Francis Case Research Articles

A Review of Zebra Mussel Biology, Distribution, Aquatic Ecosystem Impacts, and Control with Specific Emphasis on South Dakota, USA

Zebra mussels Dreissena polymorpha are a native bivalve from eastern Europe. They were first detected in North America in Lake St. Clair in 1988 and were presumably introduced via infested ballast water. Zebra mussels have spread rapidly across the United States, with 31 states reporting infestations as of 2019. Zebra mussels were first detected in South Dakota, USA, in 2015 in Lewis and Clark Lake and McCook Lake, with subsequent infestations occurring in Lake Yankton in 2017, Lakes Francis Case and Sharpe in 2019, and Pickerel Lake, Kampeska Lake, and Lake Cochrane in 2020. This review paper presents information on zebra mussel biology and control, with specific information on the waters of South Dakota, USA.

Factors Related to Age-0 Native Fish Catch per Unit Effort in the South Dakota Missouri River Reservoirs

AbstractThe Missouri River in South Dakota was dammed in the 1950s and 1960s, altering the biotic and abiotic factors that potentially affect native species in the mainstem reservoirs. Little research has been conducted regarding what factors regulate age-0 catch per unit effort (CPUE) of native fish species since the damming occurred. Thus, we examined age-0 CPUE for 13 native fish species in the four Missouri River mainstem reservoirs. We analyzed data from 1995 to 2015 in Lake Oahe, 2002 to 2016 in Lake Sharpe, 1998 to 2008 in Lake Francis Case, and 2000 to 2013 in Lewis and Clark Lake. Not all species were sampled in all reservoirs. We evaluated potential environmental (inflow, gauge height, peak flow, precipitation, air temperature) and biological (CPUE of other native species, nonnative species, and predators) factors that have documented impacts on age-0 native fish. Significant relationships existed between age-0 native fish CPUE and both biotic and abiotic factors in all four reservoirs, but were species- and reservoir-dependent. Age-0 CPUE was either stable or increasing in all species examined in all reservoirs except age-0 Emerald Shiner Notropis atherinoides in Lewis and Clark Lake. Age-0 Emerald Shiner CPUE in Lewis and Clark Lake was most supported by biological models (e.g., total age-0 nonnative and adult White Bass Morone chrysops CPUE), and a positive relationship existed between age-0 Emerald Shiner and both factors. We believe our findings provide valuable insight into successful management of native fish populations.

Long-Term Data Suggest Potential Interactions of Introduced Walleye and Smallmouth Bass on Native Sauger in Four Missouri River Impoundments

Abstract Sauger Sander canadensis, Walleye Sander vitreus, and Smallmouth Bass Micropterus dolomieu, are important sportfish in the four main stem Missouri River reservoirs in South Dakota: Lakes Oahe, Sharpe, Francis Case, and Lewis and Clark. However, native Sauger populations, once assumed to be stable, may be in decline. To identify temporal trends and potential interspecific mechanisms affecting Sauger populations, we examined their long-term abundance trends in standard gillnet surveys and angler catch and harvest trends in long-term creel surveys. We also used a robust design occupancy model to examine changes in within-lake distribution of this species. There are concerns regarding the effects of Walleye and Smallmouth Bass on Sauger, so we also described the population trends of these potential competitors. Standard gillnet surveys indicated declining abundance of both Sauger and Walleye in Lakes Oahe and Sharpe. Sauger abundance has trended down in Lewis and Clark, but upward in Francis Case. Conversely, Walleye abundance trends declined in Francis Case and increased in Lewis and Clark. Occupancy (ψ̂) of Sauger declined in all four reservoirs, indicating a contracting distribution throughout the reservoirs. Walleye occupancy remained ∼1.0. Smallmouth Bass occupancy increased in the three reservoirs with sufficient data for analysis, excluding Lewis and Clark Lake. Smallmouth Bass exhibited steady increases in angler catch and harvest, as well as abundance in long-term gillnet surveys, suggesting expanding and increasing populations. Habitat alteration is hypothesized to be a major driver of the Sauger occupancy and abundance declines. However, Walleye and Smallmouth Bass interactions could also be contributing to observed declines of native Sauger.

