174 BULLETIN OF THE BUREAU OF FISHERIES 



tember, 1915, it proved more abundant, both absolutely (on the average about 

 7,000 per square meter inside 100 meters and 11,000 outside) and relatively (an 

 average of 20.5 per cent of the vertical catch of copepods), than at any time from 

 December to August, and the average numbers per square meter rose, respectively, 

 to 9,693 and 11,205 m in October of that year, when it occurred at 88 per cent of the 

 stations, though it constituted only about 11.5 percent of the total copepods caught in 

 the vertical net during the month. 



The two maxima suggest two breeding seasons for A. clausi in the gulf — one in 

 early spring and the other in late summer— each followed by a well-marked increase 

 in the actual abundance of the species, as measured both by the number of specimens 

 existing per square meter of sea surface and by the percentage of the total copepod 

 population which it constitutes. Probably it does not breed to any extent in the 

 gulf during the autumn or winter. A. clausi is likewise at its minimum during 

 winter in north European waters and most abundant during the warm months. In 

 the southern part of the North Sea its minimum falls in February and its maximum 

 in August (Farran, 1910). It is to be noted that the seasonal distribution of A. 

 clausi in the gulf shows it to be endemic there, not an immigrant, propagating in 

 spring in the centers where some few have persisted through the unfavorable winter 

 season and extending its area of reproduction as its spreads far and wide with the 

 increase in its numbers. 



Regional distribution.— -In February and March, 1920, it occurred sparingly on 

 the eastern part of Georges Bank, on Browns and German Banks, off Machias, off 

 the mouth of the Merrimac River, near Gloucester, and off Cape Cod, but at only 

 3 stations in the basin of the gulf, all in the southeastern part (fig. 58). Thus, at 

 the season when it is at its miminum it persists in small numbers here and there 

 throughout the shoal zone but disappears from most parts of the basin. By April, 

 with the increase in its numbers just noted (p. 172), it had become sufficiently dis- 

 persed over the basin to be taken at most of the deep stations in one or other net; 

 but it still continued most abundant over a zone running offshore from the neigh- 

 borhood of Cape Sable out across Browns Bank to the Eastern Channel and to the 

 eastern part of Georges Bank, with secondary centers of abundance along western 

 Nova Scotia, off Cape Cod, and off Cape Elizabeth, just as was the case in March. 

 By May and June of 1915 we found A. clausi so generally distributed over the 

 eastern, northern, and western parts of the gulf (in numbers ranging from 1,400 to 

 25,000 per square meter) that no separation into "rich" and "poor" areas is possible, 

 except that it seems to have been scarce in the neighborhood of Mount Desert 

 Island. Curiously enough, this was also the case on Browns Bank, which was one 

 of its chief centers of abundance in April, 1920. Probably it is equally universal on 

 Georges Bank during these months, judging from its presence at all the stations on 

 the line from Cape Cod out across the western end of the bank on May 16 and 17, 

 1920; but there were only about 200 per square meter at the outermost station, 

 just outside the continental edge (Station 20129), contrasted with about 14,000 at 

 the station on the bank (Station 20128), suggesting that this was about its offshore 

 boundary, which accords with its neritic nature. 



« The counts of copepods for 1915, on which these calculations are based, are given in Bigelow, 1917, p. 319. 



