Date of this Version
The diet of shovelnose sturgeon (Scaphirhynchus platorhynchus) in the Missouri River downstream from Gavins Point Dam was characterized in 1993 (N=100 fish) and 1994 (N=103 fish) for comparison with similar data collected in 1971-72 (N=130 fish) by Modde and Schmulbach (1977, hereafter Modde). The primary difference among years in the South Dakota studies was river discharge, which was low in 1993 (9-22 kcfs; kcfs=thousand cubic feet per second), moderate in 1994 (18-31 kcfs), and high in 1971-72 (26-49 kcfs). We compared sturgeon diet weight and the weight of benthic and drifting invertebrates among years, among monthly discharges in 1993 and 1994, and among three types of river substrates. We also compared our results with those of Megargle and White (1997) who conducted a concomitant study in Montana portions of the Missouri River where discharges were relatively natural. Our working hypothesis was: sturgeon diet quantity and quality decreases as discharge increases because high discharges make the prey (benthic and drifting invertebrates) less available and less accessible to the predator (the sturgeon).
For the predator (sturgeon), we found no difference in diet weight among years of low, medium, and high flow. However, diet weight did decrease as discharges increased among months within a year, but other factors like temperature and seasonal changes in invertebrate biomass are also changing monthly. All sturgeon used in the analysis ranged in length from 408 to 675 mm (standard length). We recaptured five sturgeon that had been tagged in the same river reach 13 years earlier by Dr. Jim Schmulbach. These individuals had grown an average of 12 mm in length; two had gained weight and three had lost weight.