Timothy K. Cross

Effects of Sampling Time, Intraspecific Density, and Environmental Variables on Electrofishing Catch per Effort of Largemouth Bass in Minnesota Lakes

Abstract Effects of sampling time (day or night and fall or spring), target fish density, water clarity, water temperature, water conductivity, and lake morphometry on electrofishing catch per effort (CPUE) of largemouth bass Micropterus salmoides 200 mm total length (TL) and longer were determined. Electrofishing catch per hour (CPH) and catch per kilometer (CPK) were also compared to determine if each expression provided similar trends in CPUE. Correlations between day CPH and day CPK (r = 0.99; P < 0.0001) and night CPH and night CPK (r = 0.97; P < 0.0001) suggested that both measures provided similar trends in CPUE. Night CPH significantly exceeded day CPH, and spring CPH significantly exceeded fall CPH. Catchability (q) decreased with increasing density; therefore, CPH increased nonlinearly with density. Day CPH in fall decreased with increasing Secchi depth and water temperature but was unrelated to largemouth bass density. Day CPH in spring decreased with increasing Secchi depth and water temperatu...

Landscape factors influencing lake phosphorus concentrations across Minnesota

Abstract Total phosphorus (TP) concentrations are known to be a significant factor influencing fish populations in Minnesota lakes. Consequently, a primary focus of the Minnesota Department of Natural Resources to address fish habitat in lakes across the state has been to determine relationships between TP concentrations and watershed conditions in Minnesota lakes. Because phosphorus concentrations in Minnesota lakes vary widely corresponding to differences in geomorphology, nutrient criteria were established by the Minnesota Pollution Control Agency for specific ecoregions. To refine these relationships in Minnesota lakes managed for fishing, we gathered mean summer epilimnetic TP concentrations on 1330 natural lakes to identify where agricultural and urban development have elevated phosphorus levels. Random forest, regression tree, and generalized additive models were used to model spatial variation in lake phosphorus concentrations across Minnesota. Key landscape variables known to influence TP concentrations in lakes, including lake depth and watershed size, were used as explanatory variables in these models, along with agricultural and urban development quantified for lake watersheds from the National Land Cover Dataset. These models explained up to 60% of the variation in TP in lakes across the state and showed a critical benchmark of anthropogenic land use disturbance at 40%, that once exceeded could significantly alter TP levels and consequently fish populations. This information should be useful for fish managers to prioritize conservation efforts and to set appropriate fish population goals.

Effects of Lake Productivity, Climate Warming, and Intraspecific Density on Growth and Growth Patterns of Black Crappie in Southern Minnesota Lakes

ABSTRACT Growth and growth patterns of black crappie (Pomoxis nigromaculatus) from a diverse set of 42 lakes in the southern half of Minnesota, and effects of lake productivity, climate warming, and intraspecific density on growth were determined First-year growth increments decreased with increasing mean depth and increased with increasing chlorophyll-a concentration until concentrations reached around 100 μg/L. At concentrations above 100 μg/L, fist-year growth decreased with increasing chlorophyll-a concentration. The number of years needed to grow to 200 mm (QAGE) was lower in lakes with low (< 3.8 per lift) catch per lift (CPUE) of black crappie in gill nets set in summer. Second- and third-year growth increments explained 71% of the variation in QAGE among lakes, but first-year growth did not explain any additional variation. Black crappie with sigmoidal growth patterns (second- or third-year increments ≥ first-year increments) reached 200 mm significantly faster than black crappie with nonsigmoidal...

Spatial Habitat Dynamics Affecting Bluegill Abundance in Minnesota Bass–Panfish Lakes

Abstract A holistic approach to fisheries management requires an understanding of factors related to fish abundance over several spatial scales. We used geographic information systems to extract data describing habitat influences across three different spatial scales for a selected ecological class of Minnesota bass–panfish lakes (n = 113). These data were then analyzed by regression tree analysis to describe relationships between habitat and trap-net catch per effort (CPE) of bluegills Lepomis macrochirus. At the landscape scale of analysis, bluegill CPE increased among lakes with decreases in hydrologic connectivity (landscape position) and increases in geographic northing and easting gradients that corresponded to regional differences in geomorphology and edaphic characteristics. At the watershed–lake scale of analysis, a regression tree model with variables describing watershed area, cultivated land cover, forested land cover, and lake area explained 55% of the variation in bluegill CPE among lakes. A...

Seasonal Variation in Trap-Net Catches of Bluegill in Minnesota Lakes

Abstract We assessed the seasonal variation of trap-net catches of bluegill Lepoomis macrochirus in Minnesota lakes. Size and catch per unit effort (CPUE) of bluegill declined from June through August. The highest CPUE of bluegills longer than 149 mm total length occurred when gonads were most developed. Regression models indicated day of year explained 66% of the variation in biomass per lift, 63% of the variation in CPUE of bluegills longer than 149 mm, and 40% of the variation in CPUE for all sizes of bluegill. Similar trends in temporal variation of biomass per lift and CPUE were seen in the statewide data set of Minnesota surveys. Seasonal variation should be accounted for when data on trap-net catches of bluegill are used for management decisions.

