MIGRATIONS OF COD 
77 
THE SUMMER MIGRATION 
Spawning . — A spawning immigration, bringing foreign cod to the Chatham-South 
Channel region during the winter, may explain why the commercial catch holds up 
so well at that season. But this can not explain the summer movement of cod 
thither from Nantucket Shoals, because spawning in any amount is confined to the 
autumn, winter, and early spring. And we have no evidence from recaptures of any 
migration of cod eastward or northward from Nantucket Shoals during the spawning 
season. 
Food .- — Except in cases where cod could be observed following bait, such as sand 
eels (Ammodytes) or young herring (Clupea), it is difficult to ascertain what effect 
regional or seasonal variations in their food supply may have on their migrations. It 
has been observed that the bulk of the cod’s food on Nantucket Shoals and on the 
Chatham grounds consists of large crustaceans. No cod stomachs have been exam- 
ined in South Channel, but, being primarily a haddock ground, we can reasonably 
assume that the food supply as a whole is less attractive to the cod there than on the 
Chatham grounds or Nantucket Shoals. Two hauls with a fine-meshed shrimp trawl 
made in South Channel by the Albatross II on June 13, 1929, caught very little cod 
food such as crabs, medium-sized mollusks, worms, etc., but considerably more sam- 
pling must be done before a good picture of the food supply and bottom in the channel 
can be obtained. 
If it should be proven that the food on the Chatham grounds is more attractive 
to the cod than the food in South Channel, it might explain why the former locality 
has yielded a greater portion of tagged Nantucket cod than has the channel region. 
But it would not necessarily prove that Nantucket cod migrate to the Chatham 
grounds primarily in search of better feeding grounds. On the contrary, nearly all 
the cod stomachs examined on Nantucket Shoals held a large amount of food, or at 
least as much as the stomachs of fish caught on various other banks, and nearly all 
the fish caught by us were fat and healthy. Equally, if food were scarce on the Chat- 
ham grounds few fish would live there and the region would have yielded a much 
smaller number of tagged Nantucket cod than was actually the case. So while it is 
apparent that the Chatham grounds ranks about the same as Nantucket Shoals in 
having sufficient cod food for holding bodies of fish the year around, yet there is no 
basis for believing that the food supply affords the chief stimulus for an intermigra- 
tion between the two grounds. 
Temperature . — There is no cod ground off our coast that has the peculiar temper- 
ature variations that obtain to the southeastward of Cape Cod, for there, on Nan- 
tucket Shoals, a close similarity exists between the surface and bottom temperatures, 
due to the tidal currents sweeping over its uneven bottom contour and thus stirring 
up the water, while, in contrast to this, the waters in the Chatham-South Channel 
region are stratified as to temperature, except for very brief periods in the spring and 
the fall. Temperature, therefore, appears to offer a hopeful field of investigation for 
determining the cause of such migrations of cod as occur between these two regions, 
particularly as these have taken place, according to our tagged fish, only during the 
summer. 
