PROGRESS IN BIOLOGICAL INQUIRIES, 1926 601 
I think, perhaps, that this is sufficient introduction to the lines of attack. 
I might say that we also begin to see ahead the possibility of predicting whether, 
in any year, the stock of fish is going to be big or small—in the case of the 
mackerel, for instance, whether it is going to be worth while to go mackerel 
fishing or not. For years the mackerel fishery has been subject to tremendous 
fluctuations, the catch varying from practically none to hundreds of millions 
of pounds. Until these investigations started there was no way of envisaging 
at all whether these fluctuations reflected movements of the fish or whether they 
reflected actual fluctuations in the number of mackerel existing in the sea at 
the time. We are now in position to say that they do reflect actual fluctuations 
in the numbers of mackerel. 
COD STUDIES 
By W. C. SCHROEDER 
My problem is concerned with the life histories of the cod, pollock, and had- 
dock. The three species are found under much the same conditions, so that 
their combined study is practicable; but special attention has been given to 
the cod, and the data for this species are more complete than for the others. 
These are important food fishes, found on both sides of the North Atlantic, 
and are worth over $5,000,000 annually to our fisherman. 
Migrations, rate of growth, and age with respect to size are the chief factors 
under consideration. Doctor and Mrs. Fish have been studying the drift of 
the eggs and larve, while I am interested chiefly in fish upward of 114 inches 
in length. It is at about this size that the cod takes to the bottom and the 
first scales appear. 
We know very little about cod less than 10 inches in length or less than 2 
years of age, and this is one of the gaps we are endeavoring to fill. Fish 
of this size appear to be scarce along our immediate shores, for repeated seining 
around Woods Hole and along the entire New England coast has produced but 
very few specimens. Offshore we have done a limited amount of dragging 
with beam trawls, otter trawls, and circular nets; but on smooth, sandy bottom 
we catch next to nothing, and on rocky bottom the fishing gear is nearly always 
damaged or lost. Last summer we were fortunate in making several good 
hauls on northeast and southwest Georges Bank, where the bottom was some- 
what rocky, yet not enough so to destroy the gear. Among miscellaneous fishes, 
about a dozen cod and many haddock, 2 to 5 inches long, were caught. It 
seems logical to conclude, even on this meager evidence, that the habitat of 
very young cod includes all our offshore cod grounds. In changing from the 
pelagic to the demersal stage, those fish that happen to go down on or near 
rocky bottom ought to have a much better chance of survival than those that 
chance upon mud or sand, where they afford an easy prey for carnivorous 
fishes. 
The smallest caught along our shores are usually more than 10 inches in 
length. The shore waters of Maine form a refuge for cod of 10 to 20 inches, 
and fish of these lengths are so abundant there that some sort of migration 
from offshore evidently takes place. 
In an attempt to throw light on migrations, 41,000 fish were tagged from 
1923 to 1926, of which 28,000 were cod, 5,000 were pollock, and 8,000 were had- 
dock. Operations were carried out by the Halcyon up to 1925 and by the 
Albatross II during 1926. The fishing grounds comprised many localities from 
eastern Maine to southern Massachusetts, but most of the fishing has been 
centered off Mount Desert and on Nantucket Shoals. 
All these fish were caught with hand lines and by the same method of fishing 
as is employed by commercial vessels. Generally we fished in water less than 
25 fathoms in depth, for beyond this depth an increasing number of fish are 
unsuitable for tagging, owing to their sudden removal from the great pressure 
which exists on the bottom. In shallow water we lose from 5 to 10 per cent 
of our fish, due to injuries received when they are hooked, and in water as 
deep as 40 to 50 fathoms the losses due to pressure and hooking combined 
amount to from 15 to 20 per cent. 
After a fish has been landed it is laid on a measuring board and a tag is 
clamped to the upper part of its tail near the base. A few scales are scraped 
from along the side below the first or second dorsal fin and the fish is then 
returned to the water. These cperations require about 20 seconds per fish. 
