The Schaeffer model treats net stock growth as a quadratic function of stock size .
Net growth is positive for smaller stocks levels.
A stable equilibrium occurs at any stock level where harvest equals net growth and the stock is generating a sustainable yield . If harvests exceed net growth, stock levels decline. Continued harvesting in excess of sustainable yields will result in severe depletion or even extinction. Although biologists often recommend harvesting at MSY, this isn't economically efficient, since it doesn't account for costs of harvesting. In Tietenberg's Fig.