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BULLETIN OF THE BUREAU OF FISHERIES 
southern extremity of its range during the summer of 1916, when there was a very 
close correspondence between the limits occupied by abundant Calanus on the shelf 
south of New York, vertically as well as horizontally, and water of 4 to 7°. With one 
exception it swarmed only in water of 6° or colder (Bigelow, 1922, p. 143, figs. 45 
to 47). 
In general it may be said that along the North American seaboard C.jinmarchicus 
is abundant and dominates the plankton only in temperatures lower than 12 to 15°, 
or where it can have ready recourse to water as cool as this by sinking or by swimming 
downward a few fathoms. The fact that in 1916 Calanus was not as definitely 
concentrated in the deeper water between Marthas Vineyard and Delaware Bay in 
November as in August, is in line with this general thesis, for the equalization in 
its vertical distribution corresponds to the vertical equalization of temperature 
(and of salinity) which takes place there in autumn; and it suggested that “the 
failure of the southern Calanus swarm to migrate to the surface during the mid- 
summer nights, as it so often does in the Gulf of Maine and elsewhere, was due 
either to the very high surface temperature, or possibly to the very low surface 
density” (Bigelow, 1922, p. 145). With the advance of autumn both these barriers 
are weakened by surface cooling, until in winter, thanks to the vertical uniformity 
of the water, the only physical factors governing vertical migration are sunlight 
and geotropism. 
At the other extreme, while C. jinmarchicus probably can survive in the very 
lowest temperatures obtaining anywhere at sea, the isotherm of 2° has been found to 
mark approximately the lower level to its regular occurrence in the northern part of 
the Norwegian Sea (Damas, 1905). Damas and Koefoed (1907) found it more 
plentiful in the intermediate strata in the seas between Spitzbergen and Greenland 
at temperatures of 1 to 2° and upward than in the colder water below. 
It is probable that C. jinmarchicus requires a somewhat higher temperature for 
its successful reproduction. Thus the abundance of early postlarval stages in the 
Gulf of St. Lawrence during June, July, and August (Willey, 1919) suggests that 
breeding takes place there chiefly after the end of May, by which season the upper 
20 meters or so have warmed by several degrees from the winter minimum. This 
is certainly the case in the Massachusetts Bay region, where nauplii did not appear 
in any abundance in 1920 until the whole column of water, down to 70 meters, was 
warmer than 2.7° and the upper 25 meters warmer than 4.5°. 
The relationship between the breeding range of this copepod and temperature 
is similar around Iceland, for in spring it spawns abundantly to the south of the 
island in water of 4° and upward ; but apparently it does not do so at all to the north, 
where the temperature remains as low as 1 to 3° throughout May, though enough 
Calanus drift westward around Iceland to make this copepod extremely abundant 
along the northern coast in summer (Paulsen, 1906). As Damas and Koefoed 
(1907) have pointed out, C. jinmarchicus is therefore less Arctic in its relationship 
to temperature than is C. hyperloreus, probably finding the lower limit to its active 
reproduction at about 2 to 3° ; and the same for its rapid growth, though it is able to 
survive through long periods of lower temperature, growing slowly if at all. 