Emerging reservoir delta‐backwaters: biophysical dynamics and riparian biodiversity

AbstractDeltas and backwater‐affected bottomlands are forming along tributary and mainstem confluences in reservoirs worldwide. Emergence of prograding deltas, along with related upstream hydrogeomorphic changes to river bottomlands in the backwater fluctuation zones of reservoirs, signals the development of new and dynamic riparian and wetland habitats. This study was conducted along the regulated Missouri River, USA, to examine delta‐backwater formation and describe vegetation response to its development and dynamics. Our research focused specifically on the delta‐backwater forming at the confluence of the White River tributary and Lake Francis Case reservoir. Objectives of the research were to: (1) describe and analyze the process of delta‐backwater formation over space and time; (2) determine by field sampling and GIS mapping how vegetation has responded to development of the delta‐backwater; and (3) compare the woody plant communities of the delta‐backwater to those along free‐flowing and regulated remnant river reaches. In response to base level changes caused by reservoir filling, the thalweg of the lower 31 km of the original White River channel and adjacent floodplain aggraded by up to 12 m between 1954 and 2011. The overall channel slope flattened from 0.70 to 0.29 m/km. Riparian Populus–Salix forests increased in area by nearly 50% during the post‐dam period by colonizing new deltaic and floodplain deposits. Many of the native woody species found along natural and regulated river reaches were also found on the delta‐backwater. Woody species sorted along a fluvial to delta gradient; wetland affiliated species (Salix spp., Typha spp.) dominated the delta‐backwater near the reservoir while riparian species (Populus, Fraxinus) dominated in upstream portions of the delta‐backwater. This habitat complex supports young stands of native riparian vegetation now in decline in remnant reaches protected from flooding.

De una Iglesia occidental a una Iglesia mundial. Una interpretación de la reforma eclesial

<p>Según la tesis de Karl Rahner, de que el Vaticano II marcó el inicio de una Iglesia mundial, se habría concretado germinalmente un salto cualitativo que deja atrás el tiempo en el cual la Iglesia exportó su modelo cultural europeo de cristianismo. Dicha tesis puede aportar una clave para interpretar los procesos de reforma eclesial en desarrollo con el “caso Francisco”. El artículo analiza tres asuntos conciliares claves que revelarían la transición de una Iglesia occidental a una Iglesia mundial: se ha frenado la “reforma de la reforma” litúrgica; en el caso de la eclesiología, ha “retornado” la categoría de pueblo de Dios; y ha llegado al obispado de Roma la recepción de Gaudium et spes más coherente con la voluntad de los padres conciliares.</p>

Otolith Microchemistry Reveals Natal Origins of Walleyes in Missouri River Reservoirs

AbstractReproductive habitats are vital for sustaining fish populations, but their location and relative natal contributions are often unknown or poorly understood. We used otolith microchemistry to examine natal origins of Walleyes Sander vitreus in Missouri River reservoirs (i.e., Lake Oahe, Lake Sharpe, Lake Francis Case, and Lewis and Clark Lake) in North Dakota and South Dakota. Water Sr:Ca and Ba:Ca were spatially heterogeneous and temporally consistent in all impoundments. Otolith Sr:Ca and Ba:Ca from age‐0 Walleyes permitted the reclassification of fish to known natal habitats (i.e., tributary, embayment, main stem) and individual sites with 87% and 75% accuracy, respectively. Natal contributions were highest in tributaries, particularly those in Lake Oahe, where 32% of all adults and 77% of Lake Oahe adults hatched. Embayments and main‐stem environments had high natal contributions (67–78%) in Lakes Sharpe and Francis Case and Lewis and Clark Lake, where tributaries are less abundant. Our research demonstrates the utility of otolith microchemistry for measuring habitat‐ and site‐specific natal contributions and provides further information that can be used in managing Walleyes in Missouri River reservoirs, particularly for broodstock collection, habitat protection and restoration, and harvest regulations.Received April 17, 2015; accepted December 11, 2015 Published online March 30, 2016

Effects of a protected slot limit on smallmouth bass size structure and angler harvest

