When increasing fishing effort is applied to an exploited stock, the increase in total yield (the marginal yield) is less than might be estimated from the product of the increase in effort and the catch-per-unit-effort. The marginal efficiency, defined as the percentage that the actual increase is of the expected increase, decreases from near 100% for very lightly fished stocks to near zero or even negative for heavily fished stocks. The precise form of the marginal efficiency as a function of the catch is examined for two of the commonly used population models. The implication of the concepts of marginal yield and marginal efficiency for fisheries management, and the planning of fisheries development are discussed.