A study of the natural history of the herring in Norwegian coastal waters thus leads 
us to the conclusion that the life-cycle of the herring is limited to a more or less restricted 
area of sea, and that these waters have a common race or stock of herring. True, indi- 
viduals may, as we have seen, exhibit peculiarities of growth according to locality, as 
for instance in southern or more northerly waters; these variations in growth must, 
however, at any rate for the present, be considered as due to the influence of extraneous 
natural conditions having no permanent hereditary effects. It would seem highly pro- 
bable that the young of fish which have grown up off the west coast of Norway can 
be carried by the current up to the North Coast waters, and there grow up under the 
same natural conditions, and at the same rate, as the indigenous Nordland fish. How 
far this also applies to the more distinctly localised shoals which are found in some of 
the fiords, Trondhjemsfjord, Lysefjord, etc., is yet an open question, the solution of which, 
while presenting features of biological interest, is hardly likely to influence the great 
general law, that the race of herring in the Norwegian coastal waters forms, on the whole, 
a single independent stock. 
As will be seen in the following chapter, the full-grown herring make periodical mi- 
grations between the spawning times, moving out into the North Sea and the Norwegian 
Sea; it has already been noticed, in the foregoing, that they may penetrate as far as 
to the Faroes, and they may possibly even find their way to the sea north of these is- 
lands, the southern Bottlenose Grounds. During the course of these migrations, they fre- 
quently mix with herring of other races, (the Shetland herring); as far as is known, how- 
ever, it would seem fairly certain that the very great majority return to the Norwegian 
coastal waters on the approach of the next spawning season. 
The fluctuations of the herring fishery. 
It now remains to be seen how far these scientific results serve to elucidate the pro- 
blem of fluctuations in the great herring fisheries, and the question naturally arises 
whether any relation can be shown to exist between the varying composition with regard 
to age of the samples examined, and the variations in the hauls made by the fishermen. 
It is obvious, that no such relation need, as a matter of fact, exist. We may well 
imagine that the yield of a certain fishery could be poor, even though one year class 
might be far more numerous than other still poorer year classes, while a highly profitable 
fishery may be based upon the capture of fish belonging to several year classes, each 
of which is represented in about the same proportion. Only by-experience of the natural 
conditions which govern these phenomena can we obtain information of any value in 
this regard; we will therefore proceed to examine the facts which have been ascertained 
as to the hauls made by the fishermen, as given in the fishery reports and statistics. 
In the Norwegian fishing industry, great interest has for a long time been evinced 
as to the collection of information dealing with the size of the herring and cod. The 
necessity of this is obvious, when taking into consideration the great variation in the 
sizes due to the fluctuations in the composition with regard to age. These practical 
observations have not, however, been based on measurement of the length of the herring, 
as in the foregoing, but on collection of statements as to their weight, and on the basis 
of long experience of the fluctuations in the weight of the fish, a system of assortment 
has arisen, embracing certain size groups (weight groups), which have gradually become 
