Brett M. Johnson

Functional and Numerical Responses: A Framework for Fish-Angler Interactions?

A conceptual framework for sport fisheries in which fish—angler interactions are viewed as a dynamic predator—prey relationship was evaluated with data from the walleye fishery in Lake Mendota, Wisconsin. A detailed 7—yr creel survey data set for this fishery revealed a link between fish and angler dynamics. Angler catch rates were positively and linearly related to fish abundance over the range of data in the study. Angling effort was positively related to catch rate. Fish—angler relationships were obscured by three difficulties commonly associated with fishery data sets: seasonal responses, measurement errors in predictor variables, and time series bias. Statistical techniques removed each of these biases, but the data were still insufficient to conclusively identify an appropriate model for the interaction. Nevertheless, it is clear that assumptions of static angler response to sport fish manipulations are unwarranted and capable of seriously confounding management efforts. Interactions among anglers a...

Stocking Strategies for Fingerling Walleyes: An Individual-Based Model Approach

The success of any program for stocking walleye (Stizostedion vitreum vitreum) fingerlings is strongly dependent on growth of the stocked fish during the summer and early fall months immediately following their release into lakes, reservoirs, or rivers. An individual-based model (IBM) was developed to describe growth of young-of-the-year (YOY) walleyes in Lake Mendota (Wisconsin, USA). The IBM was used to evaluate stocking strategies for walleye fingerlings. According to the rules of this simulation model, predation by a walleye would occur only if the walleye was sufficiently large relative to the prey individual. The length-frequency distribution of the YOY walleye population at the end of the 1989 growing season was predicted accurately by the model. During 1989, walleye fingerlings with a mean total length of 50 mm were stocked into Lake Mendota on 28 June. Simulations were performed to investigate the effects of the size of stocked fingerlings and the timing of stocking on subsequent YOY walleye growth. These simulations revealed that if walleye fingerlings were stocked on 28 June, at an average total length of 60 mm rather than 50 mm, then the proportion of large (total length of @>175 mm) fish in the YOY walleye population at the end of the growing season would have increased threefold over the observed proportion. Economic cost per large walleye was minimized when average total length at stocking was 62 mm. Stocking 50-mm walleye fingerlings on 14 June instead of 28 June resulted in a tripling of the percentage of large walleyes at the end of the growing season.

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...

A model of bluegill-largemouth bass interactions in relation to aquatic vegetation and its management

Abstract Dense, slow growing populations of bluegill sunfish are common in lakes where heavy vegetation prevents thinning of bluegill numbers by their primary predator, the largemouth bass. Since bass can effectively prey on bluegill only along the periphery of dense macrophyte beds, mowing vegetation to create additional edge may improve growth rates and size structure of both species. While this concept is straightforward, determining the optimal vegetation configuration for the two species is difficult because mechanisms such as feeding, competition, and mortality together determine the population response and may interact in complex ways. This paper describes a model that simulates the interactions of bluegill, largemouth bass and their invertebrate prey in the context of vegetation structure. The model is calibrated to data from a heavily vegetated temperate lake and is used to examine the fish response to additional edge created by mowing channels through plant beds. Bass growth rates and numbers increased after most simulated vegetation removals because their access to prey increased. Bluegill grew fastest when about 30% of the vegetation was cut, but responded negatively to mowing more than about half the plants. Bluegill responded to the manipulations more by changing growth rates, while bass responded more by increasing numbers. Moderate plant removals accomplished by mowing many narrow channels are most likely to simultaneously benefit both species. The model synthesizes current understanding of the relationship between macrophytes, bluegill and largemouth bass, and suggests which vegetation manipulations are most informative to test in large scale experiments.

Piscivores and Their Prey

Not suprisingly, Lake Mendota’s fishes have been the subject of a multitude of studies since the days of Birge and Juday. However, these studies have focused almost entirely on yellow perch (Perca flavescens), white bass (Morone chrysops), and cisco (Coregonus artedii), probably because of their abundance or importance to the fishery. When this study began, relatively little was known about Lake Mendota’s piscivore community, including abundance, reproductive success, growth rates, diet, and distribution. Hence, we had some fundamental questions that needed to be addressed before we could predict the course of the biomanipulation experiment.

