PLANKTON OF THE GULF OF MAINE 



199 



mately 300 per cubic meter, paralleling the calculations for August as closely as 

 could be expected with an animal distributed so irregularly. 



Six stations between Massachusetts Bay and the mouth of the Grand Manan 

 channel gave about the same average (298) for the first week of October, with but 

 little variation from station to station (see table above), evidence that, as judged 

 by the number per cubic meter — that is, the density of aggregation and availability 

 for fishes — Calanus finmarcMcus was distributed with comparative uniformity over 

 the inner parts of the gulf during the late summer and early autumn of 1915, a year 

 probably fairly representative. Vertical hauls off Cape Cod and in Massachusetts 

 Bay on the 26th and 27th of the month yielded it in much larger numbers, rivaling 

 the denser communities of the species in spring and early summer. 



We have no data on this subject for the months of November, December, or 

 January, but the catches of the horizontal nets, at depths of 15 to 240 meters during 

 the cruise of December to January, 1920-1921, were so small that Calanus must 

 then have been distributed very sparsely, indeed, and probably in no greater numbers 

 per cubic meter than in March (if as great), judging from the volumes of the catches 

 of the horizontal hauls, which consisted chiefly of copepods (see table, p. 304, for 

 percentages of Calanus). Thus the whole Gulf of Maine supports a much sparser 

 community of Calanus in winter and until May than it does from late spring to 

 October, with the maximum density of aggregation for this copepod falling from May 

 to July, the seasonal fluctuations in this respect paralleling those of the actual 

 numerical strength of the local stock. 



Percentage of occurrence. — The degree to which Calanus finmarcMcus predomi- 

 nates over all other copepods in the Gulf of Maine basin may best be illustrated by the 

 percentages of this species in the total catches of copepods. The vertical hauls of 1915 

 1920, and 1921, combined, averaged about 55 per cent C. finmarcMcus, inclusion of the 

 surface hauls for the spring of 1920 and the horizontals made during the summers of 

 1912 and 1914 bringing theprecentage up to about 60. Furthermore, C. finmarcMcus 

 is the only copepod that has occurred at every tow-net station in all parts of the guff 

 at all seasons and in almost every haul, vertical or horizontal, and the only one that 

 w r e have ever taken in 100 per cent purity. The three instances of this among the 

 surface tows for 1920 (stations 20100, 20111, and 20112, see table, p. 303) are not 

 especially significant, the total catch being so small in each case that other less 

 common species occurring side by side with Calanus might easily have been missed by 

 the net. 



