98 
BULLETIN OF THE BUREAU OF FISHERIES 
Duff (1929, p. 16), who studied Sable Island (Nova Scotia) cod, concluded that 
adult fish in this region reach their maximum rate of growth during May, June, and 
July, and their minimum during January and February. 
Whether it be temperature or food that is the more important factor in bringing 
about fluctuations in the rate of growth of cod, it has been found that, according to 
the scales, a period of rapid and of slow growth occurs each year, alternating through- 
out the life of the fish. 
EVIDENCE FROM LENGTH FREQUENCIES 
The first attempts at determining the age of cod were based upon length fre- 
quencies. (Hjort, 1914, p. 121.) In discussing the length-frequency method of age 
determination, first applied to fish by Peterson (1892), it is pointed out by Dahl 
(1909, p. 759) that this method is workable as a rule only up to the third year. He 
says further (ibid.): 
The method in fact rests on the supposition, that the start in size which the fry of one year 
possesses compared to the fry resulting from next year’s spawning, that this start in size is retained 
also during subsequent years. 
To a certain extent this holds good, where the spawning season is short, and where growth is 
uniform. But experience shows that a long spawning season, unequal growth in different years and 
different localities, besides active and passive migrations, combine to blot out the “annual groups” in 
most species after the lapse of very few years. After the lapse of even one year the single individuals 
of a year group could not in all cases be recognized as belonging to a certain group and after a lapse 
of a few years a recognition of the year classes even as groups became almost impossible. 
It would seem that the cod falls in that category, which makes it difficult to 
determine age classes by the length-frequency method for the spawning season is long, 
the fry carry out passive and many of the adults active migrations, and there is a 
regional variation in the rate of growth. But in spite of these difficulties year classes 
up to the third, and in some cases even to the fifth, may be recognized, provided fair- 
sized samples of fish are measured from each locality that is selected for study. 
In European waters Graham (1926, p. 24) found that North Sea cod fry averaged 
about 3.6 centimeters early in July, 4.8 centimeters early in August, and 7.9 centi- 
meters late in September. 
Dannevig (1925, p. 10) found the average size of cod seined near Arendal, 
Norway, on the Skagerrack, to be 8 to 12 centimeters in October. Fry of these sizes 
were placed in a rearing pond and attained a length of about 15 centimeters (6 inches) 
by the following April when presumably about 1 year old. 
Off the east coast of Scotland, Fulton (1901, p. 227) found that cod hatched 
around April were 4% inches long by November, 5 / inches by December, and 5% 
inches by January. 
Off the east coast of England, Wallace (1923, p. 17) found that the lengths of 
yearling cod ranged from 3 to 7 centimeters (1% to 2% inches) in July and from 5 to 
14 centimeters (2 to 5 % inches) in October, but the number of cod so taken were too 
few to form dependable modes. 
In the Irish Sea, Johnstone et al. (1924, p. 8) report catches of young cod taken 
by a prawn trawler, as follows: August, 137 fish, 4 to 19 centimeters, average 8.1 
centimeters (3.19 inches); October, 64 fish, 9 to 19 centimeters, average 13.56 centi- 
meters (5.34 inches); November and December, 48 fish, 9 to 19 centimeters, average 
14.23 centimeters, (5.60 inches). 
Off the east coast of Iceland, where the bottom water temperature is around 0° C. 
a good part of the year and where cod live from the fry stage until they are several 
