The Effects of Water Level Manipulation on Native Mussels in the Upper Mississippi River

Managers in the Upper Mississippi River (UMR) are using reductions in the river’s water levels during summer to mimic historical water regimes and rehabilitate habitats for vegetation and other species.  Concerns for the unintended effects of these actions on mussel populations has threatened to halt these projects.  We followed the movement and survival of mussels associated with a 1.0’ summer drawdown in Pool 5 of the UMR.  During 2009 (non-drawdown year) and 2010 (drawdown) we PIT tagged ~460 mussels of two species (Amblema plicata and Lampsilis cardium) at reference and treatment sites.  Recovery of tagged mussels was >88%.  Mortality averaged ~5% in 2009 and ~22% in 2010 and varied between species.  During the drawdown, we observed species-specific movement behavior such that A. plicata typically burrowed into the substrate, whereas L. cardium moved horizontally and followed the receding water.  Net movement of marked mussels was ~1.5 times higher at the treatment sites than the reference sites in 2010.  Mussel movement was associated with the rate of water level drawdown and oriented perpendicular to shore into deeper water.  Results from this study are being used by resource managers to better evaluate the effects of this management tool on native mussel populations.