The relationship between the growth of the fish population and the size of the fish population 1. As a general rule, the growth and the size of the population are positively related up to some point. Beyond that point, growth, although positive, decreases as the population increases further until, eventually, the population becomes so large that growth goes to zero.
Growth is limited by the size of the fish habitat, the amount of available food, and other factors.
Hence the population must decline. Hence the population must increase. For a given harvest size, the equilibrium size of the population is the population size that corresponds to the growth rate that is, in turn, equal to the harvest rate. Alternatively, there is a single sustainable yield associated with each size of the population. Sustainable means that the harvest rate can be maintained indefinitely over time, ceteris paribus .