Steven W. Hewett

Applications of Bioenergetics Models to Fish Ecology and Management: Where Do We Go from Here?

Abstract Papers and panel discussions given during a 1992 symposium on bioenergetics models are summarized. Bioenergetics models have been applied to a variety of research and management questions relating to fish stocks, populations, food webs, and ecosystems. Applications include estimates of the intensity and dynamics of predator–prey interactions, nutrient cycling within aquatic food webs of varying trophic structure, and food requirements of single animals, whole populations, and communities of fishes. As tools in food web and ecosystem applications, bioenergetics models have been used to compare forage consumption by salmonid predators across the Laurentian Great Lakes for single populations and whole communities, and to estimate the growth potential of pelagic predators in Chesapeake Bay and Lake Ontario. Some critics say that bioenergetics models lack sufficient detail to produce reliable results in such field applications, whereas others say that the models are too complex to be useful tools for ...

Catch Rates and Catchability of Walleyes in Angling and Spearing Fisheries in Northern Wisconsin Lakes

Abstract We examined relationships between angling and spearing catch rates (catch/h) and walleye population density (number/acre) in 118 northern Wisconsin lakes to determine if walleye catchability in these fisheries was density dependent. The densities of both adult and total walleye populations were unrelated to lake surface area. Similarly, the catchability of walleyes in angling and spearing fisheries was unrelated to lake surface area. Angling catch rates of walleyes were linearly related to total walleye population density, whereas spearing catch rates of walleyes were exponentially related to adult walleye population density. Walleye catchability in the angling fishery was not significantly related to population density, whereas walleye catchability in the spearing fishery was inversely related to population density. We conclude that walleye angling is density independent and is therefore self-regulating, whereas walleye spearing is density dependent and is therefore not self-regulating.

Prediction of Angler Catch Rates Based on Walleye Population Density

Abstract Management of angler harvest of walleyes Stizostedion vitreum assumes that angling catch rates can be regulated by fish densities. Data on walleye densities in northern Wisconsin lakes obtained between 1990 and 1993 were used to develop a predictive model for walleye angling catch rates. Data collected in 1993–1994 were used to validate these models. Total numbers of walleye per acre were estimated from mark-recapture experiments conducted during April and May of each year. Creel surveys were conducted starting on the first Saturday in May and ending on 1 March of the following year for waters where population estimates were made. Linear regression indicated a high correlation between walleye densities and yearly walleye catch rates (r = 0.92, P < 0.05). Using the linear equation generated from this analysis, we predicted yearly walleye catch rates from walleye population estimates in 11 lakes for 1993–1994. All 11 angler catch rates were within the 95% prediction intervals. When data from 1993–1...

The Relationship between Growth and Consumption: Comparisons across Fish Populations

Abstract Comparisons of growth rates and consumption rates among fish populations are not straightforward and are difficult to interpret in the absence of other information about the populations. Differences in temperature regimes, reproductive timing, activity cost, prey availability, caloric densities of predator and prey, and allometric effects of body size on metabolism are factors that can alter the direct effect of consumption rate on growth rate, We developed a method to elucidate relations between growth and consumption by correcting for allometric weight effects. Bioenergetics model simulations showed that when growth rate is regressed against consumption rate (on either an absolute or a relative basis), the relationship is confounded by the allometric effects of differences in body weight among the populations being compared. Regressing growth rates on the difference between actual and maintenance consumption rate corrects for most of the allometric effects. That regression can be used to determ...

Variation in Walleye Abundance with Lake Size and Recruitment Source

Abstract We quantified the relationship between lake size and abundance of walleyes Stizostedion vitreum at two life stages, age 0 and adult, in 172 northern Wisconsin lakes. We also determined if the relationship varied with recruitment source (stocked or natural) in order to evaluate the current system of management. For adult walleyes, as estimated by mark–recapture in spring, abundance was linearly related to lake surface area. Age-0 walleye abundance estimated by fall electrofishing catch was also linearly related to the miles of shoreline surveyed. Lake surface area explained 59% of the variability in adult walleye abundance across lakes ranging in size from 93 to 15,300 acres. Lake surface area explained 61% of the variation in abundance of self-sustaining walleye populations and 65% of stocked populations. Walleye abundance was higher in lakes sustained by natural reproduction than in lakes sustained by stocking. These patterns in abundance based on recruitment source were also apparent at age 0. ...

