38 
BULLETIN OF THE BUREAU OF FISHERIES 
extended over a period of six years, from 1923 to 1928, and the results were very 
much the same during each of these years. 
It will be noted in Table 20 that nearly all of the recaptures made in the Chatham- 
South Channel region during 1924 were of fish tagged on Nantucket Shoals in 1923. 
This suggests that many of the 1923 cod emigrated eastward the spring of 1924, for 
only two of the cod tagged on the shoals in 1924 were retaken to the eastward that 
same year, probably because our first tagging was done so late (July) in the season. 
Conditions seemed to be right for a large return of tagged cod in 1925 because we 
had marked a large number in 1923 and in the summer and fall of 1924. Some of 
these were still present on Nantucket Shoals the spring of 1925, and many fish were 
tagged that year as early as April and May. Thus, there probably were more 
tagged cod present on the shoals in May, 1925, than during any other period from 
1923 to 1928. But although this may partly explain the large return of tags from 
the Chatham-South Channel region in 1925, the same line of reasoning can not 
explain the paucity of recaptures from 1926 to 1928. 
No obvious cause for the great difference in the numbers of tags reported during 
these two 3-year periods has been detected. So far as the yield of the fishery is 
concerned, the catches made during 1923 to 1925 were actually smaller than during 
1926 to 1928. This being so, it is evident that the difference in the 3ueld of tags is 
due not to fishing intensity but to a corresponding difference in the numbers of fish 
which took part in the migration from the one region to the other. 
It is not fully understood why so many tagged Nantucket cod migrated to the 
Chatham-South Channel region during 1923 to 1925 as compared with the following 
three years, but there is some indication that temperature, together with the size 
of the fish which made up the adult population on Nantucket Shoals, was a con- 
tributing cause. For example, it is rather well known that large cod tend to work 
then’ way into deep water and that they are more susceptible to environmental 
changes than are small cod. Inasmuch as mam r of the cod on the shoals in 1923- 
1925 were upward of 26 to 28 inches long, and very few fish so large were present 
there during the next three years, it is not at all unlikely that a large part of the 
former sought the deeper waters of the Chatham-South Channel region. The fact 
that the summer of 1925, when the greatest number of recaptures was made in the 
Chatham-South Channel region, was the warmest of the six years makes this all 
the more likely. 
Our experience has been that the cod living on the Chatham grounds and on 
Nantucket Shoals carry out very much the same migratory schedule, for from both 
regions some of the fish move to the westward to spend the winter, while others 
straggle to the northward. But, unfortunately, the number of cod tagged on the 
Chatham grounds has been too small to throw any light on the question of an inter- 
migration between there and the Nantucket grounds. The decided predominance 
of the summer recaptures just mentioned seems to indicate that of the Nantucket 
cod which summer in the Chatham-South Channel region very few remain to spend 
the winter, but what part of them return westward to the shoals to join the migra- 
tion into the Rhode Island-North Carolina region and what part go north is not 
known. 
The number of cod which emigrated from Nantucket to the Chatham-South 
Channel region was not sufficiently large to make a marked impression on the tag- 
ging data of the shoals. We found, for example, that even during 1923 to 1925 
