PLANKTON OF THE GULF OF MAINE 



175 



A. clausi continued universal over the northern and western parts of tho 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: 



75S9S— 26 



