David F. Staples

Spatiotemporal trends of mercury in walleye and largemouth bass from the Laurentian Great Lakes Region

The risk of mercury (Hg) exposure to humans and wildlife from fish consumption has driven extensive mercury analysis throughout the Great Lakes Region since the 1970s. This study compiled fish-Hg data from multiple sources in the region and assessed spatiotemporal trends of Hg concentrations in two representative top predator fish species. Walleye (Sander vitreus) and largemouth bass (Micropterus salmoides) were chosen for the trend analysis because they had more Hg records (63,872) than other fish species that had been sampled from waters throughout the region. Waterbody types were inland lakes (70%), the Great Lakes, impoundments, and rivers. The compiled datasets were analyzed with a mixed effects statistical model having random effects of station, year, and fish length; and fixed effects of year, tissue type, fish length, habitat, and season. The results showed a generally declining temporal trend in fish-Hg for the region (1970–2009), with spatial trends of increasing Hg concentration from south to north and from west to east across the region. Nonlinearity was evident in the general downward trends of Ontario walleye, with a shift to an upward trend beginning in the 1990s. Only ongoing monitoring can reveal if this upward shift is an oscillation in a long-term decline, a statistical anomaly, or a sustained declining temporal trend in regional fish-Hg concentrations.

Model structure adequacy analysis: selecting models on the basis of their ability to answer scientific questions

Models carry the meaning of science. This puts a tremendous burden on the process of model selection. In general practice, models are selected on the basis of their relative goodness of fit to data penalized by model complexity. However, this may not be the most effective approach for selecting models to answer a specific scientific question because model fit is sensitive to all aspects of a model, not just those relevant to the question. Model Structural Adequacy analysis is proposed as a means to select models based on their ability to answer specific scientific questions given the current understanding of the relevant aspects of the real world.

Fish Influences on Amphibian Presence and Abundance in Prairie and Parkland Landscapes of Minnesota, USA

Abstract Many amphibian populations are declining, and increased understanding of the drivers of amphibian presence and abundance will help in their conservation. In 2005 and 2006 we estimated relative abundance of larvae of two common amphibian taxa, Tiger Salamanders (Ambystoma tigrinum) and ranid tadpoles (Northern Leopard Frog Lithobates pipiens and Wood Frog Lithobates sylvaticus), in 75 shallow lakes in prairie and parkland areas in Minnesota. We used a two-step procedure in which we first modeled presence-absence with data from all lakes and then modeled abundance only in lakes where the amphibian taxa were present. For the two amphibian taxa, a generalized linear mixed effects model was used to examine the effects of dynamic factors like fish abundance and static (timeframe of study) variables like fish community type, depth, and adjacent land cover. Fish variables had the greatest influence but differed for the presence and abundance models. Salamander and tadpole presence was inversely correlated to the abundance of benthivorous fish, while salamander abundance was best explained by total fish abundance and tadpole abundance by fish community type. We did not detect influences of land cover types on the amphibian taxa we surveyed. Our findings are important because they complement previous studies documenting that negative correlations with fish extend beyond piscivores to include both planktivores and benthivores. Hydrological changes in our study landscape (e.g., installation of drainage networks, wetland consolidation) associated with agriculture and other land uses have likely increased the distribution and abundance of fish populations, thus dampening amphibian breeding success.

Changes in Relative Abundance of Several Minnesota Fishes from 1970 to 2013

AbstractLarge-scale ecological stressors such as climate change, habitat degradation, and invasive species increasingly affect fisheries across watershed or political boundaries within which they are typically managed. Though broad-scale analyses of trends in fish populations and changes in aquatic communities can be useful for detecting the effects of these stressors, they are rarely possible due to a lack of long-term data sets. The Minnesota Department of Natural Resources has used gill nets and trap nets to monitor fish communities in lakes since 1940, and the techniques have been standardized since 1970. We applied a random-coefficient mixed-effects model to mean CPUE data to describe statewide and local population trends of eight key fish species (Black Crappie Pomoxis nigromaculatus, Bluegill Lepomis macrochirus, Largemouth Bass Micropterus salmoides, Northern Pike Esox lucius, Smallmouth Bass M. dolomieu, Walleye Sander vitreus, White Sucker Catostomus commersonii, and Yellow Perch Perca flavescen...

Factors Affecting Walleye Catch in Short-Term Gill-Net Sets in a Large Minnesota Lake

Abstract Gill nets are a versatile fish sampling gear used for many species and habitats; however, their usefulness may be limited by selectivity patterns and reliance on fish movement, which can be influenced by several factors. We evaluated the effect of net length, time of day, sampling date, surface water temperature, soak time, water depth, and location on the number, length, and initial mortality of walleyes Sander vitreus caught in short-term (mean, 38.8 min; range, 10–200 min) gill nets set in Mille Lacs Lake, Minnesota. The number of walleyes caught was best described by a generalized additive model (GAM) that contained all seven covariates and explained 35% of the variation in catch. The length of walleyes caught was best described by a GAM that contained time of day, sampling date, soak time, and sampling location as covariates and explained 11% of the variation in fish length. The initial mortality of walleyes was best described by a GAM that included surface water temperature and soak time as...

