Wisconsin Lakes Research Articles

Real-time PCR and sequencing assays for rapid detection and identification of avian schistosomes in environmental samples.

Cercarial dermatitis, also known as swimmer's itch, is an allergenic skin reaction followed by intense itching caused by schistosome cercariae penetrating human skin. Cercarial dermatitis outbreaks occur globally and are frequently associated with freshwater lakes and are occasionally associated with marine or estuarine waters where birds reside year-round or where migratory birds reside. In this study, a broadly reactive TaqMan assay targeting 18S rRNA gene (ribosomal DNA [rDNA]) sequences that was based on a genetically diverse panel of schistosome isolates representing 13 genera and 20 species (the 18S rDNA TaqMan assay) was developed. A PCR assay was also developed to amplify a 28S rDNA region for subsequent sequencing to identify schistosomes. When applied to surface water samples seeded with Schistosoma mansoni cercariae, the 18S rDNA TaqMan assay enabled detection at a level of 5 S. mansoni cercariae in 100 liters of lake water. The 18S rDNA TaqMan and 28S rDNA PCR sequencing assays were also applied to 100-liter water samples collected from lakes in Nebraska and Wisconsin where there were reported dermatitis outbreaks. Avian schistosome DNA was detected in 11 of 34 lake water samples using the TaqMan assay. Further 28S rDNA sequence analysis of positive samples confirmed the presence of avian schistosome DNA and provided a preliminary identification of the avian schistosomes in 10 of the 11 samples. These data indicate that the broadly schistosome-reactive TaqMan assay can be effective for rapid screening of large-volume water samples for detection of avian schistosomes, thereby facilitating timely response actions to mitigate or prevent dermatitis outbreaks. Additionally, samples positive by the 18S rDNA TaqMan assay can be further assayed using the 28S rDNA sequencing assay to both confirm the presence of schistosomes and contribute to their identification.

Effects of a Reduced Daily Bag Limit on Bluegill Size Structure in Wisconsin Lakes

AbstractThis study evaluated the effect of reduced aggregate panfish bag limits (from 25 to 10) on the size structure of Bluegill Lepomis macrochirus populations in seven natural lakes in Wisconsin. For assessing the overall significance of treatments, each treatment lake was paired with a control lake in which the regulation was not implemented. Across all lakes, mean total length (TL) of sampled Bluegills was significantly greater in treatment lakes than in control lakes after the regulation was implemented: on average, mean TL increased by 20.3 mm. However, efficacy of reduced bag limits varied substantially across lakes as mean TL improvements ranged from −5.1 to 63.5 mm in individual lakes. This variation could be strongly explained (R2 = 0.81) by lake Secchi depth (lakes with reduced water clarity showed larger improvements in mean TL, R = −0.62) and regulation duration (size structure improved continuously with time after the reduced daily bag limit was implemented, R = 0.75). Reduced bag limits are a useful tool for providing improvements to Bluegill size structure in Wisconsin lakes, but would be most effective in more productive waterbodies and will require substantial time investments after implementation.Received September 18, 2014; accepted December 18, 2014

Muskellunge Spawning Site Selection in Northern Wisconsin Lakes and a GIS‐Based Predictive Habitat Model

AbstractSpawning habitat degradation has been linked to declines in naturally reproducing Muskellunge Esox masquinongy populations, and managers require efficient methods to identify and protect these habitats. We collected spawning habitat data from 28 lakes in northern Wisconsin to determine Muskellunge spawning habitat selection and to create a GIS‐based model for predicting the locations of spawning sites. Spawning site selection by Muskellunge may be more complex than previously thought. Muskellunge showed selection for spawning in habitats with a sheltered effective fetch and east‐facing shorelines. The strongest selection was for habitats with a combination of moderate slope, small flats, and concave bathymetric curvature. Muskellunge selected against steeply sloping shorelines; very large areas of shallow flats; developed shorelines; herbaceous wetlands; and complex‐leafed submersed aquatic vegetation. Lake trophic status appears to interact with other habit variables to determine spawning site selection; sites without submersed aquatic vegetation were more strongly selected in eutrophic lakes than in other lake types. A GIS model of spawning site selection was created using the machine learning program MaxEnt (Maximum Entropy Modeling). The model predicted that Muskellunge would spawn in areas with moderately sheltered effective fetches, moderate to small areas of shallow flats, away from outflowing streams, and (to a lesser extent) along shorelines facing east or west. The model was tested on novel lakes using area‐under‐the‐curve (AUC) analysis, in which values ranged from 0.5 (predictions no better than random) to 1.0 (perfect assignment). The mean AUCtest value (i.e., the expectation of model performance for a novel lake) was 0.637 (SD = 0.12). When the model was used to designate the best 20% of available spawning habitat area for Muskellunge in each lake (based on the relative probability of spawning), that area contained 32% of the spawning sites. The model provides an efficient method for management agencies and conservation groups to use in designating spawning habitat for conservation and in communicating with the public through spawning habitat maps.Received January 26, 2014; accepted October 7, 2014

