A phase plot (on the left) of an open access fishery dynamic system includes biological dynamics (blue isocline) and economic dynamics (red isocline). The first represents biological growth by a logistic growth equation while growth in fishing effort is proportional to obtained rent. Corresponding developments in effort and rent (rent adjustment, labeled quasi-rent by Alfred Marshall) as functions of time are shown on the right. The equilibrium solution gains no rent, while positive or negative rent is obtained outside equilibrium. Positive and negative quasi-rent are shown as functions of time in the lower panel on the right as green or red areas, respectively. The discounted sum constitutes the total quasi-rent presented.
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