Paul W. Rasmussen

Factors Affecting Growth of Northern Pike in Small Northern Wisconsin Lakes

Abstract Fish assemblages that include northern pike Esox lucius as a dominant predator were sampled in 19 small (<120 ha) northern Wisconsin lakes. The purpose of this sampling was to describe northern pike population characteristics and identify factors affecting growth rates. Fish assemblages in these lakes were dominated by centrarchids, primarily bluegill Lepomis macrochirus, and small fusiform species such as yellow perch Perca flavescens. Northern pike population size structures were typically truncated at around 53 cm. Average density of northern pike (≥35 cm) was 16.1 fish/ha and ranged 2.8–38.0 fish/ha. Growth of northern pike decreased, in comparison with a growth standard, as early as age 4. Most northern pike were less than age 6, and few fish were older than age 8. Diet of northern pike generally indicated opportunistic feeding strategies with some preference for fusiform prey. Multiple factors were identified as potentially limiting northern pike growth and size structure. Northern pike gro...

Bias and Confidence Interval Coverage of Creel Survey Estimators Evaluated by Simulation

Abstract The Wisconsin Department of Natural Resources has carried out a complete creel census on Escanaba Lake, Wisconsin for more than 40 years. We used this creel census data set as the basis for simulations of a stratified random three-stage creel survey (stages were days, shifts, and count times) in which harvest was estimated as the product of effort and harvest rate. Effort was estimated from instantaneous counts of anglers, and harvest rate was estimated from completed-trip interviews. We evaluated the bias and precision of estimators of annual angler effort and harvest with creel census data from 3 years of varying angler effort and harvest. This creel survey method resulted in excellent estimates of annual effort. There was no evidence of bias, and coefficients of variation were less than 0.10 even though the standard errors of estimates were somewhat too large, resulting in conservative 95% confidence intervals (97–99% coverage). We found no evidence of bias for a stratum estimator of harvest i...

Interactions of Anglers and Walleyes in Escanaba Lake, Wisconsin

Interactions of anglers and walleyes (Stizostedion vitreum vitreum) were analyzed by fitting difference equations to a 33—yr time series from Escanaba Lake, Wisconsin. Fish population dynamics were density—dependent, and harvest by anglers followed a sigmoid functional response. The deterministic form of the model was stable and suggested that the fishery could withstand relatively high exploitation rates. The stochastic form of the model compared management strategies using the probabilities of increased or decreased fish and angler populations and the probability that fish populations will fall below a threshold level. These calculations exposed high risks associated with management strategies based on deterministic, equilibrium equations. The probabilities of sustaining both the fish stock and angler effort increase if harvest rates are reduced below the optimum derived from deterministic models. Consideration of angler dynamics reduced the prediction error of fish stock forecasts and substantially alt...

Bioaccumulation Patterns and Temporal Trends of Mercury Exposure in Wisconsin Common Loons

A long term field study was initiated by the Wisconsin Department of Natural Resources in 1992 to elucidate patterns of common loon (Gavia immer) mercury (Hg) exposure. Analysis of loon blood and feather samples collected from recaptured adult loons in Wisconsin 1992–2000 found evidence of a decline in overall body burdens of mercury in common loons for this region. The interval between sampling individual loons spanned 2–8 years, a sufficient length of time to observe a change in tissue Hg concentrations. Loon chick blood Hg levels declined by 4.9% annually for chicks sampled on 33 lakes during the period 1992–2000. This is the first evidence we are aware of showing a recent regional annual decrease in common loon Hg exposure. Repeated captures of wild loon chicks in Wisconsin shows that blood Hg concentrations can increase during the period of rapid feather growth (weeks 2–5), although the rate of increase is very slow. Mean egg Hg levels ranged from 0.19 to 0.87 µg Hg/g wet weight (ww) in samples collected 1996–2000. Egg Hg concentration was inversely and significantly related to lake pH (p<0.0001; r2=0.55). Adults and chicks were often captured simultaneous during the period 1992–2000. Correlations were highest between sibling blood Hg levels (r=0.88) and chick blood and adult blood (male r=0.61, female r=0.52) Hg levels, likely reflecting the influence of the Hg content of prey from the natal lake on loon blood Hg levels. The relationship between feathers and blood of adults and that of chick blood and adult feathers was weaker.

Hooking Mortality of Northern Pike Angled through Ice

Abstract Short-term (48-h) hooking mortality was examined for 185 northern pike Esox lucius angled through ice on baited still lines from three Wisconsin lakes. Mortality from capture with size-4 treble hooks baited with live fish was negligible (< 1%); however, mortality associated with pike hooks (also known as Swedish hooks) baited with rainbow smelt Osmerus mordax was substantial (33%). Additionally, mortality was greater for deeply hooked fish, and for fish exposed to short handling times. Pike hooks were associated with a higher percentage of deeply hooked fish than were treble hooks, but hooking location alone did not fully explain the difference in mortality between hook types. Of fish that bled at capture, 84% survived. The length of time that northern pike were allowed to run with the bait plus the time associated with hooking and landing the fish was unrelated to either mortality or hooking location. The use of pike hooks may not be compatible with management of northern pike fisheries if minim...

