THE AGE OF FISHES AND THE RATE AT WHICH THEY GROW. 417 
vears, in the middle North Sea six years, and further north seven years. 
In the English Channel, on the other hand, the bulk of the females reach 
maturity at four years of age. The average age for the males may be 
a year earlier than for the females. 
We have now perhaps devoted sufficient attention to the Plaice, and 
will pass on to consider certain investigations into the age of the Herring, 
which bring out very clearly another application of a knowledge of the 
age of fishes, which may have an important practical bearing. These 
investigations we owe especially to the energy and enterprise of Nor- 
wegian naturalists under the leadership of Dr. Johan Hort. 
Fig. 7.—Growth zones of herring scales compared with size of ish. 
(After Hjort.) 
As with most marine fishes, the growth of the Herring comes practi- 
cally to a standstill during the winter. This winter rest is clearly indicated 
by a ring-like mark on the surface of the scale, these rings being often very 
definite and precise. 
That each ring really does represent the cessation of growth during the 
winter has been proved by: Lea, by examining samples of Herrings month 
by month during the year (12), in the same way as was done by Wallace 
with the otoliths of the Plaice. It was: possible to follow the band of 
summer growth becoming wider and wider, until as winter came on it 
ceased, and the darker ring was found at the margin of the scales. This 
being so it is obvious that an examination of the scale can tell us a good 
deal more about a fish’s history than merely its age. We can indeed infer 
the length of the fish at the end of any particular year of its life, for it has 
been possible to show that the length of the fish is always proportional 
to the length of a particular scale. If, therefore, we magnify the scale 
until its total length from the median transverse line to the edge is equal 
to the length of the fish, the distance from this line to each of the winter 
NEW SERIES.—VOL, XI. NO, 8. DECEMBER, 1917, 25 
