An individual-state configuration model was constructed that considers a size distribution of lampreys instead of an average size. Model simulations vary only in lamprey host size and bracket the observed field data. The model predicts that lamprey-induced mortality is seasonal and strongly pulsed, consistent with independent field studies and previous modeling work. Lamprey growth increases with host size. The model suggests that increases in lamprey size lead to nonlinear increases in lamprey-induced mortality, while increases in lamprey host size lead to nonlinear decreases in host mortality. The doubling in lamprey size that has occurred in the last twenty-five years could have resulted in 3 to 15 times greater lamprey-induced mortality. By allowing variance in lamprey size using the individual-based approach, predictions of lamprey-induced mortality increased by a factor of 2 or more over predictions derived from a lamprey of average size. Model modifications are suggested that would allow the model to estimate the functional response of lampreys in Lake Michigan.