Limonology of Shadehill Reservoir, SD and Food Habit of Analysis of its Walleye Fishery

The physiochemical characteristics, nutrient concentrations, primary production, and the abundance and standing stock biomass of zooplankton and zoobenthos were examined during 1994 and 1995 to determine if limnological conditions in Shadehill Reservoir, South Dakota were responsible for an observed slow growth rate of the reservoir\u27s walleye Stiostedion vitreum population. In addition, the food habits of the walleye population were examined to determine the trophic interactions of walleye in Shadehill Reservoir. Lirnnological parameters were collected from May, 1994 through May, 1995 at five locations: two within the main body of the reservoir, one in each of its two main tributaries, and one in the tailrace below the dam. Water temperatures ranged from a high of 21.4 °C in July 1994 to 1. 7 °c in February, 1995 and no summer thermal stratification was observed. Dissolved oxygen (DO) remained high and fell below 4 ppm only once during the study (1.7 ppm on July 27, 1994). Values of pH remained above neutral and ranged from 7.6 to 9.1. The relatively high summer DO concentrations and pH values allowed hydrogen sulfide concentrations to remain low and no build-up of this toxic gas was observed. Total nitrogen concentrations remained fairly constant throughout the study both among dates and between stations within dates, and ranged from 1.3 - 3.6 mg/1. In contrast, total phosphorus concentrations were erratic and appeared to vary considerably among dates and between stations within dates. Total phosphorus concentrations ranged from non-detectable to 356 μg/1. The range of nutrient concentrations observed classify Shadehill Reservoir as eutrophic. Problems were encountered with the activity levels of the radioactive NaH14C03 solution used to measure uptake of organic carbon in the determination of algal primary production Due to these problems, among-date comparisons of daily rates of areal primary production were not possible. Between station comparisons however, were possible and showed that community respiration exceeded algal photosynthetic production at both main reservoir stations during August, 1994 and at the west station during April and May, 1995. Thirteen zooplankton species were identified from 78 zooplankton samples collected during 1994 and 1995. Mean total abundance and standing stock biomass differed among dates but not between stations nor among tows. Mean total zooplankton abundance for the entire study period was 54.5/1. Mean total biomass was 264 7 μg DW II Cyclopoid nauplii were the most frequently observed zooplankter (mean 11.6/1) and comprised 21.2% of the mean total abundance. Daphnia schodleri accounted for the greatest mean biomass (85.9 μg DW/1) and comprised 32.4% of the total mean standing stock biomass. Eight macroinvertebrate taxa were identified from 78 benthic samples collected during 1994 and 1995. Mean total zoobenthos abundance and standing stock biomass differed between dates but not between stations nor among grabs. Mean total macroinvertebrate abundance for the entire study period was 587. 7/m2. Mean total biomass was 290.9 mg DW/m2. Procladius spp. larvae were the most abundant taxa (mean 48 1.4/m2) and comprised 80.9% of all macroinvertebrates sampled. Procladius spp. also made up the bulk of the zoobenthos biomass (mean 223.5 mg DW/m2}, comprising 76.8% of the total mean biomass, and contributed 84.2% of the 1994 annual macroinvertebrate production (19,031.5 mg DW/m2). Walleyes were captured at night in September, 1994 and May through September, 1995 by electrofishing and gillnetting. Of 201 walleye stomachs examined, 1 14 contained food. The relative importance (RI) index was used to evaluate the contribution of each prey taxa to the diet of three length groups of walleyes. Relative Importance analysis indicated that Daphnia spp. was the most important prey for sub-stock length (\u3c25 cm total length) walleyes. White bass Morone chrysops became increasingly important for longer walleyes and were the most important prey for quality- to preferred-length walleyes (38-50 cm TL). Aquatic dipteran larvae were the second most important prey in the diets of quality- to preferred-length walleyes, which may indicate limited prey fishes for walleyes in Shadehill Reservoir. It is recommended that future management efforts be focused toward increasing the abundance of yellow perch Percajlavescens, which when available are a preferred forage of walleyes