INTERLAKE VARIATION IN GROWTH AND SIZE STRUCTURE OF BLUEGILL (LEPOMIS-MACROCHIRUS) - INVERSE ANALYSIS OF AN INDIVIDUAL-BASED MODEL

Habitat structure alters food availability and predation risk, thereby directly affecting growth, mortality, and size structure of fish popu lations. Size structure has often been used to infer patterns of resou rce abundance and predation. However, food availability and predation risk in contrasting habitats have proven difficult to measure in the f ield. We use an inverse modeling approach to estimate food availabilit y and habitat choice parameters from changes in length distributions o f bluegill (Lepomis macrochirus). The model suggests that dynamics of bluegill length distributions primarily reflect food availability and habitat choice. Bluegill behavior minimized effects of size-selective predation on size structure. Parameters for food availability and habi tat choice were correlated. It was therefore not possible to attain un ique estimates of food availability and habitat selection when both we re free parameters. However, when one parameter was estimated independ ently, the other could be identified. In five Wisconsin lakes, seining studies were used to estimate the size at which bluegill switched fro m littoral to pelagic habitats. Using this measure of switch size in t he model, we estimated food availability for bluegill in each lake. Th ese estimates were positively correlated with observed growth (r(2) = 0.91), demonstrating the model's ability to estimate food availability .