THE AGE OF FISHES AND THE RATE AT WHICH THEY GROW. 413 
September (the squares) there has been a distinct shifting of the group of 
fish seawards as they have grown larger. 
It will be clear, therefore, that if the fish are distributed in this way 
according to size, we must take uniform samples all along the line in 
order to obtain the true average of those belonging to any age-group. 
If, for instance, in the case of this III Group we took samples only near 
the coast we should miss all the larger fish belonging to the group, whereas 
if we took samples only at the seaward end of the line we should miss all 
the small ones. In the latter case our average would be far too high, 
in the former case it would be far too low. Wallace therefore is quite 
justified in maintaining that samples of Plaice for age determination 
must be taken upon such radial lines, if accurate average sizes for the 
different years of age are to be obtained. 
One of the points which has come out most clearly in the course of 
these studies is the great differences in rate of growth which are found, 
firstly at different seasons of the year, secondly in different years, and 
thirdly in different localities. 
As regards seasonal differences we may say that in the North Sea the 
year’s growth begins in the spring, about the month of April; it goes 
on vigorously during the summer until September, slows down in Octo- 
ber, and from that time until the following April there is practically no 
growth at all, at any rate in the shallow water near the coasts. In the 
central portions of the North Sea, for example on the Dogger Bank, a 
certain amount of growth does seem to take place in the winter. 
The evidence for differences in the rate of growth of Plaice in different 
years is chiefly based on the work of the Danish naturalist Johansen (44), 
who has studied the question by means of marking experiments carried 
out off the North Sea coast of Denmark. Thus the average annual growth 
for specimens of 20 to 29 cm. liberated in the Horn Reef area in 1903 
was only about 4m., whilst in 1904, 1905, 1906 and 1907 it was from 
6 to 7-5em. In this connection Johansen notes the interesting fact that 
in 1903, when the growth was abnormally low, there was an unusually 
rich stock of under-sized fish on the Horn Reef grounds, which suggests 
that the rate of growth may depend, amongst other things, upon the 
density of the Plaice population (Johansen, 14, III, p. 37). 
It is possible, therefore, that a certain amount of fishing on grounds 
overcrowded with young fish may tend to increase the rate of growth 
of the fish that remain. 
That the rate of growth of Plaice differs widely in different localities 
may be inferred from the fact that the average length of the different 
age-groups is different in different areas, provided always that the samples 
on which the figures are based are adequately distributed or at least 
