An individual-based model of lake fish communities: application to piscivore stocking in Lake Mendota

A general individual-based fish community simulator is presented. The model tracks the daily feeding, growth, movement, reproduction, and mortality of individuals of up to six species for multiple generations in up to three spatial boxes. The version presented in this paper has been configured and calibrated for Lake Mendota, Wisconsin. Six species are followed: yellow perch, bluegill, white bass, cisco, walleye, and northern pike. The environment consists of daily temperature, dissolved oxygen, and prey densities in each of the epilimnion, hypolimnion, and littoral zone spatial boxes. Feeding parameters and larval mortality rates were calibrated until all species persisted at reasonable biomasses, with realistic mean lengths at age and diets by life stage. Two alternate baseline calibrations are presented which differ in their degree of interannual variability. Correlation analysis of survival and growth rates showed that larvae were influenced by competition, yearlings by predation, and young of the year (YOY) juveniles by both. Adult growth was density-dependent for planktivores and positively related to forage biomass for piscivores. Cisco dynamics were effectively independent of the other species. The calibrated model was used to compare the effects of a biomanipulation experiment (piscivore enhancement) versus a coincident fish die-off event, and to evaluate alternative stocking regimes for their ability to sustain improved water duality. Predicted total zooplankton consumption was used to indicate effects on algae and water quality. Cisco die-off produced similar short-term but much larger long-term reductions in zooplankton consumption than piscivore enhancement. Delayed changes in YOY juvenile and larval survival illustrated complex indirect food web responses to piscivore enhancement. None of the three alternative stocking regimes analyzed yielded ideal management results. Stocking either had little effect on zooplankton consumption, or resulted in significant changes in piscivore mean lengths or in the composition of the fish community. (C) 2000 Elsevier Science B.V. All rights reserved.