Representing calcification in distribution models for aquatic invasive species: surrogates perform as well as CaCO3 saturation state

Modeling of invasive species’ potential distributions is critical for preventing new invasions. Distribution models use environmental characteristics of observed distributions to determine the suite of conditions that allows invasion. Some variables are directly related to species’ biological needs, while others are surrogates that do not directly influence distributions. For aquatic invasive species (AIS) with calcified shells and exoskeletons, one common limiting factor is the chemical restriction on forming calcium carbonate (CaCO3), which is usually represented by conductivity or dissolved calcium. We predicted that conductivity and calcium would not serve as accurate surrogates for CaCO3 saturation state, which is more directly linked to calcification potential. To test this prediction, we fit classification trees to field surveys of northern Wisconsin lakes for three calcifying AIS—rusty crayfish (Orconectes rusticus), banded mystery snail (Viviparus georgianus), and Chinese mystery snail (Cipanguladina chinensis). Along with lake area, variables associated with calcification were the best predictors of species distributions. Even though saturation state is the most biologically mechanistic variable, it did not improve model predictions or explain more variability in species presence than its surrogates. Overall, surrogates are sufficient for species distribution modeling in these lakes that exhibit a wide range of chemical states.

Relationships among Walleye Population Characteristics and Genetic Diversity in Northern Wisconsin Lakes

AbstractThe maintenance of genetic integrity is an important goal of fisheries management, yet little is known regarding the effects of management actions (e.g., stocking, harvest regulations) on the genetic diversity of many important fish species. Furthermore, relationships between population characteristics and genetic diversity remain poorly understood. We examined relationships among population demographics (abundance, recruitment, sex ratio, and mean age of the breeding population), stocking intensity, and genetic characteristics (heterozygosity, effective number of alleles, allelic richness, Wright's inbreeding coefficient, effective population size [Ne], mean d2 [a measure of inbreeding], mean relatedness, and pairwise population ΦST estimates) for 15 populations of Walleye Sander vitreus in northern Wisconsin. We also tested for potential demographic and genetic influences on Walleye body condition and early growth. Combinations of demographic variables explained 47.1–79.8% of the variation in genetic diversity. Skewed sex ratios contributed to a reduction in Ne and subsequent increases in genetic drift and relatedness among individuals within populations; these factors were correlated to reductions in allelic richness and early growth rate. Levels of inbreeding were negatively related to both age‐0 abundance and mean age, suggesting Ne was influenced by recruitment and generational overlap. A negative relationship between the effective number of alleles and body condition suggests stocking affected underlying genetic diversity of recipient populations and the overall productivity of the population. These relationships may result from poor performance of stocked fish, outbreeding depression, or density‐dependent factors. An isolation‐by‐distance pattern of genetic diversity was apparent in nonstocked populations, but was disrupted in stocked populations, suggesting that stocking affected genetic structure. Overall, demographic factors were related to genetic diversity and stocking appeared to alter allelic frequencies and the genetic structure of Walleye populations in Wisconsin, possibly resulting in disruption of local adaptation.

