Linking Energy Expenditure to Habitat Use of Scaphirhynchus Sturgeon

The federally endangered pallid sturgeon Scaphirhynchus albus and threatened shovelnose sturgeon S. platorynchus are fluvial specialists that occupy heterogeneous flow fields within microhabitats of large rivers.  Recent studies suggest both species select for sand over gravel, yet no information exists regarding the energetic costs of occupying such microhabitats.  We used intermittent, flow-through respirometry to quantify energy expenditure (MO₂: mg O₂ kg^-1 h^-1) of age-0 pallid and shovelnose sturgeon within two experimental microhabitats, sand and gravel, at 15 cm s^-1 and 30 cm s^-1 velocities.  Differences in velocity between substrates were quantified using digital particle tracking velocimetry.  Velocities near the substrate declined over gravel compared to the sand bottom.  Substrate altered energy expenditure; MO₂ was higher over the sand substrate.  The MO₂ between sand and gravel was reduced more for pallid than shovelnose sturgeon.  Velocity did not affect MO₂.  Energy expenditure changed in a similar fashion with respect to velocity and substrate type.  Benefits of occupying sand may outweigh the increased cost of station holding.  Sand occurs in depositional areas that may hold more food and fewer predators.  Gravel may create turbulence sturgeon would otherwise avoid.  Differences in MO₂ between species suggest that segregation may occur where sturgeon overlap within microhabitats.