MIGRATIONS OF COD 
9 
was a better chance to get a true picture of seasonal fluctuations in abundance and 
size, which would reflect any migrations that might take place. 
To insure that only fish in good condition were tagged, most of them were caught 
with ordinary hand lines of a kind in general use by commercial fishermen along the 
New England coast (fig. 1), for in this way only a small percentage were lost through 
injury. The use of the otter trawl was prohibited not only by the uneven and rocky 
bottoms fished upon but because a large proportion of net-caught fish are crushed 
or drowned by the time they are hauled out of the water. A few hundred of the 
fish tagged were caught on long lines, or trawl lines. 
The most productive fishing was found in depths of less than 50 fathoms. Greater 
depths were generally avoided, for the fish taken there often are “poke blown”; 
that is, the sesopliagus forced into the mouth due to the sudden change of pressure. 
Some idea of the losses may be had from the following: In less than 25 fathoms 
37,929 cod were caught, of which 8.1 per cent were not suitable for tagging; between 
25 and 40 fathoms, 2,730 were caught with an 8 per cent loss; and above 40 fathoms, 
2,089 were caught with a loss of 17 per cent. 
Frozen herring (Clupea) was used for bait almost exclusively up to 1925, but 
after then squid and other baits were used in addition. The herring proved to be 
the best all-around bait off the New England coast, but off New York and New 
Jersey conchs (Lunatia), surf clams (Mactra), and soft clams (Mya) were found to 
be much the best. 
Immediately after its capture the fish was laid on a measuring board, its length 
recorded, a tag clamped to the tail (see fig. 2), a few scales scraped from the side, 
and then returned to the water. These operations usually required from 10 to 15 
seconds for each fish. Beginning in October, 1927, certain fish were tagged on the 
head, chiefly those measuring less than 30 inches in length. 
The desideration has always been, from the time of Fulton’s first experiments 
(1890, p. 354) in marking cod up to those of Graham (1929c), that the type of tag 
should be one that would remain on the fish for a reasonably long while, that would 
cause no injury, and that would be sufficiently conspicuous to the fishermen. The 
tag adopted for use in the present cod investigation had been used successfully by 
Dr. Charles H. Gilbert in marking Pacific salmon and is similar to the type com- 
monly used for ear-marking cattle but smaller, the length being 2 % inches when the 
tag is extended. These tags (fig. 3) were easily attached to the tail of the fish by 
means of a clamp. A tag in place is shown in Figure 4. 
It was necessary that the metal used for the tags withstand the chemical action 
by sea water for a long period and, after experimentation with various sorts, elim- 
inating silver as too costly, monel metal was finally adopted as the most satisfactory. 
Kesults obtained from this experimentation are given in Table 3. 
Table 3. — The metals used and the number of tags returned to date for each as a result of the lagging on 
Nantucket Shoals during 1923 
Metal 
Tags used 
Tags re- 
turned 
Percentage 
Silver ___ . 
1,000 
47 
4. 70 
Aluminum __ __ _ 
2, 623 
700 
102 
3. 90 
Copper 
35 
5.00 
4. 66 
Silver-plated copper. . ..... . __ . _. 
300 
14 
MoneL .. ... ... ... .. .. ... .. . 
5,621 
134 
2. 38 
