If the fish stock is to be maintanined at a constant value, then it 

 is necessary that; 



V = A + G - M, or in words % 



The catch must be equal to the increase: •that is, the natural weight 

 increase through egg production and growth, minus the loss through 

 natural mortalityo 



If more than the amount of the increase is caught, then the 

 strength of the fish s took will decline. Finally, this might 

 lead to emptying of the sea. If less than the increase is caught, 

 than the fish stock will increase in weight* The situation in 

 unfished areas — for instance in the Barents Sea during the beginning 

 of this century — shows that the fish stock does not increase in 

 strength indefinitely. Somewhere a constant strength or equilibri\im 

 is reached, whereby, if there is no fishing, it is valid that the 

 weight increase through egg production and growth is cancelled by 

 the natural mortality, or (A + G = M) • 



In such a dense fish stock there are many old fish and insofar 

 as the egg production increases progressively with age, a very 

 large number of eggs are producedo The mortality, however, is 

 very high; partly because the older fish feed more on fish than 

 the younger ones do, and partly because of the higher average age. 



If we begin to fish this stock the density decreases. If, after 

 a certain density has been reached, we do not fish away yearly more 

 than the natural increase, a new equilibrium is established that is 

 at a sub-msLximum density of the fish stock. We will now find what 

 changes appear in the stock when, for instance, by step-like increasing 

 of the fishing intensity, equilibriums are adjusted by increasingly 

 lower densities, A similar increase of fishing intensity has occurred 

 in several fishing areas. The following changes were thereby observed: 



1, By an increase of the fishing intensity the older age groups 

 decrease more in strength than the younger ones. The next table shows 

 us that, theoretically, this must be expected. It is shown here how 

 many fishes are left over from year to year from a group of 1,024 one- 

 year-old fish when the annual decrease is respectively 25 percent, 

 50 percent, and 75 percent. 



Age in years 



Yearly decrease 25^? 1,024 768 576 



n n 



» N 



50^ 1,024 512 256 



75^ 1,024 256 64 



Consequently, the increase of the degree of fishing lowers the average age. 



5 



