— 202 — 
weight of the large salmon are due to varying composition in point of age in different 
years. The years in which the yield shows large quantities of fish of over 3 winters (after 
the time when they, as emigrant young, left the rivers), exhibit a high average of 
weight for the large salmon. In the years when the two-winter fish are predominant 
among the large salmon, the average weight of the latter is low. Here again we find, 
that. the “quality” is not necessarily the outcome of temporary conditions as regards 
nourishment, but rather due to the composition in point of age, which has its origin 
some years back. 
Here also it is evident, that any investigation of the actual variations in quality 
from year to year must be based upon a knowledge of the composition in point of age 
of the stock, which alone can enable us to subject the question to critical analysis. An 
examination of Fig. 126, showing the fluctuations of the salmon fishery, at once suggests 
comparison with those apparent in the quantity of cod liver obtained in the same 
years, and considerable resemblance will also be discernible, (cf. Figs. 106 and 123). 
Speculations of this sort, however, would lead us far beyond the bounds of practical 
reality. 
CHAPTER VI 
Review of the present position of the Investigations. 
Results hitherto obtained. 
The most important result which we have been able to refer to in the present work 
may be briefly expressed as follows: The study of methodically collected material, embra- 
cing a period of many years, has demonstrated the existence of an intimate relation 
between the fluctuations in the numerical value of the stock of fish and the yield of the 
great fisheries. This applies to the Norwegian herring and cod fisheries, the herring 
and haddock fishery of the North Sea, and in all probability also to the North Sea cod 
fishery. 
The opinion generally prevalent hitherto was that the renewal of the stock of fish 
took place, as in the case of the increase of any human population, by means of a more 
or less constant annual increment in the form of new individuals; the results here arrived 
at, however, indicate that this renewal, in the case of the species investigated, is of a 
highly irregular nature. At certain intervals year classes arise which far exceed the 
average in point of numbers, and during their lifetime, this numerical superiority affects 
the general character of the stock, both as regards quantity and quality, thus again 
exerting a decisive influence upon the yield of the fisheries in both respects. 
Origin of the fluctuations in renewal of the stock. 
These results naturally lead us to enquire into the causes, or better, the natural 
phenomena which occasion these peculiar fluctuations in the renewal of the stock; the 
hydrographical or biological conditions which give rise to the occurrence of rich or poor 
year classes of fish. The problem is one of unquestionable difficulty, embracing as it 
does all the conditions which in any way affect the fish from the egg stage to the time 