Evaluation of a Management System for a Mixed Walleye Spearing and Angling Fishery in Northern Wisconsin

Abstract In response to Chippewa tribal harvest declarations, the state of Wisconsin lowers daily angling bag limits for walleye Stizostedion vitreum in an attempt to avoid a maximum adult walleye exploitation of 35% on more than 1 in 40 occasions. We asked whether uncertainty in estimates of total allowable catch (TAC) and the sliding bag limit system used to manage anglers allowed the state to meet the overharvest risk level. During 1990–1998, 20–25 lakes were randomly sampled each year to estimate adult walleye stock and walleye harvest by angling and spearing. Adult walleye exploitation was calculated as the sum of angling and spearing exploitation rates. Adult walleye exploitation rates averaged 11.83% and did not vary significantly among years during 1990–1998. Angling exploitation averaged 8.38%, and spearing exploitation averaged 3.45%. Out of 210 lakes sampled during 1990–1998, only 4 (1.9%) had total exploitation rates that exceeded 35%. Since the exploitation rates were measured with error, we ...

Testing a methodology for assessing plant communities in temperate inland lakes

Abstract We outline and test an aquatic plant sampling methodology designed to track changes in and make comparisons among lake plant communities over time. The method employs a systematic grid-based point-intercept sampling design with sampling resolution adjusted based on littoral area and lake shape. We applied this method in 72 Wisconsin lakes ranging from 6.5–245 ha in size, recording species presence–absence and depth at approximately 20,000 unique sample points. To assess how reductions in sampling effort might affect data quality, we used Monte Carlo simulations (100 iterations at each of 9 levels of sampling intensity) to reduce total lake sample points by 10% through 90% using a stratified random selection approach. Species accumulation curves were fit using the Michaelis-Menten 2-parameter formula for a hyperbola, and the predicted asymptote was similar to observed species richness. In a subset of lakes, oversampling (200% effort) did not yield significant increases in species richness. However, even a modest reduction (10–20%) in sampling effort affected species richness, while frequencies of occurrence of dominant species and estimations of percent littoral area and maximum depth of plant growth were less sensitive to sampling effort. In addition, we provide results of a power analysis for detecting changes in plant communities over time. Future applications of this protocol will provide information suitable for in-lake management and for assessing patterns in aquatic plant communities state-wide related to geographic region, hydrological characteristics, land use, invasive species and climate.

A Largemouth Bass Closed Fishery to Control an Overabundant Bluegill Population in a Wisconsin Lake

ABSTRACT We studied Round Lake, in central Wisconsin, to determine if predator-prey balance could be restored by protecting predator populations. In January 1986 the 28.8 ha lake was closed to fishing for gamefish: largemouth bass (Micropterus salmoides), northern pike (Esox lucius), and walleye (Stizostedion vitreum). Panfish seasons remained open. Largemouth bass and bluegill (Lepomis macrochirus) were the predominant predator and prey in Round Lake, with bluegill comprising 58 % by weight of the largemouth bass diet from 1984 to 1991. Numbers and biomass of stock length (200+ mm) largemouth bass increased following closure of the lake to predator fishing, and there was a reduction in the population of stock-to-quality (80–149 mm) bluegill. Bluegill biomass remained relatively stable from 1984 to 1992, while bluegill numbers decreased. Bluegill relative weight (Wr) and longevity increased. Increased numbers of quality (150+ mm) bluegill by 1988 resulted in higher fishing pressure and panfish harvest.

Whole-lake Herbicide Treatments for Eurasian Watermilfoil in Four Wisconsin Lakes: Effects on Vegetation and Water Clarity

Abstract Four pilot whole-lake herbicide treatments for extensive Eurasian watermilfoil (EWM) (Myriophyllum spicatum L.) infestations were conducted in Wisconsin between 1997 and 2001 using fluridone at a range of dosages (6–16 μg/L). Annual post-treatment data (4–7 years) were evaluated to assess (1) effects on exotic plants; (2) changes to native plant communities; and (3) effects on water clarity. Temporal shifts in treatment lakes were compared against natural fluctuations in untreated reference lakes. In conjunction with aggressive follow-up spot treatments with 2,4-D or manual removal, fluridone treatments provided between 1 and 4 years of substantial EWM relief, with the exotic ultimately re-establishing at pre-treatment levels or greater in 3 of the 4 lakes. Native plant communities shifted in all 4 lakes following fluridone treatment. The large decreases, outside the range seen in untreated lakes (first quartile of the reference lake distribution) for all treatment lakes containing EWM, Elodea canadensis, Ceratophyllum demersum, and Najas flexilis, strongly suggest a direct effect of the fluridone treatment. We observed large increases, outside the range seen in untreated lakes (fourth quartile of the reference distribution), for 1 of 2 treatment lakes with Potamogeton crispus, and 1 of 2 treatments with Chara spp. Secchi depth decreased significantly in 2 of the 3 lakes for which data were available. Future applications should consider, among other criteria, the dominant natives in the plant community, their sensitivity to fluridone, and potential impacts associated with decreased water clarity.

Comparison of a Stratified, Instantaneous Count Creel Survey with a Complete Mandatory Creel Census on Escanaba Lake, Wisconsin

Abstract Creel surveys often play a major role in fisheries management, yet there has been little research on the accuracy of such surveys. To address this shortage of information, we compared fishing effort and harvest rate estimates from a stratified, instantaneous count, access point creel survey (20 h/week) with known values from a complete creel census on Escanaba Lake, Wisconsin. The estimated total fishing effort was only 7% lower than the actual, with a relative standard error (RSE = 100s·SE/estimate) of 11%, and the 95% confidence interval (CI) of the estimate included the actual effort. Harvest rate estimates were made twice, once by using all hours of fishing and once by using only time spent fishing for specific species. The 95% CIs for all of the estimates over the entire period included the actual measurements, and RSEs ranged from 18% to 27%. However, the 95% CIs for some of the estimates from individual strata did not include actual measurements, and RSEs ranged from 20% to 71%. We found t...