Critical Swimming Speed and Behavior of Juvenile Shovelnose Sturgeon and Pallid Sturgeon

The swimming performance of hatchery-reared. juvenile shovelnose sturgeon Scaphirhynchus plarorynchus and pallld sturgeon S. albus was studied in a laboratory swim tunnel at 20°C and 1O0C. The mean 30-min critical swimming speed was not significantly different between species at either temperature (36.9 crd s for shovelnose sturgeon and 35.9 cm/s for pallid utur-geon at 20°C. 19.4 cmls for shovelnose sturgeon and 15.0 c d s for pallid sturgeon at 10°C), Free swimming (swimming without contact with the substrate) waa observed less than 18% of the time at speeds greater than 15 cmls. As speed increased, pallid sturgeon Swam gig-nificantly less in the water column at 20°C; however, speed had no effect on percent free swimming among shovelnose sturgeon at 20°C. The results of this utudy Indicate thal, over the temperature and size range tested, shovclnose sturgeon and pallid sturgcon probably do not segregate in rivers due to different swimming or station-holding abilities Shovelnose sturgeon Scaphirhynchus platoryn-char and pallid sturgcon S. alhus are sympatric in the main-stem Mississippi and Missouri rivers and are: generally associated with flowing water htlbi-tats in or near the main channel (Bailey and Cross 1954). Adults of both species are known to use relatively low-velocity areas astiociated with dike fields, sand bars, and islands (Carlson et al. (Scaphirhynchus spp.) in the wild are infrequent, and young pallid sturgeon have only recently been sampled. Juvenile (5.0-21.0 cm) shovelnose sturgeon and pallid sturgeon have been collected by bottom trawling on the sand flats of the Mississippi River, primarily in main channel border areas whcre velocities 20 cm above the b o t t ~ m ranged from 20 to 80 crnls; sand troughs were usually present in general capture locationssource Monitoring Program, personal communication). The extent of overlap in habitat use by river sturgeons remains relatively unknown (particularly for the early life stages), but a potential consequence of sharing habitat would be an in-terspecific competition for resources. Fish occupying similar habitats partition resources along a trophic gradient or through differences in the temporal or spatial use of the habitat (Roes 1986). Closely related, sympatric species may segregate spatially in lotic environments through limits in their ability to occupy increased velocities (e.g., Peakc et al. 1997). While most available information suggests that river sturgeons at a given life stage occupy the same general habitats , in some field studics adult pallid sturgeon were found to occupy faster currents and to …