Contributions of Ecosystem Modeling to Understanding Instabilities of the Fish Community of Lake Erie

After a long period of recovery, the decline of several important species during the 1990s raised concern about stability of the Lake Erie fish community. Proposed explanations for the declines included cumulative effect of reductions in phosphorus loading, effects of dreissenid mussels on pelagic productivity, and effects of predator-prey oscillations induced by the resurgence of walleye during the 1970s and 1980s. Analyses of predictions of the Lake Erie Ecological Model indicate that the instability was induced by the walleye resurgence, but also indicated that changes in lower trophic level productivity interact with fish harvest policies in complex ways. This presentation reviews solutions to unique problems associated with the formulation of the Lake Erie Ecological Model and estimation of parameters and explores the implications for more general studies of nested, hierarchical systems involving space-time behavior.