A trematode parasite alters growth, feeding behavior, and demographic success of invasive rusty crayfish (Orconectes rusticus)

Nonindigenous species can cause major changes to community interactions and ecosystem processes. The strong impacts of these species are often attributed to their high demographic success. While the importance of enemy release in facilitating invasions has often been emphasized, few studies have addressed the role of parasites in the invasive range in controlling demographic success of potential invaders. Here we examine whether a trematode parasite (Microphallus spp.) can contribute to previously documented alternate states in the abundance of invasive rusty crayfish (Orconectes rusticus) in north temperate lakes in Wisconsin, USA. Microphallus infect O. rusticus after emerging from their first intermediate host, a hydrobiid snail. As previously documented, O. rusticus reduce densities of hydrobiid snails through direct predation and destruction of macrophyte habitat. Therefore, if Microphallus substantially reduce O. rusticus fitness, these parasites may reinforce a state of low crayfish abundance, and, at the other extreme, abundant crayfish may repress these parasites, reinforcing a state of high crayfish abundance. From samples collected from 109 sites in 16 lakes, we discovered (1) a positive relationship between crayfish infection intensity and hydrobiid snail abundance, (2) a negative relationship between parasite prevalence and crayfish abundance, and (3) a negative relationship between parasite prevalence and crayfish population growth. With experiments, we found that infection with Microphallus reduced foraging behavior and growth in O. rusticus, which may be the mechanisms responsible for the population reductions we observed. Overall results are consistent with the hypothesis that Microphallus contributes to alternate states in the abundance and impacts of O. rusticus.

Vulnerability to harvest by anglers differs across climate, productivity, and diversity clines

We contrast catchability of walleye (Sander vitreus) and northern pike (Esox lucius) populations with angling fisheries across regions that differ twofold in growing-degree-days and productivity and sixfold in fish diversity. Populations of both species in Alberta, Wisconsin, Minnesota, and Oneida Lake, New York, had density-dependent catchability with approximately tenfold higher catchability in Alberta than in the other regions when density was controlled for. There is no evidence that the higher catchability estimates for Alberta walleye and northern pike are due to differential spatial distributions, enhanced hook avoidance due to catch and release or to differential size structure of the populations, or to differences in harvest regulations. We argue that the most likely explanation for the tenfold higher catchability is increased hunger resulting in enhanced foraging activity in the region with a substantially shorter growing season, lower prey productivity, and lower prey community diversity. Regardless of the proximate causes, higher catchability of fish harvested in recreational fisheries in Alberta substantially increases their vulnerability to overharvest and collapse if angling effort is unabated.

Effects of Recruitment, Growth, and Exploitation on Walleye Population Size Structure in Northern Wisconsin Lakes

AbstractWe evaluated the dynamics of walleye Sander vitreus population size structure, as indexed by the proportional size distribution (PSD) of quality-length fish, in Escanaba Lake during 1967–2003 and in 204 other lakes in northern Wisconsin during 1990–2011. We estimated PSD from angler-caught walleyes in Escanaba Lake and from spring electrofishing in 204 other lakes, and then related PSD to annual estimates of recruitment to age-3, length at age 3, and annual angling exploitation rate. In Escanaba Lake during 1967–2003, annual estimates of PSD were highly dynamic, growth (positively) explained 35% of PSD variation, recruitment explained only 3% of PSD variation, and exploitation explained only 7% of PSD variation. In 204 other northern Wisconsin lakes during 1990–2011, PSD varied widely among lakes, recruitment (negatively) explained 29% of PSD variation, growth (positively) explained 21% of PSD variation, and exploitation explained only 4% of PSD variation. We conclude that population size structure was most strongly driven by recruitment and growth, rather than exploitation, in northern Wisconsin walleye populations. Studies of other species over wide spatial and temporal ranges of recruitment, growth, and mortality are needed to determine which dynamic rate most strongly influences population size structure of other species. Our findings indicate a need to be cautious about assuming exploitation is a strong driver of walleye population size structure.

