Freshwater mussels vanishing from southeastern US rivers

About 300 species of freshwater mussel have been documented in North America, and the southeastern United States holds more of them than anywhere else. Around a tenth are extinct and a third of the rest are listed as endangered or threatened.¹ Dams caused about 20 of the 30 or so 20th-century extinctions, but dam building had largely stopped by the mid-1980s and rivers have been cleaner since the Clean Water Act of 1972, yet most populations kept falling.¹ In Kentucky's Horse Lick Creek, which had dozens of species in the late 1980s, the die-off was mostly complete by 2005, and a recent search of more than an hour found not one mussel, alive or dead.¹ Since 2016 Virginia's Clinch River has had repeated sudden die-offs of thousands of pheasantshells, cause still unknown.¹ A 2018 experiment at 17 sites in the Rockcastle River found that caged young mussels grew poorly where invasive Asian clams (Corbicula fluminea) were abundant, with no link to water chemistry.¹ A larger study of about 90 streams in 13 states, from Minnesota to Georgia, is now being analysed, and so far the clam is the one factor present in most streams with declines.¹

Who needs an answer

• State hatcheries that breed rare mussels and release them, deciding which streams will let them survive.¹
• The US Fish and Wildlife Service and its partners, spending recovery money on six rare species through a $3.4 million grant to the Tennessee Valley Authority.¹
• River and watershed managers, if the clams prove to be the cause.
• Water managers, since mussels filter large volumes of river water: those in one 300-mile stretch of the Upper Mississippi filter more than 14 billion gallons a day.¹

The domains it chains

River flow, temperature and sediment; algae and bacteria that mussels and clams both eat; an invasive clam now in 47 states that can reach thousands per square yard;¹ the host fish every mussel needs to carry its larvae; possible pathogens; and the clarity of the water downstream.

What a twin should let people try

• Competition: if Asian clams take the food young mussels need, at what clam density does a mussel bed stop replacing itself?
• Recovery: in parts of the Rockcastle where the clam population seems to have crashed, young mussels are appearing again.¹ How fast could a bed come back, and from which nearby populations?
• Releases: given host fish, flow and clam numbers, which streams are the best places to release captive-bred mussels?

Where to start

The 13-state stream study led by Wendell Haag of the US Forest Service holds mussel counts, algae, sediment, water chemistry and fish and insect surveys for about 90 streams; it is still being analysed.¹ Stream observations for US rivers are in USGS Water Data.