72 
BULLETIN OF THE BUREAU OF FISHERIES 
summer species just mentioned. It appears evident, therefore, that competition for 
food between the cod of Nantucket Shoals and other species of fish has little or 
nothing to do with causing them to migrate westward from the shoals in the fall or 
in limiting their stay on the wintering grounds. 
Enemies . — Enemies in the form of other fishes do not drive cod from Nantucket 
Shoals. Its only important and widespread enemy there is the dogfish (Squalus), 
but although the dogfish migrate southward in great hordes from the Gulf of Maine 
to at least the Chesapeake Capes, the first of them appear off New York or New 
Jersey about a week before the cod, not after them or with them, and they pass on 
and are not seen again, except for a straggler, anywhere on the cod grounds from 
Nantucket Shoals to Delaware until the following April when the water has warmed 
to about 5.5° C. (42° F.). Although both dogfish and cod may migrate westward 
from Nantucket Shoals at the same time during part of November, cod continue to 
leave the shoals until well into December, long after the dogfish have passed. 
Salinity . — No exhaustive attempt has been made here to correlate the presence 
and abundance of cod with the salinity of the water. On grounds where cod are 
present the year around the following salinity data have been taken from Bigelow 
(1927, p. 815-19): Below 150 meters (about 80 fathoms) the salinity fluctuates only 
about 0.5 per mille throughout the year and ranges around 33.7 to 34.2. At depths 
of 100 to 150 meters (about 55 to 80 fathoms), in the coastal zone between Cape Cod 
and Cape Sable, the variation runs from about 32.38 to 34.11, according to depth, 
locality, and date. In the 40 to 100 meter zone (about 22 to 55 fathoms), which 
includes most of the cod grounds off our coast, the range in salinity throughout the 
year is about 31.8 to 33.2 per mille. 
On Nantucket Shoals there appears to be very little fluctuation in the salinity 
during the summer, autumn, and winter, when, at 20 to 40 meters (11 to 22 fathoms), 
it probably is around 32 to 32.5 per mille, while in the spring it is only slightly lower. 
An indication that cod are not usually influenced to migrate by ordinary changes 
in salinity may be had from our tagging experiments in the immediate shore waters 
of Maine which showed that many of the cod remained localized from one year to the 
next, although the water freshens there in the spring more than it does offshore. In 
line with this, Needier (1929, p. 9) found that in the Gulf of St. Lawrence cod are 
often caught in a salinity around 30 per mille, which is fresher than that found on 
bottom on the banks off New England. 
Therefore there is no reason to believe that fluctuations in salinity cause cod 
living along the New England coast, particularly those on Nantucket Shoals, to 
make extensive migrations. 
Temperature . — One of the striking things about the migration of cod from 
southern Massachusetts into the Rhode Island-North Carolina region is that it begins 
each year in October. For this reason it would appear that a falling water tempera- 
ture was the stimulus which sent the fish on their journey. 
As Nantucket Shoals is the most southerly year-around cod ground along our 
coast, we might reasonably expect that seasonal differences in water temperature 
would have more influence on the migrations of the cod living there than would be 
the case in an intermediate region where the extremes of temperature would not be 
as great. However, an examination of the data for the Nan tucket-Delaware region 
is not so reassuring. 
Temperatures taken on Nantucket Shoals are given in Table 31. In general it 
can be said that the water there reaches a maximum of about 11° to 15.5° C. (52° 
