THE AGE OF FISHES AND THE RATE AT WHICH THEY GROW. 421 
series. After a number of poor or average years the fishing begins to 
improve and remains successful for three or four years in succession. In 
the case of the Norwegian Spring Herring fishery the years from 1909 to 
1913, when the fish of the 1904 class predominated, were exceptionally 
good years, with a very high yield, the year 1913 especially being the best 
fishery during the whole period from 1896 to 1913 (Fig. 9). 
We are furnished, therefore, with what appears to be a distinct step 
in advance in our attempts to find a rational explanation of the fluctua- 
tions in the fisheries. A season occurs in which the conditions are excep- 
tionally favourable for the production of young fish, either owing to an 
exceptional supply of nourishment upon which the larvee and fry can 
feed, or to the absence of enemies, or to some other cause which at present 
has not been traced. As these fish grow up they year by year come to 
form a more important factor in the yield of the fishery and the abundance 
of fish caught increases. In the case we have considered the Herrmgs 
born in 1904 dominated and rendered fruitful the fishing of the six years 
from 1909 to 1914. How much longer their influence will be felt remains 
to be seen. 
What appears to be an exactly parallel case occurred in the North Sea 
Haddock fishery, and curiously enough it was again fish of the year 1904 
that were exceptionally abundant. The young fish of 1904 began to 
show in the catches in 1905, and in 1906 they were present in extra- 
ordinary numbers (Helland-Hansen, 10, p. 33). Although the case has not 
been worked out in the same detail as for the Herring, the statistics show 
an exceptional quantity of medium-sized Haddock in 1907, and of large 
Haddock from 1907 to 1910. This is just what we should expect from the 
gradual growth of the fish born in 1904, which were so exceptionally 
abundant in 1906. 
For the Plaice also it has been shown that the abundance of the young 
brood on the nursery grounds varies greatly from year to year (Johansen, 
14, Pts. III and VI), and there is little doubt that the same sequence 
of events occurs in the case of this fish, though it has not up to the present 
been tollowed in detail. 
These investigations, then, seem to give us one of the keys necessary 
for a proper understanding of much in relation to the fluctuations of the 
fisheries which was previously difficult to understand. Moreover, they 
offer a prospect of enabling us to predict the probable course of the fishery 
some years ahead, for when the exceptional abundance of the young fish 
of any year has been discovered, we shall be able to say, from a know- 
ledge of the growth-rate of the fish, in how many years these fish will 
reach marketable size and if all goes well with them give rise to an 
abundant fishery. Information of this kind, intelligently applied, ought 
