'The smell will knock you off your feet': mass mussel die-offs baffle scientists

Mussels, the backbone of the river ecosystem because they control silt levels and filter water, are facing a mysterious affliction

Each fall since 2016, wildlife biologist Jordan Richard has returned to the same portion of the Clinch River in Tennessee, braced for the worst - tens of thousands of newly dead mussel shells gleaming from the surface of the water.

The mass die-off isn't recognizable at first. But once Richard sees the first freshwater mussel, which look quite different to their marine cousins of moules frite fame, he scans the river and finds another every five to 10 seconds.