being mature; it would therefore be natural to consider all four classes as representing 
different stages of size, developement, and age in one and the same race of fish. G. O. 
Sars expressed this opinion forty years ago; so deep-rooted, however, is the general idea 
of difference between the classes, that they are even now widely considered as being 
distinct species or races of fish. 
The careful investigations of late years as to the size, growth, and composition with 
regard to age of the different classes of herring have done much to elucidate this question. 
Size of the herring, and determination of age by means of measurement. 
The first experiments with a view to determining the age and growth of the herring 
were based upon the only method at that time known, viz. the system of measurement 
invented by Dr. C. G. Jon. Prrersen. In a sample of some hundreds of fish, the length 
of each individual was measured in centimetres. All the measurements taken were 
then arranged in order of size, whereby certain groups were obtained, and these were 
supposed to correspond to certain year classes. In a seine haul of small herring taken 
at Lofoten in March 1913, the percentage in a sample of 300 individuals for each centi- 
metre of size was found to be as follows: 
CM sore wala 7 8 9 10 zul 12 13 14 15 16 
REES, 28 28) Re 122 35 28 LS 1009 42 8 
Drawn as a graph, (Fig. 5) these figures give, as will be seen, a curve with double 
summit, representing two groups, 
the one between 7 and 11 cm., 
the other from 12 to 16 cm. 
The sample being taken in March, 
1. e., in spawning time, the group 
of smaller fish would be then just 
one year old, the other consisting 
of two year old fish. Determina- 
tion of age by means of the annual 
rings on the scales has shown this 
to be correct; the scales of the fish 
of the smaller group exhibited one 
winter ring, those of the larger 
having two. The two methods of 
6 TREE 9 10 11 127 13 % 15 16cm age determination, that by mea- 
Fig. 5. Percentage of different cm. sizes in the yield of small surement of length, and that by 
herring. Lofoten, March 1913. scale measurement, lead thus in 
this case to identical results. 
In dealing with larger fish, however, greater difficulties are encountered. Fig. 6 shows 
a curve based on the measurements of a seine haul, taken in Nordland during 
the autumn of 1909, and consisting of small herring and fat herring together. As will 
be seen, this curve exhibits four summits or groups. The herring being taken in the 
autumn, and their spawning time being in the spring, one would expect to find four 
groups of the following ages; ?/,, 12/3, 2/, and 3?/; years. Examination of the scales, 