Climate change drives coherent trends in physics and oxygen content in North American lakes

Using a 25-year record of monitoring data, we show that recent climate change has affected the thermal properties and oxygen content of seven lakes in south-central Ontario, Canada, and five lakes in north-central Wisconsin, USA. Coherent patterns in autumnal lake warming were driven by increased autumn air temperature in both lake districts. Temperature increases were restricted to the epilimnion and metalimnion of the lakes, resulting in increased thermal stability of the water column. Mixing depths also decreased over the study period. Shallower mixing depths in the Ontario lakes were due to climate-driven increases in lake-water dissolved organic carbon concentrations. Collectively, changes in the thermal regime of the lakes suggest autumn mixing of the water column may be delayed. Metalimnetic oxygen also increased in the Wisconsin lakes, perhaps in response to increased algal production as lake thermal regimes changed. The response of individual lakes to climate change was modified by lake chemistry in the Ontario lake district and by lake chemistry and morphometry in the Wisconsin lake district. Our results demonstrate coherent lake response to climate change and highlight the importance of both regional and local factors in regulating individual lake response to global climate change.

Efficacy, selectivity, and herbicide concentrations following a whole-lake 2,4-D application targeting Eurasian watermilfoil in two adjacent northern Wisconsin lakes

The herbicide 2,4-D (2,4-dichlorophenoxy acetic acid) has been used to control the nonnative aquatic plant Eurasian watermilfoil (Myriophyllum spicatum; EWM) since the 1950s. Although published research evaluates the herbicide's predicted and observed concentration and exposure times in both laboratory and field settings, few data are available evaluating selectivity and long-term efficacy as well as herbicide concentration behavior following large-scale, whole-lake applications. A controlled study was conducted on 2 adjacent oligo-mesotrophic northern Wisconsin lakes to determine the potential efficacy and selectivity of large-scale and low-dose 2,4-D applications. Initial 2,4-D concentrations in both treated lakes were approximately 100 μg/L higher than the nominal lakewide targets of 500 and 275 μg/L, respectively, and the herbicide dissipated and degraded more slowly than predicted. A lakewide regression model relating 2,4-D concentration at monitoring sites to days after treatment (DAT) found the mean half-life of 2,4-D to be 34–41 DAT, and the threshold for irrigation of plants not labeled for direct treatment with 2,4-D (<100 μg/L) was not met until 50–93 DAT. In the lake treated at the higher 2,4-D rate, EWM was not detected for 3 consecutive years posttreatment. Additionally, several native monocotyledon and dicotyledon species also showed sustained significant declines posttreatment. This study is the first to link field-collected 2,4-D concentration measurements to selectivity and long-term efficacy in EWM control following whole-lake management efforts. Although multiyear EWM control was achieved with these single low-dose applications, longer than expected herbicide persistence and impacts to native plants demonstrate the challenges facing aquatic plant managers and the need for additional field studies.

Are rapid transitions between invasive and native species caused by alternative stable states, and does it matter?

Rapid transitions in ecosystem structure, or regime shifts, are a hallmark of alternative stable states (ASS). However, regime shifts can occur even when feedbacks are not strong enough to cause ASS. We investigated the potential for ASS to explain transitions between dominance of an invasive species, rusty crayfish (Orconectes rusticus), and native sunfishes (Lepomis spp.) in northern Wisconsin (USA) lakes. A rapid transition from Lepomis to rusty crayfish dominance occurred as rusty crayfish invaded Trout Lake, and the reverse transition resulted from an eight-year experimental removal of rusty crayfish from Sparkling Lake. We fit a stage-structured population model of species interactions to 31 years of time-series data from each lake. The model identified water level as an important driver, with drought conditions reducing rusty crayfish recruitment and allowing Lepomis dominance. The maximum-likelihood parameter estimates of the negative interaction between rusty crayfish and Lepomis led to ASS in the model, where each species was capable of excluding the other within a narrow range of environmental conditions. However, uncertainty in parameter estimates made it impossible to exclude the potential that rapid transitions were caused by a simpler threshold response lacking alternative equilibria. Simulated forward and backward transitions between species dominance occurred at different environmental conditions (i.e., hysteresis), even when the parameters used for simulation did not predict ASS as a result of slow species responses to environmental drivers. Thus, ASS are possible, but by no means certain, explanations for rapid transitions in this system, and our results highlight the difficulties associated with distinguishing ASS from other types of threshold responses. However, whether regime shifts are caused by ASS may be relatively unimportant in this system, as the range of conditions over which transitions occur is narrow, and under most conditions, the system is predicted to exist in only a single state.

