Keith D. Koupal

Water quality parameters of a Nebraska reservoir differ between drought and normal conditions

Abstract Increased anthropogenic disturbance to watersheds and rivers is exacerbating the effects of oscillating wet-dry periods common in the Midwestern United States, consequently impacting the maintenance and health of lake and reservoir systems. A change in flow regime should affect water quality parameters in reservoirs, but few data exist to document such changes. During a four-year drought, the mean water volume of Harlan County Reservoir, in south-central Nebraska, dropped more than 50%, changing this irrigation reservoir from a lotic to a lentic system. We monitored changes in chlorophyll a, turbidity, temperature and dissolved oxygen over 7 months (April–October) during 2003–2006 (drought conditions) and 2007–2009 (normal conditions) at 15 locations. Chlorophyll a and turbidity were both significantly greater during drought conditions in most months. Dissolved oxygen predominately decreased during drought conditions, while water temperature did not change. Chlorophyll a and turbidity were also greater in the upper (river inflow) than the lower (near dam) reaches of the reservoir during the drought, but were similar during normal years. The observed substantial increases in chlorophyll a levels caused the reservoir to be classified as hypereutrophic, using the Trophic State Index, during drought years but eutrophic during normal years. These data emphasize the need to include precipitation and inflow patterns when examining water quality parameters and assigning trophic status to a particular water body.

Interactions among Three Top-Level Predators in a Polymictic Great Plains Reservoir

Abstract After the introduction of hybrid striped bass (white bass Morone chrysops × striped bass M. saxatilis) into Harlan County Reservoir, Nebraska, gill-net catch per unit effort (CPUE) of walleyes Sander vitreus appeared to decline while that of white bass remained stable. This result prompted the question of whether these three species can be managed collectively in reservoir ecosystems. However, despite the frequency with which these three popular sport fishes coexist in Great Plains reservoirs, we are unaware of any studies that evaluate resource overlap among them. Therefore, we compared their diets, diet overlap, isotopic composition, vertical distribution, and vertical overlap in Harlan County Reservoir from June to September 2002 and 2003. All three species consumed similar prey (i.e., gizzard shad Dorosoma cepedianum and Chironomidae), and diet overlap was high (i.e., Piankas index > 40) during all months. On no occasion did all three predators consume the same sizes of gizzard shad. Hybrid ...

Food habits and abundance of larval freshwater drum in a South Central Nebraska irrigation reservoir

During certain times of the year high densities of larval fish can impact the aquatic community. The specific role of freshwater drum (Aplodinotus grunniens) is not known but larval freshwater drum can be abundant in some systems and have been shown to feed on zooplankton. To assist with understanding the role larval freshwater drum play in reservoirs, we investigated changes in peak abundance from 2003–2010 in addition to food habits and prey electivity in 2008 in a South Central Nebraska reservoir. We found that larval freshwater drum abundance varied 100-fold during this study. Cladocerans were an important food item for larval freshwater drum. Freshwater drum >10 mm total length selected for Daphnia spp. (46–80 % of organisms found in stomach), while freshwater drum ≤10 mm total length selected for Bosmina spp. (37% of organisms found in stomach), which is a less common group of zooplankton in this reservoir. All larval freshwater drum showed a negative selectivity for copepod nauplii (the Strauss index value of ≤−0.35 for all length groups). Based on the food habits described in this study, larval freshwater drum have the potential to compete with larval sport fish species for desirable zooplankton (ie, Daphnia spp.).

Changes in relative abundance of adult walleye and egg density following the addition of walleye spawning habitat in a midwest irrigation reservoir

Cobble substrate for walleye (Sander vitreus) spawning was added at Sherman Reservoir, Nebraska, in January 2008. We evaluated changes in relative adult walleye abundance and egg density in response to the cobble substrate and estimated site fidelity of adult walleye during the spawning seasons of 2007, 2008, and 2009. A 15% increase in electrofishing catch-per-unit-effort (CPUE) of adult walleye was observed on the cobble substrate the first season it was available (2008), but a 5% decrease in electrofishing CPUE was observed the second season (2009) compared to the 2007 season. While gill nets were not run during the first season the cobble was available, a 113% increase in gill net CPUE was observed the second season. During the first season the cobble was available, a 215% increase in egg density was observed, but no increase in egg density was found in the second season. Site fidelity of adult walleye within a spawning season was 94% and was 81% between seasons. These results indicate that the addition of the cobble substrate increased the relative abundance of adult walleye and density of eggs on the cobble substrate.

Swimming and jumping ability of 10 Great Plains fish species

Stream fragmentation can be detrimental to lotic fish species by preventing important life history movements. The maximum swimming speed and jumping ability of 10 stream fish species were evaluated to describe potential water velocity and height barriers to fish movement. A 10-L swim tunnel was used to test maximum swimming performance and an artificial waterfall with an adjustable weir was used to test jumping performance. All tested fish were between 30 and 100 mm total length. Mean maximum swimming velocity ranged from 37.5 ± 1.2 cm/s for mosquitofish to 65.0 ± 1.7 cm/s for largemouth bass and jumping ability ranged from 0 cm for bluegill to 13 cm for green sunfish. Differences in swimming and jumping ability demonstrate how limits to dispersal can be taxa specific and therefore, impact the conservation of rare species and management of exotic species. The values determined in this study can be useful in creating models to predict barriers to fish passage.

