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of constant extent; even within one and the same species therefore, it will be necessary 
to observe considerable caution when drawing conelusions as to the time at which one 
or another year class should become extinct. It will be readily understood that the 
Norwegian fishery investigations have for years been keenly interested in noting how 
long the frequently observed rich year class from 1904 would continue to make itself 
apparent among the herring and cod, where it has long exerted so dominant an influence, 
and occasioned so rich a yield. As mentioned in Chap. I, we have hitherto only had 
occasion to observe the disappearance or great reduction in numbers of a single rich 
year class among the spring herring; that of 1899, which in 1908 and 1909, 1. e., when 
9 and 10 years old, still played an important part among the spring herring, but by 
1910 had dwindled to an entirely msignificant portion of the stock. This is illustrated 
by Fig. 16 and Fig. 22, Chap. I. Regarding the cod, we found that the 10 year old 
fish played a great part among the skrei in the year 1907, and we have seen, that 
the average weight of the skrei may exceed 3 kilos (gutted weight) corresponding 
to an average age of at least 11 years. (Vide Fig. 99.) 
Possibility of deducing amount of yield from the composition of year classes. 
As already frequently pointed out, no conclusions can, as a matter of fact, be drawn 
from the composition in point of percentage of the different year classes as 
to the numerical value of the stock or to the prospects of future yield. On the 
other hand, it has often been observed that a comparison of this factor with 
the actual results of the fishery may m many cases suffice to give reliable mdications 
as to the coming yield. Thus in studying the spring herring fishery, 1t was found that 
after the first discovery of the close relation between the high percentage of the rich 
1904 year class and the rich yield of the fishery, the same relation continued during several 
successive years, (1910—1913). The same applies to the Finmark fishery for these years. 
We have thus had occasion to observe the frequent recurrence of like situations which 
have at least enabled us to formulate certain predictions with a high degree of proba- 
bility. It would however, be unjustifiable and premature as yet to conclude from this 
that such predictions may always be made. I have frequently referred to the possibility, 
both as regards cod and herring, that other factors might have to be reckoned with, 
as for instance a higher mortality among older individuals than has hitherto been the 
case in the 1904 year class, or an alteration in the movements or migrations of the shoals. 
As to fluctuations in the mortality, we can only repeat that our knowledge in this respect 
must be increased by further experience. As regards the migrations, however, I may 
here add a few words to the remarks already made upon the subject in Chapters I and 
III. First of all, in considering the question of relation between the stock and the yield 
(percentage caught) it is necessary to bear in mind the fact that fishing is not always 
carried on throughout the whole possible extent of the grounds where the fish occur, in 
consequence of which it may happen that a part of the stock is entirely exempt from the 
toll levied by the fishery. If therefore, the stock make any change of ground which 
the fishermen are unable to follow, we may have a poor yield concurrent with the exi- 
stence of a rich stock. It will therefore always be of the highest importance to know 
where the fish are to be found, and the greater the extent to which this is known, or 
the more nearly the actual fishery embraces the whole area of occurrence of the fish, the 