Food web consequences of long-term invasive crayfish control

Controlling invasive species can restore ecosystems while also quantifying species interaction strengths. We experimentally removed invasive rusty crayfish (Orconectes rusticus) from a Wisconsin lake. Rusty crayfish abundance declined by 99% in 8 years and did not significantly increase 4 years postharvest, with no compensatory recruitment response observed. Native crayfish (Orconectes virilis) and sunfish (Lepomis spp.) abundances increased by two orders of magnitude as rusty crayfish abundance declined, and macrophyte cover increased significantly in 2–4 m waters. We expected benthic macroinvertebrate densities to increase as rusty crayfish were removed; however, fish consumption of invertebrates increased as rusty crayfish density declined, and macroinvertebrate responses varied among families and habitats. Total Gastropoda density increased 300-fold in cobble, while the density of one gastropod family declined in macrophytes. Ephemeroptera, Odonata, and Amphipoda densities also declined in certain habitats as rusty crayfish were removed, suggesting that they are indirectly facilitated by rusty crayfish. This study highlights the importance of considering indirect effects when assessing the impacts of invasive species and demonstrates that these impacts may be reversed over relatively short time scales.

Influence of phosphorus scavenging by iron in contrasting dimictic lakes

Internal regulatory controls of phosphorus (P) via iron (Fe) scavenging were quantified in four contrasting dimictic Wisconsin lakes: Mendota (eutrophic, calcareous), Fish (mesotrophic, calcareous), Devil’s (mesotrophic, noncalcareous), and Sparkling (oligotrophic, noncalcareous). Hypolimnetic enrichment of P was highest in Mendota and Devil’s and least in Fish and Sparkling. This enrichment was attributed mainly to internal loading in the noncalcareous lakes and regeneration of sedimenting epilimnetic P in the calcareous lakes. Differences in Fe scavenging efficiencies at fall turnover were related to hypolimnetic Fe:P molar ratios as well as Fe availability and its control by sulfate–sulfide chemistries. In the noncalcareous lakes with high hypolimnetic Fe enrichment (Fe:P &gt; 2), 45% of whole-lake total P was removed. P removal was low (&lt;20%) in the two calcareous lakes with minimal Fe enrichment (Fe:P &lt; 2). These differences in hypolimnetic P enrichment and subsequent Fe scavenging at fall turnover help to explain the differences in the amount of P available for subsequent spring and summer primary production as well as the differences in trophic state of the four lakes.

Regional-Level Inputs of Emergent Aquatic Insects from Water to Land

Emergent aquatic insects can provide inputs to terrestrial ecosystems near lentic and lotic waterbodies, producing ecosystem linkages at the aquatic–terrestrial interface. Although aquatic insect emergence has been examined for individual sites, the magnitude and spatial distribution of this phenomenon has not been examined at regional spatial scales. Here, we characterize this cross-habitat linkage for the state of Wisconsin, USA (169,639 km2). We combined GIS hydrological data with empirical data and predictive models of aquatic insect production to estimate annual aquatic emergence for the state of Wisconsin. Total emergence (lentic + lotic) was estimated to be about 6,800 metric tons of C y−1. Lentic systems comprised 79% of total estimated insect emergence, primarily due to the large amount of lake surface area relative to streams. This is due to both basic ecosystem geometry and the overall abundance of lakes in Wisconsin. Spatial variation was high: insect emergence in southwestern Wisconsin was dominated by streams, whereas for most of the rest of the state insect emergence was dominated by lakes. Lentic inputs to land were highly concentrated (relative to lotic inputs) because lakes have a high ratio of surface area to buffer area. Although less concentrated, the spatial extent of lotic influence was greater: statewide, four times more land area fell within the 100 m buffer zones of streams compared to lakes. Large waterbodies (almost all of which were lakes) were hotspots of insect emergence and input to land. Aquatic insect inputs exceed estimated terrestrial secondary production in 13% of buffer area, and by a factor of 100 or more adjacent to large lakes (>50,000 ha). The model sensitivity analysis showed that the simplifying assumptions and sources of potential error in the input variables had a minor impact on the overall results.

