interrelationships of fishing intensity, stock size, and 

 yield. 



To demonstrate how this evidence is used to study the 

 qualification of a salmon stock for abstention, let us sup- 

 pose that the relationship between nvimber of spawners 

 and the nximber of ad\ilt fish they produce resembles the 

 hypothetical examples in the following table: 



Run of Permissible 



Number of adult fish catch for 



Example spawners they produce sustained yield 



1 500 1500 1000 



2 1000 2500 1500 



3 1500 2000 500 



If, from the 1500 adxilt fish in (1) we catch 1000 fish, we 

 leave 500 spawners to perpetuate the run at an average of 

 1500 fish. If, from the 2500 adult fish in (2) we catch 1500 

 fish, we leave 1000 spawners to perpetuate the run at an 

 average of 2500 fish. If, from the 2000 adtilt fish in (3) we 

 catch 500 fish, we leave 1500 spawners to perpetuate the 

 run at an average of 2000 fish. We can say that these catches 

 of 1000, 1500, and 500 fish represent the average sustained 

 yield at the three levels of escapement. In this case the 

 maximum average yield is 1500 fish, and the optim\im 

 escapement is 1000 spawners. 



Suppose that we have a run of 2500 fish annually. If we 

 catch 1500 and leave 1000 for spawning, we would be f\illy 

 utilizing the stock while maintaining the escapement at the 

 level which, on the average, will provide the maximum 

 sustained yield. It woiold be concluded that more intensive 

 fishing would not provide a substantial increase in the sus- 

 tained yield. However, take a case where we have a run of 

 2000 fish annually, out of which we have always caught only 

 500 fish and have left 1500 for spawning. These 1500 spawners 

 provide a long-term yield of 500 fish. By increasing the 

 fishing intensity to catch 1000 fish instead of 500, the escape- 

 ment would be reduced from 1500 spawners to 1000 spawners. 

 These 1000 spawners woxild provide a long-term yield of 1500 

 fish. Here, then, is a case where naore intensive fishing 

 would provide a substantial increase in the sustained yield. 



For a halibut or herring stock, the scientific evidence 

 required to determine whether the first condition for ab- 

 stention is satisfied includes additional data on growth rates, 

 rates of mortality from natural causes, the age at which the 

 fish enter the fishery, and the nxomber of years they remain 

 available to the fishery. 



The following example illustrates how growth and natural 

 mortality rates affect the yield to be obtained from a given 

 number of fish. Suppose we have a group of 1000 fish which 

 is going to be available to us for one year, and that we could 

 catch any or all of them at any one time during the year. Let 

 us say that, at the start of the year, each fish weighs 5 



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