104 
BULLETIN OF THE BUREAU OF FISHERIES 
The scales of cod afford perhaps the most ready means for determining age and 
rate of growth. Up to the sixth or seventh year they are reasonably dependable, 
but beyond this age they become increasingly difficult to interpret. Occasionally a 
very old fish has remarkably well-defined scales, and such fish assist in placing the more 
doubtful ones in their approximate year class. 
Not only has the scale method of age determination for the cod been verified by 
tagged fish, but scales have been compared with otoliths and skeletal structures. 
Winge (1915, p. 19) found that the number of growth zones laid down on the otoliths 
agreed very well with the number of winter or slow-growing zones on the scale. The 
oldest cod examined by Winge was 13% years old according to its scales, compared 
with a determination of about 14% years according to its otoliths. Cunningham 
(1905, pp. 137-139), working with rather young cod, found that the number of annual 
zones laid down on the scales and on the otoliths was the same. He also utilized the 
pectoral girdle and the vertebrae, but found these skeletal structures to be untrust- 
worthy as a means of determining age. Saemundsson (1923, p. 8-7) used otoliths, 
the coracoid, and the pelvic bone in determining the age of Icelandic cod and was 
able to check the age of comparatively young otoliths with the scales. Graham 
(1929a, Pt. I, p. 42) concludes that by using the precise method of making scale 
tracings (Graham, 1928) the majority of cod scales will give a correct age reading. 
He found, too, that otoliths showed some degree of correspondence with the scales. 
Typical cod scales under magnification somewhat resemble a thumb print. 
They are usually oval in shape and are marked with concentric rings, or circuli, the 
first one of which is generally offcenter, away from the pigmented part of the scale. 
The numerous circuli form growth zones, each of which, with the exception of possibly 
the first growth zone which may have all its circuli about equally spaced, is divided 
into two parts — one composed of widely spaced circuli, the result of rapid growth, 
and the other of closely spaced circuli formed during a period of relatively slower 
growth. (Fig. 29.) The wide and the narrow circuli in each zone, when taken 
together, are believed to mark about one year of growth. Winge (1915, p. 10, figs. 
5 a and 5b) shows by means of tagged Faroe cod, which were recaptured one to two 
years later, that a “minimum,” or annulus, is formed during the 'winter. That 
one annulus forms each year has been found on the present investigation, too, for 
those tagged fish recaptured a year later had the additional year of growth registered 
on their scales. (Fig. 30.) 
It is the seasonal variation in growth registered on the scales that permits the 
age of the fish to be calculated by this means. And as there may be regional differ- 
ences in growth dependent upon the physical and biological conditions of the fish’s 
immediate environment, each cod region that differs appreciably from another in 
temperature, food supply, etc., offers a separate problem with respect to the interpre- 
tation of the growth zones on the scales. 
Winge (1915, p. 12) found that cod from the Faroes grew more rapidly and laid 
down widely spaced circuli on their scales in summer and closely spaced circuli in 
winter. It was his opinion that cod in other localities probably do the same. 
Saemundsson (1923, p. 27), who worked with the cod around Iceland, found, according 
to the scales and otoliths, that the most rapid growth took place on the south coast, 
with a gradual decrease as one goes to the right around the island. The slowest growth 
was found on the east coast. At Arendal, Norway, Dannevig (1925, p. 21), who 
experimented with young cod in a rearing pond which had very much the same char- 
acteristics as the sea which it adjoined, raised some of the fry to an age of 2% years 
