MIGRATIONS OF COD 
107 
the minor fluctuations in growth, circulus compared with circulus, did not appear to 
correspond. 
The first zone of growth on the cod’s scales presents difficulties, for it is often 
hard to calculate whether it represents the first full year of growth or only part of a 
year. Scales first appear when cod fry are about lj /2 inches (38 millimeters) long, 
about six to eight weeks after hatching and at about the time they take to the bottom. 
As cod larvae hatch from fall to late spring, it is possible that the formation of the 
first “annulus” might be completed on the scales of some fish when they are only 6 
or 8 months old, while others may be as old as 12 or 14 months. 
It is assumed that the widely spaced circuli which marks the beginning of the 
second year’s growth begin to form the first spring following the fall to spring that 
the fish was hatched. The demarcation between the first annulus and beginning of 
the second zone of growth is generally sharp on the scales of cod living off our coast, 
but is not so clear on the scales of some European fish. Graham (1926, p. 346), 
studying North Sea cod, measured the distance between circuli on the cod scale very 
much as did Winge (1915) in order to determine the limit of the first “winter” zone. 
He gives his technique (ibid., p. 351) as follows: 
The width of the narrowest pair of adjacent sclerites in the innermost suspected narrow zone 
is taken on dividers and fitted to the width of the widest sclerite in the adjacent wide zone outside 
it. If the dividers fall within it the narrow zone is the first “winter” ring. If they span or straddle 
the wide sclerite the criterion rejects the suspected narrow zone. 
Graham did his measuring directly on the projected scale, magnified about 100 
diameters. While this method apparently is helpful in identifying a secondary 
minimum within the first zone of growth, it, of course, can not determine whether 
the 15 or 20 circuli within the first zone represents 6, 8, or 12 months of growth. 
While the trend of growth should be the same on all typical scales found on the 
same fish, there is considerable variation in the number of circuli, depending on what 
part of the body the scale is found, as already pointed out by various investigators. 
The small scales along the back, near the head, or on the belly do not have as many 
circuli as the large scales along the side. And even two scales lying almost side by 
side may vary somewhat in the number of their circuli. For example, Winge (1915, 
p. 6) found on a fish with two distinct minima on all of its scales that a large scale 
from between the lateral line and the second dorsal had 48 circuli, a scale from the 
base of the pectoral had 45, and a scale from the dorsal area, obliquely in the rear of 
the eyes, had 32. Because of this variation, it is difficult to compare the fluctuations 
in growth as between two scales from the same fish or between scales from two fish 
of about the same size, living together, unless comparable scales having about the 
same number of circuli are selected. 
In order to gain some idea of the average number of circuli that form on the 
scales of southern Massachusetts cod during their first full year of growth, the scales 
of fish less than a year old were examined, with the following result: 
Table 39 . — Number of circuli on the scales in relation to length of juvenile fish 
Number of specimens 
Length 
Range in 
number of 
circuli 
Average 
number of 
circuli 
Number of specimens 
Length 
Range in 
number of 
circuli 
Average 
number of 
circuli 
1 ___ 
Millimeters 
21-30 
4... 
Millimeters 
71-80 
81-90 
5-8 
6. 1 
2 
31-40 
1 
1.0 
8 
5-13 
7. 5 
7. 
41-50 
1-4 
1.7 
5. 
91-100 
6-9 
7.8 
9 
51-60 
1-6 
3.4 
1 
101-110 
9-11 
10.0 
6 
61-70 
2-8 
5.0 
2 
111-120 
9-11 
10.0 
