BULLETIN OF THE BUREAU OF FISHERIES 
Applying this method of calculation to the number of cod actually caught, 
tagged, or recaptured by the tagging vessels on Nantucket Shoals as given in Table 
32, the following interpretation might be made: 
Table 32. — The ratio of marked to unmarked cod on Nantucket Shoals, as found by the tagging ves- 
sels, together with the estimated number of marked fish that on the average, were available for recap 
lure there 1 
Year 
Approxi- 
mate catch 
of cod 
Estimated 
Average 
of marked 
cod avail- 
able for 
recapture 
Marked cod 
recaptured 
by tagging 
vessels 
Ratio of 
marked to 
unmarked 
fish 
1923 
Number 
8, 100 
3, 400 
4,400 
1,850 
5, 500 
1,050 
700 
Number 
3, 750 
1,550 
2,050 
800 
2,850 
500 
325 
Number 
32 
38 
26 
10 
53 
19 
7 
1:256 
1:89 
1:170 
1:185 
1:104 
1:55 
1:100 
1924 
1925 
1926 _ 
1927 
1928 
1929 
3, 571 
1,089 
27 
1:132 
1 The number of cod caught by the tagging vessels includes the injured fish as well as those utilized for tagging, for it is upon 
the total catch that the ratio is based. The estimated number available for recapture, on the average, is approximately one-half 
of the actual number tagged during each year of record. The recaptures taken by the tagging vessels include tag-scarred fish as well 
as those bearing tags. 
From 1923 to 1929 an average of 3,571 cod were caught annually by the tagging 
vessels on the regular tagging ground, among which 27 12 bore tags or tag scars. 
This is a ratio of 1 marked fish to 132 that were unmarked. If an average of 1,689 
fish (see Table 32) were available for recapture on the tagging ground the popula- 
tion of this ground, during the summer at least, might be set at 1,689 X 132, or 
about 223,000. If the assumptions are correct regarding the density of fish and 
proportionate area of the tagging ground with respect to all of the shoals, then the 
total population might be estimated at about 3,000,000 cod of marketable size. 
This, however, should be looked upon as somewhere near the minimum number, 
based as it is on the supposition that of the cod present one summer on the tagging 
ground virtually none remain until the next. But not all the individual cod present 
one summer have left the shoals by the next, for although about 7 per cent of the 
marked fish were taken annually by the fishery (2.29 per cent were actually reported 
and the remainder include the estimated number of tag-scarred fish and those with 
tags which were not reported), part of them die from natural causes, while others 
emigrate to other regions. Recaptures made on the tagging ground one and even 
two years later (Table 23) show that part of the cod either remain for that length of 
time or reappear there. 
What proportion of the fish remain on the tagging ground from one year to the 
next is not known, and any attempt to determine this by calculations based on recap- 
tures made one or two years after marking by the tagging vessels would be subject 
to error due to loss of tags. However, the returns of tagged cod give at least a mini- 
mum idea of the carry-over from one year to the next. From Table 23 it may be 
calculated that the recaptures during the second year after release average about 
50 per cent as high as during the first season. The persistence of characteristic size 
groups in frequency distributions also indicates a substantial carry-over from one 
year to the next. If we assume that there were available for recapture not only most 
12 A large part of these were recaptured 6 mouths or more after tagging, and so fit rather well with the other data. 
