PLANKTON OF THE GULF OF MAINE 



247 



is evident that M. longa fluctuates widely in the gulf from year to year, being ex- 

 tremely rare, if not altogether absent, in some years but widespread in others. The 

 years 1912 and 1914 and the summer of 1916 were periods of scarcity, while 1915, 

 the winter of 1916-17, and 1920 were times of plenty. The relationship of tempera- 

 ture to these annual differences is discussed below (p. 252). 



Seasonal distribution.— During the years 1915, 1920, and 1921, which may be 

 taken as representative of the periods when M. longa is at a maximum in the gulf, 

 it was taken at the following percentages of the stations: 



Months 



January.. 

 February 

 March. .. 



April 



May 



Percentage 

 of stations 



100 

 17 

 74 

 87 

 72 



Months 



June 



August 



September 

 October... 

 December. 



Percentage 

 of stations 



60 

 75 

 50 

 86 

 87 



This suggests that on the whole M. longa is apt to be found most widespread 

 in the gulf during the late autumn, winter, and early spring, and least so during the 

 summer and early autumn. The low percentage of stations at which it was recog- 

 nized in February, 1920 (only station 20046), would upset this rule were it a regular 

 annual phenomenon; but it is more likely that that month marked the beginning 

 of a period of abundance which endured throughout 1920, and that still fewer stations, 

 if any, would have yielded it during the preceding January or December. In fact, 

 a February station was most prolific of this species at St. Andrews during the winter 

 of 1916-17, as noted above (Willey, 1921). 



Seasonal fluctuations in the actual abundance of M. longa, as reflected in the 

 numbers of specimens per square meter, did not parallel the seasonal rise and fall in 

 the percentage of stations at which it occurred, it being much more plentiful in the 

 vertical hauls in August and October than from March to June or in September of 

 the years 1915 and 1920, as shown in the following table: 



Date 



March, 1920 



April, 1920 



May (1915 and 1920 combined) 

 June, 1915... 



Average 

 number 

 per square 

 meter at 

 stations 

 where it 

 occurred 



990 

 1,650 

 2,504 

 3,193 



Average 

 number 

 per square 

 meter, all 

 stations 

 included 



692 

 1,429 



1,552 



Date 



August, 1915 



September, 1915 

 October, 1915... 



Average 

 number 

 per square 

 meter at 

 stations 

 where it 

 occurred 



14,850 

 2,453 

 8,601 



Average 

 number 

 per square 

 meter, all 

 stations 

 included 



13, 637 

 1,533 

 7,280 



It is unfortunate that only four vertical hauls were made during August, 1915, 

 when the species averaged so much more plentiful than we have ever found it before 

 or since in the gulf. It may have been only a chance that the net hit local swarms, 

 and more vertical hauls might have proved barren of M. longa, thus reducing the 

 month's average. However, the fact that this northern species should have been 

 so plentiful (from 10,300 to 23,400 per square meter) at three late summer stations 