Lake Sturgeon Population Status and Demography in the Kettle River, Minnesota, 1992–2007

Abstract We assessed population status and estimated selected demographic parameters for lake sturgeon Acipenser fulvescens in the Kettle River, Minnesota, using capture–recapture tagging data collected from 1992 to 2007. The lake sturgeon is an imperiled species of special concern in Minnesota, with population declines attributed to overfishing, pollution, and establishment of dams. In 1995, the recreational fishery was closed and the only dam on the Kettle River (Sandstone Dam) was removed, in part to promote lake sturgeon recovery. Recent incidental capture of lake sturgeon by anglers is prompting interest in re-opening the recreational fishery. We gathered information to determine (1) status and demography of the Kettle River population, (2) the effect of dam removal, and (3) the feasibility of re-opening the recreational fishery. Open-population capture–recapture models were used to examine population status and estimate survival, seniority, and the relative importance of survival and recruitment for...

Do Water Level Fluctuations Influence Production of Walleye and Yellow Perch Young-of-the-Year in Large Northern Lakes?

AbstractMany ecological processes depend on the regular rise and fall of water levels (WLs), and artificial manipulations to WL regimes can impair important ecosystem services. Previous research has suggested that differences in WL between late summer and early spring may alter the suitability of shoals used by Walleyes Sander vitreus for spawning. Other species, such as the Yellow Perch Perca flavescens, are unlikely to be affected in the same way by WL fluctuations because their spawning requirements are quite different. We used 11–23 years of data from six northern Minnesota lakes to assess the effects of WL fluctuations on the abundances of young-of-the-year (age-0) Walleyes and Yellow Perch. In two lakes (Rainy Lake and Lake Kabetogama), a change in WL management occurred in 2000, after which these lakes saw increased age-0 Walleye abundance, while the other study lakes experienced decreases or no change. Rainy Lake and Lake Kabetogama also had increases in age-0 Yellow Perch, but another study lake ...

Impacts of Copper on a Lotic Benthic Invertebrate Community: Response and Recovery

ABSTRACT We examined the impact of copper on invertebrate community composition in a small stream (Pelican Brook) originating from a zebra mussel infested lake in central Minnesota. One bay of the lake was treated weekly with chelated copper during the summers of 2004 and 2005, resulting in copper levels in Pelican Brook ranging from 0.1 to 0.8 mg/L for three months each summer. Benthic macroinvertebrates were collected from two sites in Pelican Brook in spring and fall over four years (2004–2007), including one pretreatment collection (spring 2004). Samples were also collected from two reference streams (2005–2007). Ephemeroptera, Trichoptera, and Plecoptera comprised over half of the community in pre-treatment and reference stream samples. During treatment seasons, mayflies were eliminated and the community shifted to only a few taxa (Hydropsychidae and Chironomidae, primarily Orthocladiinae). Reference stream communities showed seasonal fluctuations but not the massive shifts in taxa abundance seen in the impacted sites. Invertebrate communities began to recover in the first year post-treatment (2006), and by fall 2007 Ephemeroptera and Trichoptera again accounted for over half of the taxa collected. Ephemeroptera, particularly Heptageniidae, appear to be good signal taxa for copper impacts.

Evaluation of Two Different Stocking Rates of Small Walleye Fingerlings in Minnesota Lakes

AbstractWe compared the performance of two differing stocking densities of small (mean TL = 72 mm) fingerling Walleyes Sander vitreus in 19 Minnesota lakes. Fingerlings were stocked annually from 2008 to 2011 in June at densities of either 30 (low) or 60 (high) per littoral hectare. Although gill nets yielded low sample sizes, we found no significant difference between gill-net catch rates at age 3 between treatments or years. On average, small Walleye fingerlings contributed 15% (0–72%) to the overall gill-net catches in stocked lakes. In lakes where small fingerlings have had a record of success, we recommend managers stocking this life stage use the lower treatment density (30/littoral hectare) and consider it a maximum rate when seeking stocking efficiencies. Due to low sample sizes, we also recommend following stocking events more closely with other sampling methods, such as fall electrofishing.Received April 26, 2017; accepted August 2, 2017Published online October 20, 2017

Accurate estimates of age at maturity from the growth trajectories of fishes and other ectotherms

Age at maturity (AAM) is a key life history trait that provides insight into ecology, evolution, and population dynamics. However, maturity data can be costly to collect or may not be available. Life history theory suggests that growth is biphasic for many organisms, with a change-point in growth occurring at maturity. If so, then it should be possible to use a biphasic growth model to estimate AAM from growth data. To test this prediction, we used the Lester biphasic growth model in a likelihood profiling framework to estimate AAM from length at age data. We fit our model to simulated growth trajectories to determine minimum data requirements (in terms of sample size, precision in length at age, and the cost to somatic growth of maturity) for accurate AAM estimates. We then applied our method to a large walleye Sander vitreus data set and show that our AAM estimates are in close agreement with conventional estimates when our model fits well. Finally, we highlight the potential of our method by applying it to length at age data for a variety of ectotherms. Our method shows promise as a tool for estimating AAM and other life history traits from contemporary and historical samples.