MIGRATIONS OF COD 
67 
Nantucket Shoals at this time, evidently on their way westward, while in the fall of 
other years virtually no “foreign” cod could be recognized on the Nantucket tagging 
grounds at the time we fished there. 
3. The relationship of lengths (corroborated by tagging experiments) from year 
to year indicates that ( a ) an appreciable part of the Nantucket Shoals cod do not 
migrate westward over the winter, but remain stationary; (6) many of those Nan- 
tucket cod which do migrate westward and survive the winter return to Nantucket 
Shoals the next spring; (c) cod which migrate from such banks as Georges or Stell- 
wagen into the Rhode Island-North Carolina region pass by Nantucket Shoals, and 
in the spring such “foreign” fish return to the eastward and do not tarry on the shoals, 
else at that time they would have revealed themselves by their size distribution. 
4. Six distinct bodies of cod were found on Nantucket Shoals during our tagging 
operations from 1923 to 1929. One of these was present when we began fis hin g the 
spring of 1923; one appeared in the fall of each of the years 1923, 1924, and 1925; 
and two the fall of 1928. By the year following their first appearance each of these 
schools in turn formed a dominating size and age group. 
5. The dominant sizes of five schools of cod when they first appeared in the 
length-frequency distribution were 21 to 23 inches, 15 to 17 inches, 14 to 16 inches, 
17 to 19 inches, and 14 inches. These appeared, respectively, during the years 1923, 
1924, 1925, and the last two during 1928. 
6. Cod below 16 inches were scarce in our catches, due perhaps partly to the 
selectiveness of our hook-and-line gear, but the sudden appearance of cod as large 
as 17 to 19 inches and 21 to 23 inches, as just noted, indicates that such fish migrated 
from some other region rather than that they grew up on the shoals from the fry or 
yearling stage. 
7. The smaller cod on Nantucket Shoals are not as migratory as the larger fish. 
Cod less than about 24 inches long are apt to remain there for an extended period, 
while fish about 28 to 30 inches long and larger tend to move into deeper water. The 
scarcity of large cod on the shoals is, therefore, not due entirely to depletions caused 
by the fishery. 
8. Length frequencies have shown that fish belonging to the same group may 
average 1 or 2 inches longer on the southern part of Nantucket Shoals than on the 
northern, due probably to faster growth. 
POSSIBLE CAUSES FOR THE MIGRATIONS MADE BY SOUTHERN NEW ENGLAND COD 
What is known of other species of fish suggests that spawning, food, and tempera- 
ture are the most probable stimuli which induce cod to migrate. Unquestionably 
all cod, taken as a whole on both sides of the Atlantic, do not carry out the same 
migratory schedule. In fact, the cod is a poor example of a migratory fish, for all 
tagging experiments that have been made in the past have shown that a large part of 
the cod living on a ground one season will be found there a year later and sometimes 
longer. Obviously only mature fish carry out a spawning migration, but whether 
or not cod migrate in order to spawn depends on local circumstances such as depth, as 
it is believed that to deposit their eggs they usually seek water shoaler than 35 fathoms. 
Many fishes along our shores are present during only part of the year, making 
their appearance and disappearance regularly at certain seasons. Apparently tem- 
perature has either a direct or an indirect influence on such migrations. The cod, 
taken by and large, is not one of the “disappearing” fish, for on most of the grounds 
which it frequents it is found throughout the year. Whether or not cod shift ground 