In an effort to improve size structure of smallmouth bass (Micropterus dolomieu) in Lake Sharpe, a large Missouri River impoundment, the South Dakota Department of Game, Fish and Parks instituted 2 protected slot limits: restricted (305–457 mm) beginning in 2003 and relaxed (355–457 mm) beginning in 2008. We examined the effects of these regulations on smallmouth bass harvest and population characteristics and compared creel and population trends of Lake Sharpe smallmouth bass to adjacent reservoirs where smallmouth bass harvest was not regulated by length limits. Prior to the slot limit, the majority of the smallmouth bass harvest from Lake Sharpe was for individuals 250–400 mm in length (P < 0.001). Estimated harvest after regulation implementation was higher for smallmouth bass 250–350 mm (P < 0.001). Additionally, an increase in proportional stock density–preferred (PSD-P), number >355 mm, and angler catch of trophy smallmouth bass was observed, suggesting an effective regulation. However, a before-after-control-impact (BACI) study design and analysis indicated changes in size structure were observed in abutting Lakes Oahe and Francis Case, suggesting a Missouri River system-wide affect was likely responsible for observed population changes. Because of the lack of biological support and the negative feedback by anglers, the protective slot limit regulation was removed from Lake Sharpe in 2012. Further, when considering length limit regulations, we recommend gathering data on similar “control” systems in an effort to document changes resulting solely from the impacts of the regulation.

Evaluating Effects of Exploitation on Annual Apparent Mortality Rates of Paddlefish Using Mark–Recapture Data

AbstractKnowledge of the effects of exploitation on population dynamics is critical to effective conservation and management of fish populations. To improve our understanding of the effects of sport fishery exploitation on populations of Paddlefish Polyodon spathula, we tested eight additive and compensatory harvest mortality hypotheses for the Paddlefish population in Lake Francis Case (LFC), South Dakota, with 33 years of tag recovery data using dead recovery models in Program MARK. We evaluated additive and compensatory harvest mortality hypotheses by modeling the effects of sport fishery closures on annual apparent survival and tag recovery rates. Our most‐supported hypotheses indicated that exploitation prior to the functional closure of the sport fishery (i.e., when exploitation became negligible) was an additive source of mortality. Annual apparent mortality estimates (i.e., 1 − annual apparent survival) of our most‐supported model decreased from 0.237 (SE = 0.036) to 0.110 (SE = 0.063) following the functional closure of the sport fishery and coincided with a decrease in recovery rate from 0.095 (SE = 0.015) to 0.010 (SE = 0.004). These results suggest that relatively unregulated sport fishery exploitation can substantially increase mortality rates of Paddlefish populations. The effects of exploitation on mortality likely differ between populations and temporally within populations due to differences in density‐dependent mortality rates.Received January 7, 2014; accepted November 19, 2014

Strata-bound vein array in the basal Pierre Shale, Lake Francis Case, South Dakota, U.S.A.

A distinctive strata-bound vein array occurs in the basal Pierre Shale exposed along the shores of Lake Francis Case, a reservoir on the Missouri River in south-central South Dakota. Typically 2–4 meters in thickness, the array consistently outcrops over a >50-km distance, a significant areal footprint. Ash layers define the upper and lower bounds of the vein array. Two, suborthogonal, preferred directions of vertical veins (northeast and southeast strike) define a regional pattern. By volume, vertical veins comprise 1–2% of the rock. Thinner, more discontinuous, and irregular horizontal veins also occur. Comparisons between array orientations and the joint/vein pattern in the immediately underlying marls of the top of the Niobrara Chalk identify distinct differences. Traverse data suggest that the vein arrays are characterized by uniform horizontal extension. Vertical veins in the array are typically 1–2 centimeters thick and contain massive jarosite, selenite, and fibrous gypsum. The abundance of jarosite and fibrous gypsum distinctly correlates with position in the weathering profile, and these phases are interpreted as due to replacement of original selenite during modern weathering. However, for initial vein array formation, the following suggests that they are not related to modern weathering and formed at depth: (1) a lack of correlation of vein width/frequency with position in the weathering profile; (2) the regional extent; (3) the consistent preferred orientations; (4) the uniform horizontal extension; and (5) the coarse-grained character of the selenite. The consistent strike pattern suggests influence of a regional stress field. The mechanism/timing of vein array formation is unclear. Formation due to diagenetic processes, which are especially significant in mud rocks, would explain the strata-bound character and isotropic horizontal strain and is considered most likely. Formation during glacial loading is one intriguing possibility. Localization of the vein array may be due to the organic-rich character of the host Burning Brule Member of the Sharon Springs Formation.

