MIGRATIONS OF COD 
101 
the picture there. Furthermore, in spite of certain unknown factors that must of 
necessity be involved in a calculation of this sort, the data have an unusual degree 
of reliability because they include the records of thousands of cod living under natural 
conditions during all seasons. 
Table 37. — Rate of growth of cod caught on Nantucket Shoals f rom 1924 to 1928, as determined from 
length frequencies 
Symbols on figs. 16 to 23 
Average date 
of capture 
Average 
length 
of domi- 
nant size 
group 
Average date 
of capture 
Average 
length 
of domi- 
nant size 
group 
Increase 
in length, 
inches 
Time 
interval 
in days 
Rate of 
growth 
per 
month of 
30 days 
B 
July 14,1924 
Sept.' 10, 1924 
May 6, 1925 
Tune 9, 1925 
Aug. 23,1925 
23.9 
Sept. 10,1924 
24.6 
0.7 
58 
0. 35 
B. 
24.6 
May 6, 1925 
June 9, 1925 
25.6 
1.0 
239 
.15 
C 
19.0 
19.0 
.0 
34 
.00 
C 
19.0 
Aug. 23,1925 
Oct. S, 1925 
20.2 
1. 2 
75 
. 50 
c 
20.2 
21.3 
1. 1 
46 
.70 
c 
Oct. 8, 1925 
21.3 
Sept. 8, 1926 
24.4 
3.1 
335 
.30 
T> _____ 
__ _do_ 
15. 1 
do 
18.3 
3.2 
335 
.30 
D 
Sept. 8, 1926 
May 5, 1927 
18.3 
May 5, 1927 
June 20, 1927 
20.4 
2. 1 
239 
.25 
D 
20.4 
21.0 
.6 
46 
.40 
I> 
June 20,1927 
21.0 
Sept. 1, 1927 
21.5 
.5 
73 
.20 
D 
Sept. 1, 1927 
21.5 
July 17,1928 
24.0 
2.5 
319 
.25 
It was interesting to find, as might be expected, that there was a seasonal differ- 
ence in the rate of growth. Thus in Table 37 the fish included in the two spring rec- 
ords (0.00 and 0.40) averaged 0.20 inch of growth per month, in the four summer rec- 
ords (0.35, 0.50, 0.70, 0.20) 0.44 inch, in the two fall to spring records (0.15, 0.25) 0.20 
inch, and in the three records which embraced nearly a year’s time (0.30, 0.30, 0.25) 
0.28 inch. Accordingly, these Nantucket Shoals cod made their slowest growth from 
the fall to spring and their fastest during the summer. 
At an average rate of growth of 0.28 inch per month the growth per year would be 
about 3.4 inches. Making allowances for somewhat faster and slower growth, it 
might be said that, based on the length-frequency method of determination, cod from 
about 15 to 26 inches long living on Nantucket Shoals increase in length about 2 % 
to 4 inches a year. 
EVIDENCE FROM TAGGED FISH 
Our records of growth made by recaptured tagged cod have yielded perhaps the 
most dependable information, for they are based more on fact than on theory. Of 
course some degree of error may obtain even here, for we can not be sure that in all 
cases the growth of a tagged fish was the same as it would have been if the fish had 
never been tagged. The suppuration which often occurs around the point where the 
tag is attached to a fish has already been described. It is probable that in a case of 
excessive irritation normal growth is curtailed ; in fact, we have a few instances where 
cod recaptured a year or so after tagging, in poor condition, had gained scarcely any- 
thing in length. But eliminating such records from our calculations and considering 
only the fish that were in reasonably good to fine condition when recaptured, we are 
justified in accepting the growths as being almost the same as they would have been 
if the fish had not been tagged. 
The growth records of tagged cod obtained by the tagging vessels Halcyon and 
Albatross II and those furnished by fishermen were at first separated in order to 
determine whether the results varied appreciably, for while all of our recaptures were 
measured by the same standard and usually by the same person, those of the fishermen 
may have been measured by a number of different methods. It was found, however, 
that the two groups of data agreed very well, and, therefore, the records were com- 