Physical responses of small temperate lakes to variation in dissolved organic carbon concentrations

We used a mechanistic physical model to examine the effect of variability in dissolved organic carbon (DOC) concentrations on the physical properties of small temperate lakes. The model was validated on eight small (6 × 10−4 to 3.8 × 10−2 km2) lakes in Wisconsin and Michigan, with a standard error &lt; 1°C for seven of eight lakes. Attenuation of photosynthetically active radiation (400–700 nm) in these lakes was regulated by DOC concentrations and was important in the vertical structuring of water temperatures. Heat exchange below the surface mixed layer was near the molecular rate, increasing the importance of water clarity as a control on the heat content of deeper waters. To understand the thermal effects of changing DOC concentrations, we applied scenarios of a 50% increase and 50% decrease in DOC concentrations for one lake (Trout Bog) and found water temperatures to vary in response, with the seasonally averaged temperatures in the increased DOC scenario being &gt; 2°C colder than the reduced scenario. We found a nonlinear relationship between DOC and temperature, with clearer (lower DOC) simulations being more sensitive to climate variability, suggesting that DOC may act as a buffer against a warming climate. Our model showed that DOC also influenced epilimnetic depths, as nocturnal mixing (related to the vertical partitioning of heat) was more important than wind‐driven mixing. Small lakes are globally important regulators of biogeochemical cycles and are structurally different from larger lakes. Important feedbacks to physical processes must be accounted for when understanding the effects of changing DOC and climate on small lakes.

Reevaluation of lake trout and lake whitefish bioenergetics models

Using a corrected algorithm for balancing the energy budget, we reevaluated the Wisconsin bioenergetics model for lake trout (Salvelinus namaycush) in the laboratory and for lake whitefish (Coregonus clupeaformis) in the laboratory and in the field. For lake trout, results showed that the bioenergetics model slightly overestimated food consumption by the lake trout when they were fed low and intermediate rations, whereas the model predicted food consumption by lake trout fed ad libitum without any detectable bias. The slight bias in model predictions for lake trout on restricted rations may have been an artifact of the feeding schedule for these fish, and we would therefore recommend application of the Wisconsin lake trout bioenergetics model to lake trout populations in the field without any revisions to the model. Use of the Wisconsin bioenergetics model for coregonids resulted in overestimation of food consumption by lake whitefish both in the laboratory and in the field by between 20 and 30%, on average. This overestimation of food consumption was most likely due to overestimation of respiration rate. We therefore adjusted the respiration component of the bioenergetics model to obtain a good fit to the observed consumption in our laboratory tanks. The adjusted model predicted the consumption in the laboratory and the field without any detectable bias. Until a detailed lake whitefish respiration study can be conducted, we recommend application of our adjusted version of the Wisconsin generalized coregonid bioenergetics model to lake whitefish populations in the field.

Protecting in partnership: the Mukwonago River Basin Protection Plan

AbstractThe Wisconsin (USA) Lakes Partnership is a coalition of academic, advocacy and regulatory entities focused on ensuring effective conservation of the State's natural and water resources. This report summarizes a successful application of the Partnership concept through a case study describing the process that led to the development and implementation of a River Protection Plan for the Mukwonago River Watershed. In addition to the actions of individual landowners, a planning programme sponsored by special purpose units of government, funded in part by the State of Wisconsin and in part by non‐governmental organizations, and conducted by a regional planning authority in partnership with local universities and governmental agencies led to the development of the Mukwonago River Watershed Protection Plan, the contents of which were validated and guided by stakeholders through a Watershed Team and ad hoc Advisory Group. The Watershed Team secured and provided in part the necessary financial support for the conduct of the planning programme, while the Advisory Group identified concerns and validated recommended responses to address the shared issues of concern. The resulting watershed protection plan sets forth a strategy for the maintenance and protection of the high‐quality water resources of the Mukwonago River Basin.

