— 203 — 
when they are caught. It will therefore appear most probable that the renewal of the 
fish stock, as in the case of any stock on land, is dependent upon many factors, all neces- 
sary, and all more or less variable. Thus in each individual instance, the missing 
factor will appear to be that which determines the ultimate result. Only experience 
extending over a long period of time can enable us to ascertain with certainty 
the variation of the different determining factors, and to discover how far one and 
the same factor is in all cases of decisive importance, or whether, in the course 
of the developement of the fish, there may be several, each varying so greatly as to pos- 
sibly determine the character of the year’s production. It is evident from this, that 
a closer study of the laws which govern the renewal of the stock will naturally necessitate 
extended observation of the natural conditions which prevail in the sea, and even of 
conditions which at first sight have little or no connection with fishery questions. I 
have therefore always endeavoured, as far as lay in my power, to fix the scheme of the 
Norwegian fishery investigations upon the broadest general scientific basis. This is 
especially important in the case of the question at present under consideration, owing 
to the length of time which must necessarily elapse before the results of the year’s pro- 
duction make themselves apparent. In the course of the actual investigation, for instance, 
of the spawning, it is impossible to say what factors may be of decisive importance, 
and neglect or omission of data at the time can never be repaired. 
Under these circumstances, it is evidently of the highest importance to carefully 
determine which factors may be considered as of greatest significance, and what material 
is to be regarded as most essential for the purpose of investigation. The great task here 
presented for future consideration is, in my opinion, still at so early a stage that | am 
far from considering myself in a position to exhaustively discuss the questions raised. 
I will, however, endeavour to indicate the points which appear to me as of principal 
importance. 
Stage at which the numerical value of a year class is determined. 
The greater the difficulties which any task presents, the more important must it 
seem to confine it within the closest possible limits. First of all, it would seem natural 
to endeavour to ascertain at what stage in the life of the fish the numerical value of a 
year class is determined. We have seen in the previous chapters, that the rich 1904 
year class already made itself apparent among the cod taken with the fine-meshed trawl 
in Finmark waters during the autumn of 1905. Fish of the same year, 1904, were also 
particularly numerous in the catches of haddock in the North Sea and Skagerak in the 
autumn of 1905 and 1906, and finally, the 1904 year class played an important part 
in the stock of fat herring on the North Coast in the autumn of 1907. The rich year classes 
thus appear to make their presence felt when still quite young; in other words, the numerical 
value of a year class is apparently determined at a very early stage, and continues in approxi- 
mately the same relation to that of other year classes throughout the life of the individuals. 
An illustration of this is afforded for instance by the percentage curves for the relative 
numbers of the different year classes of spring herring in the years 1908—1913 (Fig. 16). 
The above statement, despite the general terms in which I have, for the sake of con- 
venience, preferred to formulate it here, is naturally only intended to apply to the data 
and material dealt with in the present work, without prejudice as to the possibility of 
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