18 
BULLETIN OF THE BUREAU OF FISHERIES 
proportions of males and females found among the mature fish taken inside the 
river at different times during the season is so variable as to make a calculation of 
this nature very imsatisfactory on the basis of the data at present available. No 
attempt has been made, therefore, to do this. The males undoubtedly show a de- 
cided tendency to mature younger than the females, and for this reason the total 
percentage of immature fish taken outside is somewhat less than the percentage of 
immature females. Just how much allowance should be made for this factor it is 
impossible to say, but in any event it would not alter materially the general con- 
clusions arrived at from a study of the females alone. It may be mentioned that, 
from the standpoint of conservation, a knowledge of the effect of a fishery upon 
the supply of females is of much more importance than a similar knowledge of 
the effect upon the supply of males. 
In the preliminary report mentioned above an outline of the method used in 
determining the relative maturity was given, together with a brief summary of the 
more important results that had been obtained at that time. The following report 
embodies the results of more recent studies and gives in detail the data on which 
the conclusions are based. 
The writer wishes to acknowledge his indebtedness to Dr. C. H. Gilbert, of 
Stanford University, whose advice has been constantly available throughout the 
preparation of this report. Many helpful suggestions have been obtained from Dr. 
F. W. Weymouth, of the physiology department at Stanford University. Some 
of the earlier work was done at the Hopkins Marine Station at Pacific Grove, 
Calif., but most of it was done in the laboratories of the zoology department of 
Stanford University. 
METHODS 
DETERMINATION OF AGE 
Although a detailed study of the life history and scale growth of the chinook 
salmon still remains to be made, the main features are sufiiciently known to per- 
mit of a reasonably accurate determination of age by the usual examination of the 
scales. 
The determination of the age of fish by means of a microscopic examination 
of their scales has been used so extensively during the past two decades that a 
detailed description of the method seems unnecessary. It depends upon the fact 
that the rate of growth of the fish varies materially at different times of the year. 
During the spring and summer, in general, growth is more rapid, and during the 
fall and winter it is slower. The scales of many fish, including the salmon and 
trout, bear series of concentrically arranged ridges on their outer surface. On 
account of their concentric arrangement these are known as rings or circuli. The 
scales increase in size with the growth of the fish, and circuli are added at the mar- 
gins. The scales are never normally shed, but increase in diameter by these accre- 
tions to their margins, and in thickness by additions to the inner surfaces. The 
markings formed by the circuli are therefore persistent throughout the life of the 
fish. The arrangement of these circuli is characteristically modified by variations 
in the rate of growth of the fish. During the more rapid growth of the spring 
and su mm er the rings are spaced relatively widely, but during the period of slow 