Invasive crayfish Orconectes rusticus (Decapoda, Cambaridae) is a more effective predator of substrate nesting fish eggs than native crayfish (O. virilis)

Non-indigenous crayfish pose substantial threats to native species, community structure, and ecosystem function in invaded lakes across the globe. The well-documented range expansion of the rusty crayfish Orconectes rusticus is an ideal system to study potential relationships between the attributes that contribute to invasive success, and the ways in which non-indigenous species alter invaded systems. In Michigan and northern Wisconsin (U.S.A.) lakes where O. rusticus is replacing its native congener O. virilis, it is thought that O. rusticus invasion may account for decreased fish abundance. However, not all of the potential mechanisms that could drive this process have been explored. We examined the hypothesis that O. rusticus are better benthic egg predators than O. virilis, and tested the prediction that O. rusticus would consume substrate bound fish eggs at a higher rate than its native congener. Support for this hypothesis would suggest that O. rusticus may be able to access a high-energy food source better than O. virilis, and exert predation pressures that could limit the abundance of fish species which might otherwise limit crayfish populations through top-down control. In laboratory experiments, we determined that O. rusticus exhibit a positive correlation between feeding rate and carapace length not displayed by O. virilis, and that larger O. rusticus feed at a higher rate than their native congener. We also found that O. rusticus in laboratory feeding trials engaged in longer feeding events than crayfish foraging in situ in a substrate-spawning fish nest, a factor which could give O. rusticus an advantage in resource competition with its native congener O. virilis. Collectively, these findings support the hypothesis that O. rusticus is a better predator of substrate bound fish eggs than O. virilis, and suggest that direct predation by crayfish may influence fish reproduction in lakes where O. rusticus replaces O. virilis.

Spatial heterogeneity strongly affects estimates of ecosystem metabolism in two north temperate lakes

To characterize the spatial variability of metabolism estimates (gross primary production [GPP], respiration [R], and net ecosystem production [NEP]) in two Northern Wisconsin lakes, we collected data from 27 and 35 dissolved oxygen sensors placed in a two‐dimensional array throughout the upper mixed layers over a period of 10 d per lake in midsummer. Averaged over the deployment, aerial metabolism estimates among sensor locations varied 1–2 orders of magnitude and were largely unrelated to physical habitat within the lake. For all sites and days, 76–90% of the explainable variance in GPP and R was attributable to location in the lake rather than day of the deployment. NEP, on the other hand, was less affected by location, with 79–93% of the explained variance attributable to the day of the deployment. Single‐location estimates can yield errors of more than an order of magnitude in estimates of daily GPP and R and can mischaracterize the trophic status of the lake. Using a rarefaction approach, we found that using four randomly placed sensors increased the precision of the resulting daily metabolism estimates fourfold over single‐location measures in both lakes.

Simulated Equilibrium Walleye Population Density under Static and Dynamic Recreational Angling Effort

AbstractUnderstanding the dynamics of angling effort and how fish populations are affected by angler effort is an important and often unappreciated component of fisheries management. Our objective was to determine the extent to which angling‐induced mortality limits walleye Sander vitreus population density in northern Wisconsin lakes. We developed a simulation model to evaluate the effects on long‐term equilibrium walleye population density caused by effort limitations, density‐dependent angling effort, and angling regulations. Equilibrium density was limited by angling effort when effort was held constant but was generally lower when effort was density dependent. Equilibrium density ranged from 3.46 to 21.79 adults/ha when effort was density dependent, similar to the observed median density in lakes sustained by natural reproduction (8.7 adults/ha). Median equilibrium density was 23.3 adults/ha when effort was zero. Equilibrium density was higher when harvest was regulated by a minimum length limit as opposed to no length limit and with no postrelease mortality than with postrelease mortality. Population collapse was more likely and equilibrium density was lower when the population density and effort/ha relationship was strongest. Because the observed relationship between adult walleye population density and effort/ha was weaker, we conclude that open‐access walleye fisheries in northern Wisconsin are generally self‐regulating. However, populations likely persist at lower population densities than would be expected if angling effort were limited to lower levels than those currently observed. Our results are intuitive; however, they suggest that density increases may be on the order of 100% when effort is severely restricted compared with open‐access fisheries with unlimited effort. Other fish species may experience different expected density increases with restricted angler effort depending on a number of factors, but our results suggest that population density changes can be dramatic and that similar modeling exercises may be useful to managers of other fish species if increasing density is of interest.Received December 19, 2011; accepted June 13, 2012