PLANKTON OF THE GULF OF MAINE 
175 
A. clausi continued universal over the northern and western parts of the gulf 
during November and October, 1915 (this, as just remarked, being its season of 
maximum abundance), and across the whole breadth of the continental shelf off 
Marthas Vineyard, varying in abundance from 6,000 to upwards of 40,000 speci- 
mens per square meter of sea area at most of the stations. Nor do our records for 
the midwinter cruise of 1920-1921 suggest any shrinkage in its range during the 
later autumn, for it occurred at nearly all the stations during that December and 
January. But if the picture presented by the early spring hauls of 1920 be normal, 
A. clausi must disappear from the basin of the gulf later in the winter as its numbers 
decline. 
A. clausi has always averaged a larger percentage of the total copepod popula- 
tion in the coastwise belt of the gulf and over the offshore banks than in the deeper 
parts. In 1920 it formed 10 to 20 per cent of the copepod catch in the vertical hauls 
at most of the stations on the eastern part of Georges Bank, on Browns Bank, in 
the Cape Cod-Massachusetts Bay region, off Cape Elizabeth, and along western 
Nova Scotia from February to May, but usually less than 5 per cent at the stations 
in the deeper basin and channels where it occurred. From June to October in 1915, 
the area in which A. clausi usually constituted 10 per cent or more of the copepods 
was continuous around the whole periphery of the gulf and around Cape Cod and 
Nantucket to the westward (fig. 59). In December, 1920, and January, 1921, it 
amounted to less than 10 per cent at all but one of the stations. Thus, this species 
is only of minor importance in the general planktonic community in the more oceanic 
parts of the gulf and negligible outside the continental edge in the open Atlantic, 
but in shoal waters, both inshore and on the banks, it is usually an important factor 
and may locally equal as much as half the total catch of copepods of all kinds. 
Vertical distribution. — The hauls have not been adapted to show the vertical 
distribution of A. clausi, and the fact that all but one of the percentages of 30 or 
more were in hauls shoaler than 75 meters can not be taken as meaning a concentra- 
tion of this species in the upper water layers because associated with the fact that 
the species is most plentiful in the shoal zone. On the whole, however, A. clausi 
was a slightly larger element in the copepod community on the surface than in the 
vertical hauls during the spring of 1920 (March, 13 per cent; April, 15.5 per cent; 
and May, 14 per cent, on the average) ; and on two occasions — that is, Eastern 
Channel, March 17 (station 20073), and off the northern slope of Georges Bank, 
March 10 (station 20063) — we found them congregated so close to the top of the 
water that each of the surface hauls yielded about 1,200 specimens, whereas the 
vertical hauls took none in the one case and only 3 in the other. On the other hand, 
A. clausi has repeatedly proved more plentiful at some deeper level than on the sur- 
face, of which the following cases are typical : 
Locality 
Date 
Station 
Number 
per 
square 
meter 
from 
vertical 
haul 
Number 
taken in 
surface 
haul 
Southeast basin 
Mar. 3,1920 
Apr. 9, 1920 
Apr. 12,1920 
Apr. 16,1920 
do - 
20053 
20091 
20100 
20106 
20108 
20115 
600 
1, 125 
475 
3,000 
21, 262 
800 
0 
31 
0 
2 
225 
0 
Off Cape Ann 
Northeast basin-., 
Browns Bank 
Eastern part of Georges Bank 
Western "basin 
Apr. 18,1920 
