80 
BULLETIN OF THE BUREAU OF FISHERIES 
the three years from 1926 to 1928, when fewer fish emigrated, the following result is 
obtained: 
From 1923 to 1925 the weighted mean length of the 14,629 cod tagged on Nan- 
tucket Shoals was 26.6 inches. Recaptures of these fish reported from the Chatham- 
South Channel region throughout the same period amount to 83 fish, or 0.56 per 
cent of the total number tagged. (See Table 28 for length distributions.) From 
1926 to 1928 the weighted mean length of the 7,599 cod tagged on the shoals was 
22.43 inches. Recaptures of these fish reported from the Chatham-South Channel 
region throughout the same period amount to 7 fish, or 0.09 per cent of the total 
number tagged. 
Although the percentages of recapture in both cases are very small, they are 
based on a large number of fish and involve a mean period of about one and one-half 
years in each case; hence are significant. Of chief importance is the fact that the 
proportion of cod which are known to have made this journey during 1923-1925, when 
their average size was 26.6 inches, was about six times as great as during 1926-1928, 
when the average size was only 22.4 inches. That this difference in the percentage of 
recaptures was not due to a corresponding difference in fishing intensity for the two 
3-year periods has been pointed out on page 38. 
Further evidence showing that there is a tendency for the larger Nantucket cod 
rather than the smaller to emigrate to the Chatham-South Channel region may be had 
from the following data: Out of 37 cod tagged on the shoals in 1924 and recaptured 
there in 1924 and 1925, 21 fish were 25 inches long or less, while 16 were 26 inches or 
more at the time they were tagged; the average size was 25.3 inches. Out of 18 cod 
tagged on the shoals in 1924 and recaptured in the Chatham-South Channel region in 
1924 and 1925, 5 fish were 25 inches long or less, while 13 were 26 inches or more; 
the average size was 27.7 inches. Of the cod tagged on the shoals in 1925 good rec- 
ords were obtained for 25 local recaptures, and 15 of these fish were 25 inches long or 
less at the time they were tagged and 10 were 26 inches or more; the average size was 
24.5 inches. In contrast to this, of 17 cod tagged on the shoals in 1925 and recap- 
tured in the Chatham-South Channel region that same year, 5 were 25 inches long or 
less, while 12 were 26 inches or more; the average length was 27.4 inches. 
According to these results, when Nantucket cod average upward of about 26 
inches in length a larger proportion of them immigrate eastward into the cooler water 
of the Chatham-South Channel region than when the fish average smaller than this, 
and when in addition the summer is a warm one, as in 1925, it would seem that opti- 
mum conditions for this immigration prevail. 
SIZE OF THE COD POPULATION ON NANTUCKET SHOALS 
One of the most desirable results that can come from an investigation of this sort 
is a knowledge of the size of the cod population in the locality under consideration. 
Fortunately, for the Nantucket Shoals region we have obtained what seems to be 
sufficient data to give some idea of the general order of magnitude of the stock of 
grown cod that were present there from 1923 to 1928. In order to make such an 
estimate, there are, of course, both known and unknown factors that must be dealt 
with. Under the known we have the number of cod both tagged and recaptured by 
>ur own vessels, while under the unknown there are the reductions in the numbers of 