Predicting the Occurrence and Success of Walleye Populations from Physical and Biological Features of Northern Wisconsin Lakes

Abstract We developed a linear discriminant function (LDF) using surface area, mean depth, fetch, and three substrate descriptors to discriminate the presence and absence of walleyes Sander vitreus (formerly Stizostedion vitreum) in 120 northern Wisconsin lakes. The resulting LDF correctly classified 90% of walleye lakes and 85% of nonwalleye lakes. For model validation, the model classified 90% of walleye lakes and 93% of nonwalleye lakes in an independent set of 100 randomly selected lakes. In contrast, within walleye lakes, an LDF using the same variables correctly classified only 60% of 30 self-sustaining walleye lakes and 50% of 30 stocked walleye lakes. For model validation, the model classified 30% of self-sustaining walleye lakes and 73% of stocked walleye lakes in an independent set of 40 randomly selected lakes. To further assess the differences between walleye lakes with self-sustaining and stocked populations, the angler catch rates for largemouth bass Micropterus salmoides, smallmouth bass M....

The role of density dependence in growth patterns of ceded territory walleye populations of northern Wisconsin: Effects of changing management regimes

Abstract We assessed density-related changes in growth of walleye Sander vitreus in the ceded territory of northern Wisconsin from 1977 to 1999. We used asymptotic length (L ∞), growth rate near t 0 (ω), and body condition as measures of walleye growth to determine the relationship between growth and density. Among lakes, there was weak evidence of density-dependent growth: Adult density explained only 0–6% of the variability in the growth metrics. Within lakes, growth was density dependent, L ∞, ω, and body condition of walleyes changing with density for 69, 28, and 62% of the populations examined, respectively. Our results suggest that walleye growth was density dependent within individual lakes. However, growth was not coherently density dependent among lakes, which was possibly due to inherent differences in the productivity, surface area, forage base, landscape position, species composition, and management regime of lakes in the ceded territory. Densities of adult walleyes averaged 8.3 fish/ha and di...

Effect of Measurement Error on Tests of Density Dependence of Catchability for Walleyes in Northern Wisconsin Angling and Spearing Fisheries

Abstract We sought to determine how much measurement errors affected tests of density dependence of spearing and angling catchability for walleye Sander vitreus by quantifying relationships between spearing and angling catch rates (catch/h) and walleye population density (number/acre) in northern Wisconsin lakes. The mean measurement error of spearing catch rates was 43.5 times greater than the mean measurement error of adult walleye population densities, whereas the mean measurement error of angling catch rates was only 5.6 times greater than the mean measurement error of adult walleye population densities. The bias-corrected estimate of the relationship between spearing catch rate and adult walleye population density was similar to the ordinary-least-squares regression estimate but differed significantly from the geometric mean (GM) functional regression estimate. In contrast, the bias-corrected estimate of the relationship between angling catch rate and total walleye population density was intermediate...

Trends in Wisconsin's Muskellunge Fishery

Abstract Wisconsins populations of muskellunge Esox masquinongy provide an important recreational fishery. Our objectives were to (1) evaluate progress of the Wisconsin Department of Natural Resources muskellunge management plan, (2) examine subsequent changes in the fishery, and (3) evaluate Wisconsins muskellunge waters classification system. With the goal of maintaining viable populations and a trophy fishery, the 1979 plan endorsed more restrictive harvest regulations, an increased supply of muskellunge fishing opportunities, and increased data collection. Since the 1980s, muskellunge fishing opportunities have increased 75% in terms of lake acres and 51% in terms of stream miles. Hatchery production and stocking efficacy have improved to the point where the department needs to reevaluate current stocking practices. With the establishment of a shorter season, a higher statewide minimum length limit, and an increased use of special regulations, harvest regulations have become progressively more restr...

Evaluation of a 15-Inch Minimum Length Limitfor Walleye Angling in Northern Wisconsin

Abstract In 1990 the Wisconsin Department of Natural Resources implemented a 15-in (1 in = 2.56 cm) minimum length limit for walleyes Stizostedion vitreum on waters throughout the state, excluding some lakes where walleyes demonstrated slow growth or high mercury contamination. To determine if management goals for the regulation had been met, we examined the effects on lakes with self-sustaining walleye populations in the ceded territory of Wisconsin, specifically effects on population structure, exploitation, and angler catch and harvest. We compared creel survey data from up to 19 lakes sampled before and after the regulation had been implemented, including lakes affected by (N = 13) and exempted (N = 6) from the 15-in regulation. We also compared postregulation (1990–1998) creel and biological data from up to 76 lakes that were both affected and exempted. Effects of tribal harvest, which occurred on all of these 76 lakes but was not subject to the same length limit, were also examined using postregulat...