210 BULLETIN OF THE BUREAU OP 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.JinmarcJiicus 

 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. finrnarchicus 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.finnmrchicus 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, 0. finrnarchicus is therefore less Arctic in its relationship 

 to temperature than is C. hyperboreus, 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. 



