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The concept of a sustainable yield (SY, Gulland, 1983; Lannan et aL, 1989) has dominated fisheries management for almost 50 years. The central idea is that each stock has a harvestable surplus, and that fisheries that do not exceed this will not compromise the stock's natural perpetuation. A basic assumption is that the fishery targets a unit stock with definable patterns of recruitment and mortality. Although it is difficult to reach agreement on what constitutes a stock (Gauldie, 1991), the notion of population units with varying degrees of temporal or spatial integrity stimulated a quest to characterize and identify such assemblages. It became apparent that few species form single homogeneous populations, but rather that fish species are often composed of discrete stocks, and that these stocks may react to harvesting more or less independently.