All additions to the fish stocks must come from roe spawned by the 

 adult females of the populations, and the fish are exclusively dependent 

 on the food present in their living areao 



Although we have seen that the size of a fish s took is limited, we 

 still do not have to believe that the fishery can influence the production 

 rate, when it cannot be shown that the percentage of the fish stock which 

 falls into the hands of the fishery is of considerable size compared 

 to the percentage which dies a natural deatho Therefore, we have to 

 know which part of a year class dies annually by fishing and which part 

 dies from natural causes <> The mortality through fishing starts only 

 after the fish have reached a size whereby it is no longer possible 

 for them to escape through the meshes of the nets. By checking the 

 strength of a year class in the catches from year to year with the aid 

 of age observations, one can find out by wliat percentage the year 

 class declines annually on the average^ as a result of fishing and 

 natural causes togethero For the plaice, for instance, this was 

 56 percent during the period 1925-30 (Thursby=Pelham, 1939). 



From the percentage of tagged fish recaught one can establish 

 the mortality caused by fishingo For the plaice this was 45 percent 

 during the period 1929-32 (Hickling„ 1937) » The natural mortality 

 among this catchable-sized plaice was therefore 56^ - 45^ = 11^ a 

 yearo Since many tags were not returned because they are lost by 

 the fish, or because they are not noticed by the fishermen, this 

 percentage will in reality be lowero Indeed, one may say that the 

 mortality through fishing is about 4 times larger than the natural 

 mortalityi for the other economically-important fish species one 

 will find ratios of a similar order© The fishing, therefore, 

 must be of very great influence on the fish stocko 



The size of a fish stock can be represented by its total weight. 

 The total weight of a fishstock will increase by the addition of 

 the young fish developed annually from the eggs, and by the growth 

 of the various individuals « The fish stock will decrease through 

 mortality caused by natural factors or by fishingo If we represent 

 the annual increase of the number of individuals as a result of 

 egg development as A, the increase by p;rowth as G, the natural 

 mortality as M, and the mortality as a result of fishing as V, 

 then the size of the fish stock at the end of a year (S^) compared 

 with the size at the beginning of the year (S) can be shown by the 

 formula s 



S = S-'- + G-M-Y 



The change in the strength of the fish stock is therefore* 



1 / 



S - S = A + G=M-Y^ 



l/ Editor's note; Seemingly there is a mistake in these formulae, 

 i.e., S and S are switched around » As we see it, the formulae 

 should be • 



S^ = S + A + G-M=V 



and 



S^ - S = A + G-M-V 



