Seasonal variation in sampling indices for shovelnose sturgeon, river carpsucker, and shorthead redhorse collected from the Missouri river below fortRandall dam, South Dakota

Gill nets and electrofishing were utilized in the Missouri River below Fort Randall Darn, South Dakota to sample river carpsucker (Carpiodes carpio), shorthead redhorse (Moxostoma macrolepidotum), and shovelnose sturgeon (Sca phirhynchus platorynchus). Shorthead redhorse, shovelnose sturgeon, and riv er carpsucker all exhibited significant monthly variation in mean relative weight (WR) during at least one sample year (P < 0.05). In general, Wr valu es for all species tended to be highest early in the year followed by a ste ady decline to a seasonal low during the later sample months. Shorthead red horse and river carpsucker catch per unit effort (CPUE) followed a similar bimodal pattern of spring and fall peaks with lower CPUE during summer. Cat ch per unit effort of shovelnose sturgeon was highest in the fall at the lo wer site (P < 0.001, October 1998; P = 0.006 and P = 0.009, September and O ctober 1999) with no differences found at the upper site. Within-site varia tions in size generally were not significant for any species analyzed (P > 0.05). However, the median length of fishes tended to be higher at the uppe r site during early sample months and higher at the lower site during the l ater sample months, suggesting a migratory pattern for adult fish.