Stocking Success of Paddlefish in Lake Francis Case, South Dakota: Population Characteristics and Sport Fishery Potential

AbstractLake Francis Case, South Dakota, a Missouri River reservoir, supported a paddlefish Polyodon spathula sport fishery shortly after impoundment, but this fishery closed in 1986 owing to a lack of natural recruitment. Lake Francis Case has been stocked intermittently since 1974 and annually since 1990 to maintain the population and potentially sustain a sport harvest. To evaluate the success of these stockings, we assessed the current status of the Lake Francis Case paddlefish population (i.e., population characteristics and dynamics) and evaluated the potential effects of exploitation on this population. Advanced fingerling stockings have been effective, as 56% of sampled fish were of hatchery origin (indicated by coded wire tags), and may sustain a harvestable population. Year‐class strength was negatively correlated to water temperature inputs from Big Bend Dam and positively correlated to White River discharge. Downstream emigration is probably responsible for most of the variability in year‐class strength, and the rate of downstream emigration may be greater during years with warmer water temperatures. Harvest simulations suggested that growth overfishing was possible under a 660‐mm minimum length limit but not under 840‐mm and 1,016‐mm minimum length limits. The proportional size distribution of memorable‐length fish (PSD‐M) was reduced 28.6% and 14.3%, respectively, by 5% exploitation under simulated 660‐mm and 840‐mm minimum length limits. Under a simulated 1,016‐mm minimum length limit, PSD‐M was unchanged by 10% exploitation. At 5% exploitation, broodstock availability was reduced by more than 30% under the simulated 660‐mm and 840‐mm minimum length regulations and was reduced 16% under the 1,016‐mm minimum length limit. Our results suggest that this population could support a limited (e.g., 5% exploitation) sport fishery under a high (e.g., 1,016‐mm) minimum length limit without substantial changes in size structure or the availability of broodstock if stockings are continued.

Age‐Structured Assessment of Walleyes, Saugers, and Naturally Produced Hybrids in Three Missouri River Reservoirs

AbstractWalleye Sander vitreus and sauger Sander canadensis naturally hybridize in many populations, but the factors influencing the recruitment of hybrids are unknown. We determined relative year‐class strength for 1,454 saugers, walleyes, and naturally produced hybrids from three Missouri River reservoirs. Hybridization rates varied from 4% in Lakes Sharpe and Francis Case to 21% in Lewis and Clark Lake. Hybrids comprised multiple year‐classes in each system, indicating that hybridization does not occur in erratic pulses but at a consistent low‐level recruitment rate. The year‐class strength of hybrids was not correlated with that of either walleyes or saugers. Our models suggest that higher than average recruitment of walleyes, saugers, and hybrids can be expected during years with warmer spring and early summer water temperatures in Lake Sharpe and Lewis and Clark Lake and during years when inflow (from main‐stem discharge for hybrids and tributary inputs for saugers and walleyes) is reduced in Lewis and Clark Lake.

Comparison of Creel Survey Methods on Missouri River Reservoirs

AbstractWhen creel survey methods such as noncontact aerial designs are applied to large water bodies, they are often biased because of scheduling difficulties and flight reductions made to reduce cost. An aerial creel survey has been used to estimate fishing pressure on South Dakota's Missouri River reservoirs since 1975. We compared fishing pressure estimates from aerial and bus route designs and examined the effect of count frequency on variance and point estimates. A total of 71,400 (observed and unadjusted) and 94,298 angler‐hours were estimated on Lake Francis Case during June 2000 by use of the aerial and bus route methods, respectively. Aerial boat and shoreline fishing pressure estimates were 65,845 angler‐hours (95% confidence interval (CI) = 25,770) and 5,554 angler‐hours (95% CI = ±2,436), respectively. Bus route boat and shore fishing pressure estimates were 82,671 angler‐hours (95% CI = ±14,878) and 11,627 angler‐hours (95% CI = ±6,618), respectively. Analyses indicated that reservoir zone, day type, and type of fishing were significant main effects (P &lt; 0.001), which supported spatial and temporal stratification in the sampling design. Within‐strata comparisons of fishing pressure estimates from the two surveys did not reveal any significant differences (0.09 &lt; P &lt; 0.99). Although the bus route method led to 22,898 more angler‐hours of fishing pressure than the aerial method, the bus route method estimated total fishing pressure with higher precision because the day‐to‐day variation in fishing pressure estimates was comparatively lower than that of the aerial estimates. The aerial and bus route methods both estimated shoreline fishing pressure with lower confidence than for boat fishing pressure; this was probably the result of overall low shoreline fishing pressure and high day‐to‐day shoreline fishing pressure variability. Bootstrap analysis and nonlinear regression indicated breakpoints in variance estimates between six and seven counts for both survey methods. Therefore, a minimum of eight counts per month was required to produce stable variance and point estimates with an acceptable level of confidence for either survey design.

