PLANKTON OF THE GULF OF MAINE 195 



combined, this will at least give an idea of the general status of the species. The 

 average numbers of Calanus jinmarchicus per square meter for all parts of the gulf 

 combined have been as follows: July and August, 1912, about 45,000; August, 1913, 

 about 2S,000; July and August, 1914, about 55,000— results probably not far from 

 the truth, judging from the evenness of the frequencies from summer to summer. 

 About 30,000 to 40,000 specimens of this copepod would then be a reasonable expec- 

 tation for the average frequency below each square meter of the surface of the gulf 

 in midsummer, though actually with extremely wide variations from station to sta- 

 tion — that is, from hardly a trace to upward of 200,000. This is a decrease by more 

 than one-half from the most prolific period and region of May (p. 194) and a con- 

 siderable shrinkage from the stock existing generally in the gulf in May and June. 

 Correspondingly, the richest Jul}- or August catch for the period 1912 to 1914 was 

 less than half the richest May catch, and while we have never found less than 7,000 

 Calanus per square meter in May, several August catches have contained fewer than 

 100. In some summers, however, the stock remains very high or may even con- 

 tinue to increase until well into July, as exemplified by the year 1916, when vertical 

 hauls yielded an average of about 147,000 Calanus" (approximately 71,000 of them 

 being large adults) among 210,000 copepods of all kinds for six stations in Massachu- 

 setts Bay, off Cape Cod, and in the southwestern part of the basin (Bigelow, 1922, 

 p. 136). 



In September, 1915, for which month vertical hauls were made at nine stations, 

 including the Massachusetts Bay region, the average per square meter (about 35,900), 

 with frequencies per square meter of 4,400 to 138,400, about equaled the expectation 

 for August; but the individual counts, station by station, show a tendency toward 

 dispersal of the local shoals of Calanus by the general circulation of water in the gulf 

 during early autumn, resulting in equalization of the stock, a phenomenon which 

 often accompanies, though is not necessarily a sign of, a cessation of active repro- 

 duction. 



If the counts for 1915 may be taken as typical, Calanus may be expected to 

 increase again in numbers from September to October, the average per square meter 

 being about 51,000 for the latter month with three of the vertical hauls more produc- 

 tive than 100,000 and none producing less than 7,500. This period of reproduction, 

 if it be one, must be brief, with the stock dwindling rapidly later in autumn, for the 

 yields of the horizontal tows taken during December, 1920, and January, 1921, were 

 uniformly scanty. The volume of the catches, however, suggest that 0. finmarcliicus 

 was more evenly distributed over the inner parts of the gulf at that season than we 

 have usually found it during its period of greater abundance in spring and summer. 

 Unfortunately, however, these stations do not afford numerical data. 



Density of aggregation. — Calanus finmarchicus, being the most plentiful copepod 

 in the Gulf of Maine, and, thanks to its comparatively large size coupled with its 

 numbers, by far the most important source of crustacean food for the plankton- 

 feeding fishes, the local abundance in which it gathers is of importance in the natural 

 economy of the region. The numbers present per square meter are not a direct 

 index to this, for the specimens living under that or under any other unit of the 



" Assuming Calanus to have constituted 70 per cent of the catch, which is probably below the actual figures. 



