PLANKTON OF THE GULF OF MAINE 205 



not cover its constant presence on the surface in the gulf at all hours of the day 

 in spring, contrasted with its absence from the surface by day in autumn (p. 204), 

 the illumination being about as bright at the one season as at the other. 



It is possible that temperature, combined with light, may be a factor in the 

 case — that is, Calanus may tend to sink in warm, brightly illuminated water, but 

 to rise in pale illumination, irrespective of its temperature — but until this interesting 

 subject has been studied more thoroughly I need only emphasize that the reactions 

 of Calanus in their local application to the gulf result in its being far less plentiful 

 in the surface stratum than below 10 meters or so by day, and often by night, during 

 the half of the year when the temperature is highest and the solar illumination 

 brightest. 



Horizontal hauls locate the zone of chief abundance for this copepod in the 

 gulf at 25 down to about 100 meters depth during the months of July, August, 

 and September, showing that it tends to avoid the deepest waters of the gulf in 

 summer as well as in winter and to congregate in the mid depths. I have pointed 

 out elsewhere (Bigelow, 1915, p. 290) that in the summer of 1913 much larger catches 

 of Calanus usually were made in hauls from 30 to 40 meters than from 100 to 170 

 meters at stations where we towed at three levels — surface, intermediate, and deep — 

 with the shallower catches "usually two to four times as large in bulk as the deep 

 ones, a difference too great to be charged to the difference in mouth area between 

 the 4-foot and the Helgoland nets. And this source of error was further checked 

 by occasionally alternating the two nets." The only exceptions to this rule during 

 that cruise were at three stations in the eastern half of the gulf (10093, 10097, 

 and 10100), where Calanus was about equally abundant in the deep and shallow 

 hauls and plentiful right up to the surface. Again, on July 19, 1916, a much larger 

 quantity of C. finmarchicus (upward of lj^ liters) was taken in Massachusetts 

 Bay in the 30-0 meter haul than at 83-0 meters. The next day a 40-0 meter haul 

 off Cape Cod (station 10344) yielded upward of 2,500,000 large Calanus (Bigelow, 

 1922, p. 136), 5 not to mention smaller ones, while the 8S-0 meter haul took not 

 over one-twelfth as many, estimated by their bulk (6 liters in the one case and less 

 than one-half liter in the other). 



The catches of Calanus in the open horizontal nets likewise averaged from two 

 to three times larger from above 100 meters than from greater depths during the 

 cruise of July and August, 1914; and though stations 10246, 10248, and 10254 were 

 exceptions, with several times as many Calanus and other copepods taken in tows 

 at 150 to 225 meters as at 50 to 75, it was only above 100 meters that notably large 

 catches were made (Bigelow, 1917, p. 312). 



The chief zone of abundance for C. finmarchicus in the Norwegian Sea also 

 lies above 200 meters (Damas, 1905, p. 11), with about 400 meters as its lower 

 limit. Around Iceland Paulsen (1909) found it in great abundance down to 500 

 meters; Nordgaard and J0rgensen (1905) record it as most plentiful at 200 to 300 

 meters in the Norwegian fjords in winter; and Damas and Koefoed (1907) found 

 it down to at least 1,200 meters depth between Norway, Spitzbergen, and Greenland. 



» Our largest catch of large Calanus. 



