Fish Population Dynamic and Stock Assessment

6.2.2. Concept of MSY

Fundamental to the notion of sustainable harvest, the concept of MSY aims to maintain the population size at the point of maximum growth rate by harvesting the individuals that would normally be added to the population, allowing the population to continue to be productive indefinitely. Under the assumption of logistic growth, resource limitation does not constrain individuals’ reproductive rates when populations are small, but because there are few individuals, the overall yield is small. At intermediate population densities, also represented by half the carrying capacity, individuals are able to breed to their maximum rate. At this point, called the maximum sustainable yield, there is a surplus of individuals that can be harvested because growth of the population is at its maximum point due to the large number of reproducing individuals. Above this point, density dependent factors increasingly limit breeding until the population reaches carrying capacity. At this point, there are no surplus individuals to be harvested and yield drops to zero. The maximum sustainable yield is usually higher than the optimum sustainable yield and maximum economic yield.

MSY is based on a simple and easily understood concept of how a stock reacts to fishing. It is calculated on a simple physical measure. i.e., weight of fish caught. In a steady state fishery, biomass declines as effort increases. The effort level corresponding to MSY being referred as FMSY. FMSY is used as biological reference point. MSY and FMSY could be estimated by surplus production models and by prediction models (Refer chapter on surplus production and prediction models)