Grain size and sorting effects on size-dependent responses by lotic crayfish to high flows

We examined how sediment grain size and sorting mediate flow-induced dislodgment, movement (i.e., time spent crawling), and postural changes of small (10.5–19.9 mm carapace length [CL]), medium (20–29.9 mm CL), and large (30–42.4 mm CL) crayfish (Orconectes obscurus) subjected to incrementally increased flows in a laboratory flume. We also compared size-specific distributions of O. obscurus among three habitat types (deep pools, shallow pools, and riffles) with different current velocities and grain properties in a 4th-order stream in northeastern Ohio, USA. Number of crayfish dislodged increased with current velocity and crayfish body size on all grain sizes (small pebble, large pebbles, and small cobbles) and all sorting treatments (well, moderately, and poorly sorted) in the flume. Small crayfish were dislodged at significantly higher average current velocities than medium and large crayfish on all grain sizes and sorting treatments, in part because they were small enough to crawl into stone interstices to avoid high hydraulic stress. Movement by all crayfish decreased as current velocity increased on all grain sizes and sorting treatments. However, activity responses differed in a size-specific fashion. Crayfish of all sizes flattened their bodies against grains, flattened or tucked their telson under their abdomen, and attempted to use their chelipeds to grasp grains to resist dislodgment or aid in movement on the grain patch at increased velocities. Field collections showed that small crayfish occupied riffle habitats characterized by faster current velocity and larger grain size, whereas larger crayfish occupied deep and shallow pools characterized by slow current velocity and smaller grain sizes. Our results imply that body size is a critical factor in determining crayfish habitat distributions and resistance to high-flows in streams with stony beds.