76 
BULLETIN OF THE BUREAU OF FISHERIES 
middle of April the majority of the cod have left the New York-New Jersey region, 
although at that time the water there is still much cooler than it is in the fall when 
the first cod arrive. A comparison of the Cholera Bank temperatures obtained in 
November with those taken off Cape May in mid-April will illustrate this (reference 
numbers 42 to 47 and 57 and 58). A further example is furnished by our result's off 
Atlantic City, N. J., in 1928, when on April 1 it was 4° C. (39° F.) in 7 fathoms 
where cod were being caught and only 5.5° C. (42° F.) in the same place on April 13, 
by which time most of the cod had departed (as noted by the almost daily catches of 
the fishermen). 
Even in May the water is still comparatively cool along the New Jersey shore, 
for during the middle of that month temperatures of 4.7° to 7.3° C. (40.5° to 45.1° F.) 
were found off the coast about 10 to 60 miles, while off Long Island, late in the month, 
a reading of 5° C. (41° F.) was obtained. Further eastward, in the vicinity of No 
Mans Land, the temperature in 16 fathoms taken late in the month was 7.7° C. 
(45.8° F.) (reference numbers 1, 30 to 32, 61 to 64, Table 30). 
In all these cases the water temperature was well below the maximum which 
obtains on Nantucket Shoals and on many of the cod grounds off New England 
during the summer, so it is apparent that cod leave the region west of Rhode Island in 
the spring at least two months before the bottom water approaches the warmth that 
exists on their summering grounds on Nantucket Shoals. 
During the summer, although there are virtually no cod caught west of Rhode 
Island, there are places which are presumably good cod ground where the tempera- 
ture is as low and even lower than that on Nantucket Shoals. For instance, in the 
region 40 to 112 miles southeast by east from New York City (reference numbers 35 
to 39) and 40 to 66 miles east of Cape May Light N. J. (reference numbers 68 and 69) 
temperatures ranging from 4.5° to 8.1° C. (40.1° to 46.6° F.) prevailed in August, 1916. 
The fact remains, however, that during the summer a large proportion of the best cod 
ground to the westward of Nantucket Shoals is covered by water, the temperature of 
which approaches or exceeds the maximum ordinarily tolerated by cod. 
Neither the spawning instinct, the availability of food, changes in salinity, nor 
the presence of enemies appear, therefore, to be the cause of the annual migration of 
cod from southern Massachusetts into the Rhode Island-North Carolina region. 
Our present knowledge indicates that ordinarily cod tend to spread and occupy all 
suitable grounds unless prevented by depth or temperature barriers. As no depth 
barrier exists between Nantucket Shoals and the grounds to the westward, it would 
seem that temperature is the more direct cause, particularly as the migration is 
seasonal and the departure of cod from the shoals begins each fall when the water 
commences to cool. This assumption does not cover the return migration in the 
spring quite as well, because the grounds are vacated well in advance of temperatures 
high enough to constitute a barrier, as judged by the degree of warmth tolerated by 
cod on Nantucket Shoals in the summer. Nor does it explain why there is not an 
extensive spread of Nantucket Shoals cod to the north and east. 
