FISHERY BULLETIN: VOL. 71, NO. 2 



capacity of its environment, it will tend to 

 increase. In the simplified case the rate of 

 increase will depend solely on the abundance of 

 the population (or the abundance of the ex- 

 ploitable part of the population). 



On the basis of this simple model, it is clear 

 that if the annual removals by man (catch) are 

 equal to the annual natural increase, the size 

 of the population will remain unaltered, and 

 such a catch can be sustained indefinitely. For 

 such a situation the sustainable yield can be 

 defined as being equal to the net rate of natural 

 increase and represents the yield which can be 

 maintained indefinitely while also maintaining 

 the stock size at the same level. 



The rate of increase, as a proportion of the 

 population, generally decreases as the popula- 

 tion increases. The absolute value of this natural 

 increase, and hence the sustainable yield, is 

 small for very small populations and is also 

 small for large populations as they approach 

 the limiting value. It will be greatest for some 

 intermediate level of population which is the 

 population abundance giving the maximum 

 sustainable yield (MSY) (see Figure 1). 



If the population is less than the level giving 

 the MSY, no catch greater than the sustainable 

 yield can be maintained for more than a short 

 time. The population will be reduced each year 

 by an amount approximately equal to the 



difference between the catch and the sustainable 

 yield. The sustainable yield will in turn be 

 reduced, leading to an ever faster reduction, 

 which, if the amount caught is maintained by 

 fishing harder and harder, will in a fairly 

 short time lead to the "commercial extinction" 

 of the stock. 



On the other hand, if the population is 

 above the MSY level, catches greater than the 

 sustainable yield corresponding to that popu- 

 lation level, can be maintained indefinitely, 

 provided that they are not greater than the 

 MSY. This is because, as these catches reduce 

 the population toward the MSY level, the 

 sustainable yield will increase. When the 

 population is reduced to the level at which the 

 sustainable yield is equal to the catch, then 

 the catch can be maintained indefinitely with- 

 out further change. The maintainable yield 

 may then be defined as the largest catch that 

 can be maintained from the population, at 

 whatever level of stock size, over an indefinite 

 period. It will be identical to the sustainable 

 yield for populations below the level giving the 

 MSY, and it will be equal to the MSY for 

 population at or above this level. In the latter 

 case harvesting the maintainable yield will 

 involve for a short transitional period a change 

 in population abundance as it is thinned out to 

 the MSY level. 



Mointoinable yield 



Stock Abundance 



Figure 1. — Sustainable yield and maintainable yield as function of 



stock abundance. 



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