In a fishery where underfishing were taking place, there would be a lot of large old fish. These would not be growing very much themselves but they would be taking a lot of food from the younger ones that could, in the right circumstances, grow rapidly. On the other hand, if a fishery were overfished, few individuals would reach maturity and the breeding stock would fall dramatically. There must be an optimum somewhere between these two extremes. This is the maximum sustainable yield.