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In the mainstem, 5572 fish of 18 species were captured in seines and 727 fish of 13 species were captured in traps. Trap netting missed species of small fish, i.e. fathead minnow, emerald shiner, sand shiner (Notropis stramineus), and longnose dace (Rhinichthys cataractae), and also missed the one (each) white bass and largemouth bass that were collected by seining. Traps were most effective on river carpsucker (158 fish), channel catfish (111 fish), and shorthead redhorse (Moxostoma macrolepidotum) (88 fish). In the seine catch, the sand shiner usually had the highest CPUE values, which exceed 2 fish/m in some reaches (Table 3). Shorthead redhorse, flathead chub (Platygobio gracilis), longnose dace, plains minnow, and western silvery minnow usually had CPUE values ranging from 0.1 to 1.0 fish/m, but the CPUE for the remaining 12 species was usually less than 0.1 fish/m. The CPUE values varied greatly for most species by reach and between years (Table 3). For example, CPUE for channel catfish ranged from 0.01 at reach 4 to 1.09 at reach 9 in 1995. In 1996, CPUE at reach 4 increased to 0.08, but at reach 9 declined to 0.04.

Mean W_r values by size category for channel catfish were 103 ± 1 (substock lengths), 81 ± 1 (stock to quality lengths), and 92 ± 3 (quality to preferred lengths). The skewed U-shaped W_r pattern for river carpsucker was similar to that of channel catfish (Fig. 3), but mean W_r values for most size categories were lower: 88 ± 2 (substock lengths), 84 ± 1 (stock to quality lengths), 78 ± 1 (quality to preferred lengths), and 80 ± 2 (preferred to memorable lengths). The W_r pattern for white sucker was opposite that of the channel catfish and river carpsucker (Fig. 3). The W_r values for white sucker were 78 ± 4 (substock lengths), 81 ± 3 (stock to quality lengths), 71 ± 2 (quality to preferred lengths), and 67 ± 2 (preferred to memorable lengths).

Figure 3.  Actual and predicted relative weights for channel catfish, white sucker, and river carpsucker collected in seines and trap nets from the Moreau River, South Dakota in 1995 and 1996. Arrows indicate ranges for substock length group (SS), stock to quality length group (S-Q), quality to preferred length group (Q-P), preferred to memorable length group (P-M), memorable to trophy (M-T), and trophy (T).

River flow characteristics varied somewhat among reaches and between years. Three river width measurements increased in the downstream direction. Between 1995 and 1996 the mean water surface width increased by 0.1 to 4.2 m at reaches 2 through 11. The change in width was probably a result of discharges that were about 0.4 m³/s higher in 1996 than in 1995 (Fig. 2). Discharges during our study were about the same as the historical mean during the months of June, July, and August. High discharges during February and May during our study were similar to the maximum historical discharges. Thus, we were working during a relatively wet period. For example, the maximum historical mean monthly discharge in May is 62 m³/sec, and the mean monthly discharge during May of 1996 was 60 m³ /sec. Water quality was similar among reaches and between years at the mainstem reaches (Table 2). Secchi disc depths ranged from 11 to 69 cm and were negatively correlated to discharge (r = 0.60, df = 20, P < 0.01). Reach gradient ranged from 2 to 24 cm/ 100 m in 1995 and from 3 to 23 cm/100 m in 1996. The dominant macrohabitat in the main river was the run, an area of relatively uniform depth and velocity, which made up 77 to 78% of our sample reaches depending on year. From 10 to 13% of the transects crossed riffle macrohabitats, 5 to 7% crossed pools, and about 4% were classified as mixed macrohabitats each year. A mix of clay, silt, and sand (termed "fines") covered 34 to 77% of the river bottom depending on station (Fig. 4). Silt made up most of the fines in 1995 (51%), whereas sand made up most in 1996 (56%). Very fine to very course gravel was the next most common substrate class (37% in 1995 and 29% in 1996), followed by the cobble-boulder category (Fig. 4). In-stream and riparian vegetation was sparse. Emergent macrophytes were usually rushes (Juncus spp.) occurring in stands covering less than 1 m², but there were no emergent macrophytes at 65% of transects. Banks were primarily vegetated with grasses and forbs (57%), sedges (Carex spp.) and rushes (20%), and willows (Salix spp., 7%), but 16% of the banks had no vegetation. This vegetation community of grasses and forbs, interspersed with occasional cottonwood (Populus deltoides) and willow trees extended from the top-of-bank into the upland vegetation community. In 1995, 55% of the transects showed no livestock use. Where grazing was apparent that year, 16% of the transects were classified as heavily grazed, 6% moderately grazed, and 78% lightly grazed. In 1996, all transects were grazed; 82% were judged to be lightly grazed and 18% moderately grazed.

Figure 4.  Percentage of three size categories of substrate at 11 stations on the upper Moreau river in South Dakota in 1995; fines are particles less than 1 mm in diameter, gravel diameter is between 2 and 64 mm, cobble is greater than 64 mm in diameter.