AFPENDIX G 



FISHERY YIELD CONCEPTS 



Maximum Sustainable Yield (MSY) is defined as "the greatest 

 harvest that can be taken from a self -regenerating stock of ani- 

 mals year after year while maintaining constant average size of 

 the stock" (Holt and Talbot, 1978). Important elements of this 

 definition include the concept of sustainability , i.e., a level 

 of harvest that can be maintained year after year because it does 

 not affect the average abundance of the resource, and the con- 

 cept of maximization, i.e., conditions that produce the greatest 

 sustainable yield. The MSY concept applies only to stocks that 

 are self -renewing . In practice this means that management for 

 MSY should cover all or almost all of a breeding population. 



The MSY concept is derived from hypotheses supported to some 

 extent by observation. The fundamental assumption is that an ani- 

 mal population increases rapidly when the initial population size 

 is small, but that the rate of increase slows as the population 

 size increases, until a point is reached at which the rate of in- 

 crease is zero and the population size remains steady. The graph 

 of such a population size against time is an S-shaped or sigmoid 

 growth curve. When the population size ceases to grow, births 

 and deaths still occur, but losses from the population are ex- 

 actly replaced by additions to it. 



Another assumption associated with MSY calculation is that 

 the population size at which birth and death rates balance in an 

 unexploited population is the carrying capacity, or the population 

 size which the environment can support. Populations which have 

 never been exploited are assumed to be at carrying capacity. Since 

 it depends on the condition of the habitat, carrying capacity can 

 change over time in response to environmental changes such as cli- 

 mate variation, pollution, etc. When exploitation begins the av- 

 erage standing stock of a resource population will be reduced and 

 will fall below the carrying capacity level. 



The relationship of birth and survival rates to population 

 size is assumed to be a density dependent one. This means that 

 birth and/or survival rates are higher when total population is 

 lower. Higher birth or survival rates may result from more favor- 

 able conditions due to less crowding, from behavioral responses, 

 or from increased food availability in proportion to number of 

 individuals in the population. (A given amount of food divided 

 ten ways provides a larger individual portion than if it were 

 divided among a hundred members of a population) . 



G-l 



