PLANKTON OF THE GULF OF MAINE 269 



a month earlier in the season; probably more than in summer, though perhaps no 

 more than in May. This parallels its seasonal periodicity off northern Europe, for it 

 is usually most plentiful in the English Channel in autumn (Farran, 1910), with 

 its plurimum falling in late summer and early autumn in the northeastern Atlantic 

 up to Iceland (With, 1915). 



Another fact clearly brought out is that this species, like most other copepods, 

 may be decidedly streaky in its distribution at times. For instance, when we made 

 one of our richest catches of it (24,450 per square meter at station 10338) on October 

 27, 1915, there were hardly one-sixth as many a few miles inshore (station 10339; 

 about 4,040 per square meter). As a less striking example, there were respectively 

 3,600 and 3,400 at two stations (10321 and 10324) at the mouth of Massachusetts 

 Bay on September 29, but only 850 per square meter at a third station (10320). 

 This makes it impossible to draw any but the most general conclusions from the 

 numbers of specimens taken until a much larger body of information has been 

 accumulated. 



I have purposely refrained from discussing seasonal periodicty for P. parvus on 

 the offshore banks for want of sufficient data. Until something is known of its 

 status there during the summer and autumn all that can be said is that it was slightly 

 more plentiful on Browns Bank on June 29, 1915 (470 per square meter, station 

 10296) than on March 13, 1920 (60 per square meter, station 20072), but both catches 

 were so scanty and the difference between them so small that it is not significant. 

 On the eastern part of Georges Bank it was not taken at all at two stations on March 

 11, 1920 (stations 20065 and 20066), but was comparatively plentiful on April 16 and 

 17 (3,400 per square meter at station 10310; 1,640 at station 10311). Off the south- 

 western slope of the bank, on the contrary, it was much more numerous on February 

 22 (5,000 and 3,000 per square meter, respectively., at stations 20044 and 20045) than 

 on May 17 (only 400 per square meter at station 20129), contradictory observations 

 from which no conclusions can be drawn. 



Vertical distribution. — With (1915) has described the species as usually near the 

 surface in the northeastern Atlantic, and the majority of records of it in other seas 

 have been from shoal towings. In the Gulf of Maine, however, it showed no ten- 

 dency to congregate in the uppermost strata during the spring of 1920, for it was 

 detected in a smaller percentage (10 per cent) of the surface hauls than of the vertical 

 hauls, and only in small numbers at these few (table, p. 303). Little can be said of 

 its vertical distribution in other months of the year because the copepods have not 

 yet been listed from any of the surface hauls for 19 15 or subsequently, and a record from 

 a vertical haul merely locates the specimen somewhere between the top and the bottom 

 of the water. It is probable, however, that most of the specimens collected by the 

 Halcyon in 1920-1921 (table, p. 304) came from the general level at which the nets 

 were working horizontally — that is, from depths varying from 20 to 240 meters. 



The average depth of all the vertical hauls which had more than the average 

 number of P. parvus is 127 meters, and the four richest catches of all — that is, those 

 with more than 20,000 P. parvus per square meter (stations 10332, 10333, 10336, and 

 10338) — were, respectively, from 50-0, 80-0, 50-0, and 80-0 meters, locating the zone 

 of chief abundance for the species as shoaler than 100 to 125 meters. 



