the river,” said Angie Rodgers, eastern aquatic nongame coordinator for the Commission. “Eventually, the larvae drop off the fish and onto the river bottom. If the habitat is suitable, that location becomes the mussels’ new home.” [see life cycle here]

The Commission has no plans to restock any species in the upper section. It may consider reintroduction strategies if biologists don’t see any signs of recolonization after the next survey.

Species recolonization is the most appealing prospect for two reasons: it indicates that a species can survive and thrive without human intervention; and it shows that water quality in the reach is suitable to support even the most finicky of aquatic fauna.

“Being sedentary filter feeders, mussels are extremely sensitive to changes in water quality and will not survive if the water quality is poor,” Rodgers said. “Because of their sensitivity, mussels are often referred to as ‘biological indicators of water quality.’”

Diverse mussel populations, as biologists found in the lower reach, indicate that the water is clean enough to support other aquatic wildlife.

In addition to serving as water-quality monitors, mussels act as a natural water filter by straining out suspended particles and pollutants. They also are a vital link in the aquatic food web, serving as a food source for a variety of wildlife, including muskrats, otters, raccoons, herons and some game fish.

Among the nine mussel species that the Wildlife Commission detected in its survey, five of them are state-listed as either threatened or endangered. Their presence in the lower reach is particularly encouraging because they often are less tolerant of poor water quality than their hardier, more common counterparts.

Although more than 60 species of freshwater mussels call North Carolina home, 50 percent are state-listed as endangered, threatened or special concern. Funding for this survey comes from the Nongame and Endangered Wildlife Fund, the primary source of state funds for the Commission’s Faunal Diversity and Aquatic Nongame programs. The Wildlife Commission uses this fund, which supports nongame species research and management, to generate matching money from federal grants.

You can support this effort as well as other nongame species research and management projects in North Carolina through the Tax Check-off for Nongame and Endangered Wildlife. This tax check-off allows taxpayers to designate part or all of their state tax refunds to this fund. Since 1984, taxpayers have given more than $7 million for wildlife conservation through this funding mechanism.

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