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A costly biological monitor: Mussel populations decline as water quality declines. Many species now endangered, extinct

By Jess Jones, U.S. Fish and Wildlife Service restoration biologist

Since mussels are long-lived -- typically 20-50 years -- and sensitive to particular contaminants, they serve as excellent indicators of water quality. Sadly, they are giving their lives to show us a grim picture of water pollution. The Clinch, Powell, and Big South Fork Cumberland rivers in far Southwest Virginia and eastern Tennessee collectively contain the greatest number of endangered mussel species in North America.

From 1979 to 2007, biologists from state and federal agencies have collected quantitative data in the three rivers to document the population status and trends of more than 40 mussel species.

Clinch River

• The Tennessee side of the Clinch River exhibited a 2.5-fold increase in population abundance. There are high numbers of juveniles across a range of species, including endangered ones.

• Mussels on Virginia side have declined by 70 percent in abundance, with a complete loss of numerous species from the river in the past 50 years.

• At Pendleton Island in Scott County, Va., densities of 25 mussels per quarter meter in 1979 declined to 4.6 mussels in 2004.

• Upstream, from Nash Ford to Carbo, Va., mussel populations appear to be in recovery, and the reach below Carbo is exhibiting early signs of comeback despite serious spills of fly ash and other contaminants during the 1960s and 1970s.

Powell River

• In Tennessee and Virginia, mussel populations in the Powell River have declined by 63 percent. In 1979, there were 8.8 mussels per quarter square meter; in 2005, there were 3.2 and seven species are endangered.

• In a remote and very scenic section of the river along the Virginia-Tennessee border, mussel diversity (some 30 species) remains high, but annual recruitment is low and unlikely to sustain viable populations for some species.

• Mussels are now rare or absent in the upper 25 miles of the Powell River from Woodway upstream to Appalachia, Va.

• Reintroduced juvenile mussels in the Powell are not surviving at a high rate.

Big South Fork

• The upper Big South Fork Cumberland River in Tennessee and Kentucky, one of the last natural free-flowing rivers remaining in the Cumberland River system, harbors an excellent mussel variety of 26 species that is showing signs of recovery. This is the last stronghold in the Cumberland River system of the endangered tan riffleshell, littlewing pearlymussel, Cumberland combshell, and Cumberland bean.

• Reintroduction of mussels upstream of the monitoring sites appears to be beneficial.

• Populations inhabiting some of the Big South Fork’s major tributaries, such as the New and Little South Fork rivers, show severe decline and little to no evidence of recovery. All 26 species are in decline in the Little South Fork.

Questions

A detailed assessment of water quality and physical habitat factors is needed to investigate why mussels are declining in certain river reaches. An additional key question is the effect of agriculture, forestry, urbanization, and mining on the small- and medium-sized tributaries and how water quality of those tributaries affects the main river. It is important that we inventory the tributaries and prioritize what we do.

Jess Jones, who is stationed in the Department of Fisheries and Wildlife Sciences at Virginia Tech, has conducted and published numerous studies on mussels.

 

The Cumberland combshell. Photo by Nicholas A. King

Retired U.S. Geological Survey mussel biologist Steve Ahlstead and Dan Hua, research specialist at Virginia Tech's Freshwater Mollusk Conservation Center (FMCC), survey mussels in the Tennessee portion of the Clinch River. Photo by Nicholas A. King