Development of a Warmwater Fish Community Model

James E. Breck

Michigan Department of Natural Resources
Institute for Fisheries Research
1109 N. University Ave.
212 Museums Annex Building
Ann Arbor, MI 48109-1084
e-mail: breckj@michigan.gov

Abstract.-An individual-based model, LakeMod, was developed to synthesize information and to help answer questions raised in the management of freshwater fish communities, particularly ponds and lakes containing bluegills and largemouth bass or walleyes. The daily model accounts for size-specific interactions between predators and prey, selection among multiple prey types, energetics of growth, changes in body composition, and seasonal water temperature. Sources of fish mortality include predation, starvation, and other causes. Because of the importance of energy density to fish growth and survival, a new model has been developed for dynamic changes in energy density, body composition, and relative weight. This model explains the common observation that both percent lipid and energy density show a linear relationship with percent water, and also explains the difference in vulnerability to starvation among fish of different sizes. Because of the abundance of forage fish compared to their predators, each "individual" bluegill in the model represents many identical fish in a lake or pond, whereas each largemouth bass in the model may represents one or more fish in a pond. The model has been used to explain patterns in the density-dependent growth of bluegills and largemouth bass, and to examine predator-prey interactions. Simulations of pond experiments involving different densities of largemouth bass show that the numbers and mean lengths of surviving bluegills reflect size-dependent and density-dependent processes that are readily represented in an individual-based model. Simulations show the strong interactions between growth of forage fish and growth and survival of piscivores. Understanding these complex response patterns should improve the ability of fisheries managers to determine appropriate sizes and densities of predators and prey in warmwater fish communities.