Delayed Mortality of Tournament‐Caught Walleyes

AbstractWe quantified the delayed mortality of walleyes Sander vitreus during three tournaments (April–June 2003) on Lake Francis Case, South Dakota. Weigh‐in procedures (e.g., air exposure of fish during weigh‐in, distance traveled, and so forth) and lake conditions (wind speed and wave height) were also measured to investigate how these factors affected delayed mortality. Delayed mortality was similar for control and tournament fish during the April tournament but increased for tournament fish as lake water temperature increased. Delayed mortality was 0% for control fish, even when tournament fish experienced high mortality; thus, the mortality of tournament fish was attributed to the tournament. Tournament procedures were generally similar for all tournaments, except that anglers traveled farther during later tournaments. The higher mortality experienced by tournament‐caught walleyes during the June tournament was probably due to warmer water temperatures (18–20°C). These results will allow fisheries managers to better predict the effects of delayed mortality on walleyes caught in tournaments.

Distribution and Movement of Juvenile Paddlefish in a Mainstem Missouri River Reservoir

ABSTRACT Hatchery-raised paddlefish (Polyodon spathula) were implanted with ultrasonic transmitters and tracked from August to November 1998 and March to November of 1999 and 2000 to determine diel movements and seasonal distribution in Lake Francis Case. South Dakota. Juvenile (340 to 432 mm eye to fork length) paddlefish (N=32) exhibited variable movement patterns but remained in the upper reservoir reaches throughout the three-year study. Movement rates ranged hm 35 to 3,464 m/h with 90% of all locations in the upper third (51 kml of the reservoir. Increased movement rates were weakly correlated with increased water temperature (r=0.16, P=0.04). Daytime paddlefish movement peaked in July and August. However, diel movements were variable among seasons. In spring, movement rates did not differ among diel periods, whereas summer movements were lower during crepuscular periods and fall movements were lowest during the day. Although paddlefish were stocked at two locations in Lake Francis Case, they appeared to disperse and use reservoir areas similarly.

Use of a Minimum Length Limit to Manage Walleyes in Lake Francis Case, South Dakota

Abstract Walleyes Stizostedion vitreum are the primary predator fish in Lake Francis Case, a Missouri River impoundment located in south-central South Dakota. Because of increasing angling pressure in the mid-1980s, this walleye sport fishery was nearing the point of overharvest. In 1990, angling regulations were changed to address management concerns. A partial-year (April–June), 356-mm minimum length limit was implemented, the daily bag limit was reduced from 6 to 4, and the possession limit was reduced from 12 to 8. We evaluated the effects of the length limit regulation on recruitment, population structure, growth, and angler use and harvest by comparing a 5-year preregulation period with a 9-year postregulation period. Walleye recruitment, growth, condition, and abundance did not change significantly from preregulation to postregulation periods. Abundance of 356-mm and longer walleyes and proportional stock density increased significantly in the years following implementation of the minimum length li...

Seasonal Migration Patterns and Site Fidelity of Adult Paddlefish in Lake Francis Case, Missouri River

We used ultrasonic telemetry to determine the distribution, seasonal migration patterns, and site fidelity of adult paddlefish Polyodon spathula in a main-stem Missouri River impoundment. Thirty-two adult paddlefish collected from two different spring congregation areas in Lake Francis Case, South Dakota, were surgically implanted with ultrasonic tags and relocated monthly from March to November 1994–1996. Paddlefish tagged in the Big Bend tailwaters were captured in this area and in another possible spring congregation (the White River) during prespawning and spawning periods, whereas paddlefish tagged in the White River were never located in the Big Bend tailwaters or in areas above the White River. Male and female paddlefish used reservoir habitats similarly except during the prespawning period, when male paddlefish used the White River more than would be expected by chance and all females implanted at the White River site remained below the White River. In both the postspawning and winter periods, all paddlefish moved downstream and congregated in the lower reservoir reaches. Up to 62% of males and 36% of females returned to their original capture site during the presumed spawning period at least 1 of the 2 years after tagging. Thirty-one percent of males at large for 2 years returned to their initial spring capture location each of the following years, but only 9% of females returned. Our results indicate that some paddlefish exhibited site fidelity to the areas where they were captured (i.e., the White River) and presumably attempted to spawn, although winter distribution was similar among all tagged paddlefish. Restoration and stocking efforts may need to focus on habitat management at the locations where paddlefish may spawn, given adequate substrate.