Factors Affecting Trap-Net Catchability of Black Crappies in Natural Minnesota Lakes

Abstract We quantified the effects of length-group (150–199, 200–249, and 250–299 mm), sampling period (September, October, early spring, and late spring), lake, and intraspecific density on catchability q of black crappies Pomoxis nigromaculatus in trap nets (modified fyke nets) set in eight natural Minnesota lakes during 1996–2001. The catchability of the two larger length-groups exceeded that of the smallest length-group in all lakes. Catchability in early and late spring exceeded that in September or October in most lakes; otherwise, q did not differ among sampling periods. Catchability differed among lakes during September, October, and early-spring sampling periods but did not differ among lakes in late spring. For 150–199-mm fish, q increased with increasing lake surface area during all sampling periods, decreased with increasing shoreline development ratio during all periods, decreased with increasing maximum depth during fall sampling periods, and was unrelated to the percentage of surface area o...

A Fish Habitat Conservation Framework for Minnesota Lakes

Lakes in Minnesota face a number of large-scale ecological stressors that threaten critical aquatic habitat and fish populations. We developed a fish habitat conservation framework to guide protection and restoration efforts for lakes of the state. Surrogate measures of habitat quality were used to assess fish habitat conditions in more than 1,800 Minnesota lakes. Two fundamental fish habitat types in lakes were described (physical and water quality) and geographic information system-based surrogate measures of habitat condition (shoreline and watershed disturbance) were quantified for each habitat type. Simultaneous consideration of the two habitat types were used to develop a bivariate classification of habitat condition. Habitat condition classifications were identified using data from previous studies to categorize lakes into protection and restoration classes. Appropriate protection and restoration actions was then tailored for each classification of habitat condition. The conservation framework is a...

An Evaluation of Mark–Recapture Estimates of Black Crappie Population Size Determined by Trap-Netting during Fall and Spring in Minnesota Lakes

Abstract We evaluated mark–recapture population estimates of black crappies Pomoxis nigromaculatus 200 mm and longer made with trap-net catches during fall or spring in eight Minnesota lakes (18–168 ha). We estimated population size N during fall by use of marking and recapture in fall (FF estimates) and marking in fall and recapture during the following spring (FS estimates). We estimated spring N by use of marking and recapture after ice-out in early spring (ES estimates) and marking after ice-out and recapture during late spring (LS estimates). The FF and ES estimates were multiple-census estimates, whereas the FS and LS estimates were two-census estimates even though sampling occurred over many days. The most precise estimates were the LS estimates (median 95% confidence limits (CLs) were within 36% of 1/N), and FF estimates were least precise (median 95% CLs were within 51% of 1/N) among sampling scenarios. The median trap-net effort needed to achieve 95% CLs within 25% of 1/N estimates ranged from 1...

Comparison of Three Mark–Recapture Sampling Designs for Estimating Population Size of Largemouth Bass in Minnesota Lakes

Abstract We compared population estimates of largemouth bass Micropterus salmoides (>200 mm long) made with three different designs in 13 lakes in southcentral Minnesota. Sampling was by shoreline electrofishing, estimators were either Chapman–Petersen or modified Schnabel methods, and analysis consisted of fall marking and fall recapture (FF), fall marking and spring recapture (FS), and spring marking and spring recapture (SS) designs. Most (90%) estimates made with FS designs exceeded those made with FF designs. Although estimates made with SS designs should not exceed estimates made with FF or FS designs, 80% of those made with SS designs exceeded those made with FF designs. Conversely, few (15%) estimates made with SS designs exceeded estimates made with FS designs. Several estimates made with FF designs were negatively biased because fall-marked largemouth bass were more vulnerable to capture than unmarked largemouth bass. Therefore, FF designs should not be used to estimate population size of largem...

Use of Wind Fetch and Shoreline Relief to Predict Nearshore Substrate Composition in a North Temperate Lake

AbstractSpawning habitat assessments often focus on substrate composition, but few studies have predicted shoal substrates by using environmental factors. We developed a model for predicting shoal substrates in Belle Lake, Minnesota, using wind fetch and shoreline relief characteristics. Percent composition of four substrate classes (silt, sand, gravel, and rock), water depth estimated at 1 m from shore (shoal slope), effective wind fetch measured using a GIS model, and riparian bank height derived from LIDAR imaging were determined at 50 transects. Classification and regression tree (CART) analysis grouped substrates into categories, and general additive modeling described the effects of three predictor variables on the percent composition of substrate classes. The CART analysis correctly grouped 39 of 50 transects into four categories, and misclassifications primarily resulted from the movement of sand. Effective fetch most influenced silt (low fetch) and rock (high fetch) substrate classes, shoal slope...