Water depth, flow variability, and size class influence the movement behavior of freshwater mussels (Unionida) in Great Lakes river drainages

The flow regime shapes lotic environments, influencing local instream habitats that limit the distribution, abundance, and long-term persistence of aquatic macroinvertebrates, such as freshwater mussels. Mussels are sedentary in nature and as such are exposed to an array of flow conditions over time. They require stable habitat at high flows as well as adequate water levels during low flows. To persist within suitable habitats across a hydrologic regime, mussels use vertical and horizontal movement. Mussels will either burrow vertically into the riverbed or use their muscular foot to move horizontally to new areas as habitat conditions change. Using mark-recapture techniques, we investigated how instream depth-flow conditions and other aspects of hydrology influence vertical burrowing behavior and horizontal movement rates of freshwater mussels in rivers of the Great Lakes region (Michigan, USA) and also examined differences among species and size classes. We found that water depth was the most important variable for predicting mussel subsurface probability, and burrowing into the substrate decreased as water depth increased. We also found that shell length was an important predictor of burrowing, and we observed subsurface probability to rise with an increase in shell length. We observed horizontal movement rate to increase when flow variability was low. Our results indicate that movement responses of mussels differ depending on short-term hydrology, localized water depth, and mussel size class, suggesting that these factors should be considered when implementing conservation strategies, such as mussel sampling for population assessments. Climate change and human water demands are projected to further influence the parameters we found to be important for mussel movement and thereby may negatively affect mussels in the future. Understanding and accounting for movement responses could improve freshwater mussel conservation and lotic water management.