Relations between Environmental Variables and Age-0 White Bass Abundance in Four Missouri River Reservoirs

ABSTRACT Age-0 white bass (Morone chrysops) catch per unit effort (CPUE) in standardized seine samples from 1983 to 1995 was correlated with environmental data in the four South Dakota Missouri River reservoirs (Lakes Oahe, Sharpe, Francis Case, and Lewis and Clark). Environmental parameters that were significantly (P≤0.05) correlated with CPUE varied among reservoirs. In Lake Oahe, age-0 abundance was higher in years with high flushing, low discharge from its upstream dam, and warmer air temperature during June and July. In Lake Sharpe, age-0 abundance was higher in years with cooler air temperature during January, April, and May, and less discharge and lower inflow during July. In Lake Francis Case, age-0 abundance was higher in years with higher spring precipitation, evaporation and temperature, and in years with greater decline in elevation from May to July. In Lewis and Clark Lake, age-0 abundance was higher in years with high spring and annual inflow, high reservoir elevation in March and elevation increase from April to May, and increased annual storage. Multiple regression models were developed for each of the four reservoirs with age-0 white bass CPUE as the dependent variable; coefficients of determination ranged from 0.90 to 0.94. Although the relations we observed were correlations and not necessarily cause and effect, the results of this study provide insights into white bass biology in the Missouri River reservoirs.

Prey Size Selectivity and Food Partitioning among Zooplanktivorous Age-0 Fishes in Lake Francis Case, South Dakota

Zooplankton prey selection and food partitioning were examined among cohabiting age-O walleyes Stizostedion vitreum, yellow perch Percaflavescens, white bass Morone chrysops and gizzard shad Dorosoma cepedianum. As these fishes grew, they consumed progressively larger zooplankton prey. Prey size appeared to be the primary factor governing zooplankton prey selection by the visual predators walleyes, yellow perch and white bass. Prey evasiveness was not important in prey selection for the visual predators but was important for the nonvisual predator, the gizzard shad. Age-O fishes partitioned food resources by prey size. Walleyes usually selected the largest prey at a given time, followed by white bass, yellow perch and gizzard shad. Diet overlap was highest between yellow perch and white bass, which had the most similar mouth gapes.

Ecological Characterization as a Foundation for Prediction of Plains Village Tradition Site Locations in Central South Dakota

Upland Plains Village archaeological sites in Central South Dakota located by pedestrian survey were described in terms of physiography and vegetation. Randomly selected, non-archaeological sites were similarly described and served to define the universe of sites available for habitation along the east bank of the Missouri River/Lake Francis Case Reservoir. Physiographic differences between actual and “simulated” archaeological sites suggested that several factors were important in predicting the location of upland archaeological sites. These included location in Agropyron smithii dominated plant communities with less than 5° of slope, and southern exposure. The model accounts for nearly 70 per cent of all variability associated with location and distribution of archaeological sites.

Spring and Summer Water Levels in a Missouri River Reservoir: Effects on Age-0 Fish and Zooplankton

The effects of water level on the early life history of fish in Lake Francis Case, South Dakota, were studied by comparing the abundance, growth, diet, and food supply (zooplankton) of selected species in a low-water year (1974) and a high-water year (1975). Young of 21 fish species were collected. Yellow perch (Perca flavescens), white bass (Morone chrysops), buffaloes (two Ictiobus species), and centrarchids were significantly more abundant in net catches in 1975 than 1974. Flooded shoreline vegetation in 1975 apparently enhanced spawning success by providing a more suitable substrate and protective cover for early life stages. First-summer growth of the four most abundant fishes (yellow perch, buffaloes, and white bass) did not change significantly between years. Standing crops of the larger zooplankton (calanoids and daphnids) decreased during the high-water year, perhaps because of predation by age-0 fish. Standing crops of the smaller zooplankton species such as Bosmina longirostris and Chydorus sphaericus increased during the high-water year, as did the overall diversity of the zooplankton community. We conclude that management of water levels in Missouri River main-stem reservoirs to include one high-water year out of every three would greatly enhance the lakeˈs fishery resources.