12 
BULLETIN OF THE BUREAU OF FISHERIES 
wounds or healed scars on a cod’s body, hence the destruction of adult cod by enemies 
may be greater than we now believe and must be considered a factor of some 
importance. 
Some cod no doubt are killed by parasites and disease — their most apparent 
external parasite, which attaches itself to the gills, being Lern&a branchialis L. Cod 
living in less than 20 fathoms of water are most afflicted with this pest, but we also 
found it on cod caught in a depth of 47 fathoms on the northeast part of Georges 
Bank and in 40 fathoms on Browns Bank. Commonly three out of four cod from 
shoal water have from one to four of these bloodsucking parasites on the gills, and 
although many of these fish appear to be healthy it is possible that in time they 
become weakened and that some of them die, for as a rule the gills of thin fish are 
covered with this parasite. Sumner et al. (1913, p. 644) remark that they are “often 
so numerous as to affect the health of the fish.” Cod are commonly infested with 
other parasites both internal and external, such as nematodes and caligids. External 
cancerous growths are occasionally seen, but deaths from this cause must be very 
small, for out of about 45,000 cod caught only 1 or 2 fish were afflicted with growths 
of this sort. About 1 fish in 1,000 of our catch has been extremely thin, while occa- 
sional fish, particularly large cod, may be weak and emaciated. 
Considering that most of the cod which were tagged on Nantucket Shoals were 
neither very small nor very large, that almost all of them were sound and healthy, 
and that they were not unduly afflicted with parasites, the number of marked fish 
which died from old age, enemies other than man, parasites, and disease probably was 
not more than 10 to 12 per cent during the first year after tagging. The percentage 
would increase each year thereafter as the same stock of fish became older. 
This is an arbitrary percentage, but the proportion of larger and therefore older 
fish, actually found among representative stocks of cod suggests that it can not be 
much too small or much too large. 
Percentage oj fish which lose their tags within the first year. — The fact that many 
fish have been caught showing tag marks 7 or the scar on the side where the scale 
sample had been taken proves that a considerable percentage of the tagged fish lose 
their tags while at liberty. This was to be expected, for in the North Sea experiments 
it was also found that some of the cod lost their tags. Concerning those marked on 
the operculum, Graham (1924, p. 51) writes: 
If the mark was tight the skin and flesh rotted under the button until the button almost fell 
out in the worst specimens I have, and no doubt did fall out in others which were, consequently, 
not returned. If the mark was loose the wire gradually worked the hole larger and larger. Some, 
however, have been returned in a perfectly healthy condition. In these the mark seemed to be 
just firm, neither tight nor loose, a condition hard to achieve in practice. 
Graham mentions that a new tag designed to minimize weight and resistance 
to water was tried later but proved a failure. 
The loss of the tag from its place of attachment requires a certain period of 
time, depending upon the exact point where it is fastened and upon the thickness of 
the tail. Cod recaptured one week from the time of tagging show practically no sore 
around the tag. One month later soreness has set in, but usually there is no evident 
sign that the tag will soon be lost. Three to four months later, on some fish the flesh 
is in good condition around the tag, on some suppuration has occurred while others 
already have lost their tags. About one year later the condition of the tagged fish 
7 By “tag mark” is meant the fresh wound, or healed scar, left when the tag has been lost by “eating” its way through the 
skin and flesh of the tail. 
