MIGRATIONS OF COD 
83 
of the fish marked the same year but in addition about 50 per cent of the fish that 
were marked the year before, then the summer cod population of the shoals could 
be estimated at 4,500,000. Probably the average population of the shoals lies 
between 3,000,000 and 4,500,000 cod. 
It is of course, not practicable to set a numerical value for the average number 
of cod present on Nantucket Shoals during each year of this investigation as many 
unknown factors were involved, but if the deductions just given are substantially 
sound they will give a general idea of the population’s general order of magnitude. 
It would be interesting to know what proportion of the grown fish are lost to 
Nantucket Shoals each year by deaths and emigrations, for whatever their number 
may be they seem to be replaced by other fish, thus keeping the population at some- 
where near an equilibrium. (See catches per unit of effort made during the summer, 
Table 25, p. 44.) It appears, therefore, that immigrants and small fish growing to 
market size on Nantucket Shoals are enough to maintain the stock there, so at the 
present time there is no apparent reason for believing that this ground is overfished. 
ORIGIN OF NANTUCKET SHOALS COD 
The means by which the cod population may be kept up on Nantucket Shoals or 
on any other cod ground are (a) local production, ( b ) the drifting of fry from other 
regions, (c) the immigration of bottom fry, and ( d ) the immigration of older fish. 
Any one or two of these sources may prove to be of considerably more importance 
than the others, depending on various factors, but particularly on the geographic 
location of the ground in question and on the hydrographic conditions which obtain. 
The important part played by these latter— that is, temperature and currents — - 
in the distribution of fish eggs and larvae is well known to all who have worked on 
such problems. (See Bigelow, 1926, p. 69-78.) The fact that cod eggs and larvae 
may be carried long distances from the place they were produced, has been illus- 
trated by Schmidt’s (1909, p. 22) results when he found large numbers of cod eggs 
and fry on the north and east coasts of Iceland, although spawning takes place only 
on the south and west coasts. And, as the bottom fry of the cod have been found in 
large numbers in the Baltic and in the White Sea, where adult fish seldom, if ever, 
spawn, the number of larvae transported by the currents must be vast in some 
cases, as Damas (1909, p. 127) points out. 
The same currents which carry the eggs and larvae passively along also probably 
control to a large extent the destiny of the young fry, although as these near the 
bottom stage they evidently are able to govern their vertical migrations, if not their 
horizontal. Sars (1869), in his classic account of the Norwegian cod, found that 
the first few days after hatching the larvae are kept at the surface by the yolk 
sac, but after this is absorbed they begin a more independent existence, although 
they are not able to resist the currents. Schmidt (1909, p. 20) found that the 
youngest or earliest stages are found nearest the surface and the larger ones farther 
down. M’lntosh (1897, p. 194), speaking of cod larvae in Scottish waters, states 
that by the time they are as small as one-half to three-fourths of an inch, they may 
descend considerably in the water. This general thesis has since been corroborated 
by many observers, both in Europe and in America. The smallest cod taken on 
bottom in the North Sea by Graham (1926, p. 12) were 30-52 millimeters (l%-2 inches) 
long. 