Forecasting Effects of Harvest Regulations and Stocking of Walleyes on Prey Fish Communities in Lake Mendota, Wisconsin

Abstract Two commonly used simulation models were combined to assess the effects of stocking and harvest regulations on the consumption dynamics of walleyes Stizostedion vitreum in Lake Mendota, Wisconsin. An age-structured population model was used to estimate the effects of five harvest scenarios on walleye population and fishery characteristics. Implementation of a 15-in minimum size limit resulted in increases in total yield (by weight), average weight of fish harvested, and walleye biomass remaining in the lake, Changes in walleye age structure resulting from various harvest scenarios were used as inputs to an energetics model to estimate how prey consumption by walleyes would vary under different harvest regulations. Simulations indicated that a stocking program that produced 8,000 yearling recruits annually would double walleye predation on planktivorous fish compared with predation estimated in 1987, when the study began. The modeling further indicated that a 15-in minimum size limit in conjunctio...

Development, Evaluation, and Transfer of New Technology

A selling point of the Lake Mendota biomanipulation project was the evaluation of two technologies developed by the Center for Limnology at the University of Wisconsin (UWCFL)—bioenergetics modeling and hydroacoustic assesment of fish populations—and the subsequent transfer of the technologies to the Wisconsin Department of Natural Resources (WDNR). An important result of the Lake Mendota project has been a large increase in contact and cooperation between the university and WDNR accomplished partly through monthly discussion group meetings followed by important after-hours informal discussions at the Student Union Terrace on the shores of Lake Mendota. The increased contact between UWCFL and WDNR facilitated technology transfer and we are increasingly dealing with collective evaluation and development rather than with a transfer of technology from the university to fisheries management.

Growth and Size-Selective Mortality of Stocked Muskellunge: Effects on Size Distributions

Abstract Sport fisheries for muskellunge Esox masquinongy are often sustained by stocking. Size-selective mortality has been identified as an important factor affecting stocked muskellunge, However, this mortality is difficult to assess because its effects on the population can be confounded by growth. To partition observed shifts in length frequencies of stocked muskellunge into growth and mortality effects, two lots each of approximately 1,000 hatchery-reared muskellunge fingerlings were sorted into 10-mm size-groups, marked with size-specific fin clips, and stocked into two northwestern Wisconsin lakes. Beginning 30 d after stocking, we used electrofishing to examine length distributions and estimate abundance of stocked fish. Shifts in the length-frequency distributions after stocking were found to be caused by size-selective mortality and growth. Mortality was highest for the smallest fish. A size-based analysis indicated that growth in fall contributed far more than size-selective mortality contribu...

Food Web Structure of Lake Mendota

Although some classic studies (Hrbacek et al. 1961; Brooks and Dodson 1965) have shown strong effects of planktivorous fish on lower trophic levels, it is only in the last decade that effects of interactions among trophic levels on lake ecosystems have come to the forefront of limnological research (Andersson 1984; Carpenter et al. 1985; McQueen et al. 1986; Northcote 1988; Gulati et al. 1990) A present challenge is to understand the interplay bewtween food web effects and nutrient loading (Benndorf 1988; Persson et al. 1988; Vadas 1989), a challenge that will require comparing experimental manipulations in lakes with varying nutrient loadings (Carpenter and Kitchell 1988; Carpenter et al. 1991). The objective of the food web manipulation in eutrophic Lake Mendota is to establish a large population of piscivorous fish (walleye and northern pike) and, through a cascade of trophic interactions, reduce the planktivorous fish, increase the herbivorous zooplankton, decrease algae, and increase water transparency (Kitchell, Ch. 1). In this chapter we describe the structure of the open-water food web in Lake Mendota and the dramatic changes in the planktivorous fish community that occured during the summer of 1987. The following chapters in this section describe and interpret the behaviour of different trophic levels during the first 3 years of the food web manipulation (1987–89).

Modeling the Lake Mendota ecosystem: Synthesisand evaluation of progress

The models presented in the three preceding chapters were planned as elements of an integrated ecosystem approach from phosphorus to fishes. The modeling problem was broken into three parts in order to maximize our rate of progress and make best use of the people involved. The modules—piscivory, planktivory, and herbivory-algae-nutrients—have fundamentally different time scales yet strong vertical interactions (Figure 22.1). Within a given nutrient and weather regime, differences in return time cause the upper modules to act as constraints on lower ones (O’Neill et al. 1986). Piscivore dynamics have return times of years (Post and Rudstam, Ch. 19). Stock and harvest policies as well as resource levels must be considered in modeling piscivory. Planktivory by fishes has return times of years, while that by the zooplankter Leptodora has return times of weeks (Luecke el at., Ch. 20). Herbivory, algal growth, and nutrient fluxes have rapid dynamics and short return times of a few days (Vanni et al., Ch. 21).