THE AGE OF FISHES AND THE RATE AT WHICH THEY GROW. 405 
spawning is at its height in February and continues into March. The 
youngest bottom stages of the Plaice, immediately after the transforma- 
tion, are found in quite shallow water along the margins of sandy bays 
and are often taken by shrimpers, working with push nets on the shore. 
According to the researches of Dutch naturalists (Redeke, 20, p. 40) 
these small Plaice appear at the beginning of April on the Dutch coast 
and they become numerous in May. UH, therefore, we allow two to three 
weeks for the eggs to hatch, we are left with about ten weeks for the 
larval stages. It must be admitted that this is a very indefinite statement, 
but it is, I think, as near as we can get on the evidence at present avail- 
able. The times will doubtless vary considerably in different localities 
and also in different years, owing to differences of temperature and of the 
food-supply available. 
In the case of the Sole, which was reared by Fabre-Domergue and 
Biétrix (6) in the laboratory at Concarneau, the pelagic larval stage 
lasted about seven weeks, but this of course does not give us a reliable 
figure for the time taken under natural conditions in the sea. 
The length at which the transformation 1s complete and the bottom 
life commences is in the case of the Plaice from 14-15 mm., for the Sole 
it is 10-l1lmm. The sizes at the time of hatching are 6-7-5 mm. for 
Plaice, 3-2 mm. for the Sole. 
We may say in general, then, that the adult characters in most fishes 
are established at the end of about three months from the time of spawn- 
ing, and when the length is from a quarter to half an inch. 
In order to determine the rate of growth subsequent to this period 
various methods have been employed and for some years past considerable 
attention has been given to the subject. It 1s possible, of course, to study 
the matter directly by keeping fishes in confinement and measuring them 
from time to time. But we can get little really valuable information in 
that way, as it is soon seen that the rate of growth depends very largely 
on the conditions in which the fish are placed, on the volume of fresh sea- 
water supplied to the tanks, on the temperature of the water, and on 
the amount and nature of the food which is supphed. This is an interest- 
ing study in itself, and from some points of view may prove to be of 
practical importance, but it really tells us little or nothing as to the rate 
at which the fishes grow under natural conditions in the sea, which is the 
point of main importance. 
Dr. Petersen of Copenhagen was the first naturalist to place the study 
of the subject on a sound scientific basis (Petersen, 19) and the method 
he employed is still perhaps the most useful for dealing with the first one 
or two years of a fish’s life. It consists simply in making a large and 
representative collection of the fish to be studied, from a particular 
