— 174 — 
We thus see, that fluctuations in the composition with regard to age (the variation in 
numerical value of the year classes, as noted in the previous chapters) may occasion 
fluctuations in the size, and consequently in the quality, of the fish. It might be advisable 
to investigate the question whether the fluctuations in the French sardine fishery may 
not be due to similar causes. 
Comparison of different species of Clupeide. 
We have seen in the foregoing, that a characteristic feature of both herring and sprat 
was the existence of certain distinct periods in the development of fatty deposit. First 
30 ea 
25 
AL 
F Glupea pilchardus [ Gornwal ] 
20 
Clupea sprattus [norgel- 
15 y 
©) 
ir 
10 
P 
5 
69,868, SO PE Ses 6 9.909 m98 ® 
Fig. 105. Growth rate of herring, sprat and pilchard. 
F = ister stage. S = spawning stage. 
of all, in the case of quite young fish, a period when little fat is found, then the ister stage, 
in the older but still immature herring and sprat, and finally, that of the grown, mature 
fish, in which the developement of the genital organs plays so great a part in determining 
the quality. As the different species differ widely both in growth, and as regards the 
age at which maturity is reached, it is evident that the alterations in the process of nu- 
tritive assimilation and general change of matter must differ in like degree. This will 
best be seen by comparing the rate of growth and period at which the ister stage and 
that of maturity are reached in the case of some closely related species. In figure 105, I 
have endeavoured to give a comparison, entirely schematic, of the three species; herring, 
sprat, and pilchard. 
From the figure here given, it is possible to compare the growth rate of the three 
species, not only from year to year, but from month to month. The letter F, and a 