Large-scale dispersal patterns and habitat use of plains topminnow Fundulus sciadicus: implications for species conservation

Movement behaviors attributed to spawning, dispersal, or altered habitat availability are essential to the ecology of many lotic fishes and, although considerable research has described movements of sport fish, little is known about the movement patterns of nongame species. Streams and rivers, wherein plains topminnow Fundulus sciadicus are prevalent, occur in a nonequilibrium state in which habitat patches are sporadically created and lost due to hydrologic variability. This results in regular extirpation and need for recolonization of many plains fishes, including plains topminnow. Species persistence, therefore, is dependent on tolerances to fluctuating habitat conditions and life-history traits that allow dispersal over large areas. To better understand the regional distribution of plains topminnow, we monitored large-scale dispersal patterns and habitat use of two introduced populations in Nebraska. In 2011 and 2012, plains topminnow were marked using visible implanted elastomer (VIE) marks and released at the center of two 3000-m study reaches. Populations were sampled monthly from April to November in 2011 and April through September in 2012 to describe movement patterns and habitat use. Plains topminnow were highly mobile, consistently associated with select habitat features, and the estimated individual home range exceeded standard sampling reach distances by nearly four times. The movement of plains topminnow occurred at greater rates and to distances further than known for similar species. These large-scale movements likely help maintain connectivity among populations within stream drainages and facilitate recolonization of regularly extirpated habitat patches. In this context, species movement may be critical to the maintenance and potential recovery of populations of this and other rare lotic fishes.

Sex-specific Changes in Walleye Abundance, Size Structure and Harvest Following Implementation of Regulation to Protect Broodstock

Abstract The popularity of walleye Sander vitreus fishing has resulted in the development of specialized regulations that are designed to protect these fisheries. In the case of Sherman Reservoir, Sherman County, Nebraska, the walleye population provides a sportfishing opportunity and serves as broodstock for the state. In 2009, for the primary purpose of protecting female broodstock, the regulation changed from a harvest limit of four walleye with 457-mm minimum length to allowing a reduced harvest limit of two walleye within a harvest slot (381–508 mm) and one walleye more than 711 mm. This study examined existing data sets to assess the percentage of spawning walleye protected with each regulation, sex-specific differences in relative abundance and size structure during broodstock collection and angler effort, total catch, and harvest of walleye. The new regulation has increased protection of female walleye by more than 90%, but decreased protection of male walleye by more than 60%. The relative abunda...

Using Electrofishing Catch Rates to Estimate the Density of Largemouth Bass

AbstractCatch rates are commonly used to indirectly index population density, e.g., nighttime electrofishing to survey Largemouth Bass Micropterus salmoides populations. However, the relationship between catch rate and population density must be known in order for catch rate to serve as an accurate index. We sought to determine if Largemouth Bass electrofishing catch rate was linearly or nonlinearly correlated to population density in high density populations using measurement-error models and Monte Carlo simulations. Previous studies of the relationship between Largemouth Bass electrofishing catch rate and density have occurred in low-density populations (<50/ha). We hypothesized that high-density Largemouth Bass populations may result in gear saturation during electrofishing sampling, i.e., too many fish for dipnetters to capture resulting in missed captures. This would therefore underrepresent the populations density. The nighttime electrofishing catch rate of 66–294 Largemouth Bass/h was linearly rel...

Long-term changes in biotic and abiotic factors influence larval gizzard shad (Dorosoma cepedianum) annual peak density

ABSTRACT Gizzard shad (Dorosoma cepedianum) are an influential forage fish and driver of zooplankton resources in many reservoirs. The ability to identify the biotic and abiotic factors that influence the timing of elevated gizzard shad densities can be important to better utilize this forage fish and manage sport fish. The objective of this study was to investigate which biotic and abiotic factors influence larval gizzard shad annual peak density. We used combinations of six variables (CPUE of adult gizzard shad from the prior fall, mean zooplankton density, turbidity, chlorophyll a, relative reservoir elevation, and water temperature) from a long-term (2003–2014) monitoring program to create 28 a-priori candidate models to assess the relative support of explanatory variables using Akaikes information criterion (AIC). During the course of our study, larval gizzard shad annual peak densities were best explained by zooplankton density and relative reservoir elevation. Zooplankton density provides greater food availability which has been shown to increase larval gizzard shad growth. Relative reservoir elevation was negatively associated with higher densities of larval gizzard shad. This study describes two important factors that may result in greater larval gizzard shad densities, which can assist managers in anticipating available forage for sport fish and identify strategies to improve system management.

Assessment of Seasonal Changes in Abiotic and Zooplankton Communities in Highly and Moderately Alkaline Sandhills Lakes

Regionally proximate waters in the Sandhills of Nebraska are known to have distinctive alkaline conditions. However, little is known about the seasonal variability that exists for abiotic conditions and zooplankton communities within these same waters. This study assessed two highly alkaline and four moderately alkaline lakes for seasonal trends in conductivity, turbidity, pH, temperature, dissolved oxygen, and nitrate, as well as zooplankton biodiversity and density. These findings indicate that each water has seasonally dynamic conditions and communities. Future reporting should identify the season of collection and expand the temporal collection to more accurately characterize these waters. Additionally, managers should not assume that proximate waters demonstrate similar seasonal variation.