ee 
year, but for all years, and not only for a certain part of the coast, but for its whole ex- 
tent. Investigation has shown that the rate of growth varies in different years and in 
different waters, there being a difference, for instance, between West Coast and Nord- 
land fish in this respect. A further difficulty, moreover, is in particular presented by 
the fact that the herring exhibit a tendency to move in shoals consisting of approx- 
imately the same sizes. It thus frequently happens that the small individuals of a year 
class keep to younger year classes, (e. g. in the fat herring shoals), while the larger 
specimens of the same year class associate themselves with older fish, (e. g. large herring 
and spring herring). An examination of all the individuals of one year class in a certain 
shoal will thus give a more or less correct average for that year class as represented in 
the shoal in question, but not for the year class as a whole. 
For purposes of closer study of the growth of the herring, another method is there- 
fore employed. 
The Scales of the herring as a means of determining the age and growth 
of the fish. 
As already mentioned, it is possible, from the scales of the herring not only to cal- 
culate the age of the fish, but also, by examination of the graphical view exhibited by the 
Fig. 10. Growth zones of herring scales compared with size of fish. 
growth rings, to determine how much the fish has grown in the different periods of growth. 
If therefore, we draw a picture of a scale, enlarged to such a degree as to make the di- 
stance between the centre of the so-called basal line to the edge of the scale equal to the 
length of the fish (see Fig. 10) then the distances to the different winter rings will imme- 
diately show the size of the fish during each winter of its life. 
In order to avoid the arduous work of drawing the scales thus enlarged, a labour- 
saving method is employed, as follows: With the aid of a prism, the microscopic picture 
is thrown on to a piece of paper on the table beside the microscope. On this picture, 
a slip of paper is laid, upon which are marked off the different distances from the centre 
