closer will be the relation between the size of the stock and the amount of the yield. 
The fishery investigations have therefore, as noted in Chap. III, devoted great attention 
to studying the locality of the spawning grounds, most species being especially fished 
for during their spawning time, and the study of the pelagic eggs furnishing an easy 
method of ascertaining the whole extent of the area of occurrence. As a result of these 
investigations, it has been found that the cod species are as a rule fished for — though 
possibly with varying intensity — throughout almost the whole of their area of distribu- 
tion during spawning time. The same doubtless also applies to the herring. We arrive, 
therefore, at the conclusion that it is easiest and more reliable to employ our method 
as regards the mature, spawning fish. In the case of the young, immature fish, and at 
other times than the actual spawning season, considerable difficulties are apparent, which 
must be left to future research to dispose of. 
We thus arrive at the conclusion that the method here described may already be 
employed with every prospect of success as regards the mature, and particularly the 
spawning fish. I should however, add some qualifying remarks in order to avoid any 
possibility of misunderstanding. In the fishing industry, as in any other branch of human 
activity, the attention of the majority is mainly, if not exclusively concentrated upon 
their own immediate prospects and plans. There will therefore be many who will not 
exhibit the slightest interest in such a question as that at present before us unless it is 
possible to give them definite information as to the prospects of the fishery at this or 
that particular spot where their work in the immediate future will lie. No such predic- 
tions can be formulated by the method in question, for the simple reason that the migra- 
tion of the shoals towards the fishing grounds is subject to certain natural conditions 
which always exert a considerable influence upon the movements of the fish. The ear- 
liest information as to these conditions which is based upon accurate scientific methods 
of investigation we owe to the Swedish investigators G. Exman and O. PETTERSSON*). 
These writers have, in the course of an interesting series of investigations as to the water 
layers of the Kattegat, Skagerak and North Sea, demonstrated that the movement of 
the herring towards the coast of Bohuslän may be hindered by the presence of the cold 
fresh water layers which in winter pour out from the Baltic, whereas the coastward move- 
ment of warmer, salter water from the North Sea, (the Jutland Bank) favoured the pro- 
gress of the fish in that direction. In other words, these currents have the effect, either 
of leading the herring in towards the coast (the bank water from the North Sea) or of 
barring their progress thither (the Baltic water). I have in a former work**) endeavoured 
to show that similar conditions prevail in the case of the spring herring fishery on the 
south-west coast of Norway (Flekkefjord-Sognefjord), where the spawning grounds are 
washed now by salt and comparatively warm water, now by colder, fresher layers, partly 
from the fiords and partly from the same Baltic current which makes itself apparent 
in the Skagerak. I have also shown, that this agrees with the long recognised fact that 
the herring in some years when the water is cold (down to 0°) spawn at greater depths, 
as low as 100 fathoms, while in other years the spawning may take place quite close 
*) Orro Prrrerson: Studien ueber die Bewegungen des Tiefwassers und ihren Einfluss auf 
die Wanderungen der Heringe. Fischerbote, 7, 8 u. 9, 1911. 
**) Hydrographical and Biological studies of the Norwegian Fisheries: Videnskabsselskabets 
Skrifter, No. 9, 1895. 
