A model simulating the population dynamics of the stocks of salmon spawning in a large river system is constructed, incorporating (1) a series of theoretical reproduction curves operating in successive stages during the life history of each stock producing compensatory or depensatory effects, (2) a device for simulating environmental variability (extrapensatory effects), (3) the effects of a joint fishery on mixed stocks, (4) the effects of multiple age of spawning on fluctuation in abundance of a single stock, (5) the consequences to yield of various degrees of stabilization of the fishery, and (6) the inheritance of age at maturity in salmon. Application of the model is illustrated by two examples. The first simulates the production of cyclic dominance by the inheritance of age of maturity and depensation in the freshwater stage. In the second example, five stocks differing in relative abundance; age composition, environmental variability, and rate of compensation are subjected to a common fishery which is selective for age and provides for complete stabilization of the escapement for the combined run as a whole. Comparisons between various combinations of the five stocks show the effects of these factors on the various stocks and on the run as a whole over a simulated period of 120 years.