280 BULLETIN OF THE BUREAU OF FISHERIES 



in the data here offered to indicate any tendency on the part of P. elongatus to keep to 

 the deepest levels, nor can I offer any evidence of diurnal vertical migration on its part, 

 though this is so common a phenomenon among copepods that more detailed study 

 of the occurrence of the species is likely to show it in some degree. 



Seasonal cycle. — Pseudocalanus can not be described as definitely seasonal any- 

 where within the gulf. This appears both from the percentages of stations at 

 which it has been taken in different months, the variation from month to month 

 being no greater than the chances of the hauls, and from the distributional chart 

 (fig. 83), which proves Pseudocalanus present in all parts of the gulf both in the 

 summer-autumn and in the winter-spring seasons. 50 However, if the records be 

 considered by locality, the following regional differences appear: In the coastwise 

 zone out to the 100-meter contour, from Cape Cod to Grand Manan, the frequency 

 of occurrence (percentage of stations) has been about the same for one season as 

 for another, 51 and Pseudocalanus was taken with equal regularity (70 to SO per cent 

 of the stations) over the western half of the basin west of the longitude of Mount 

 Desert Island (long. 6S° 30' W.) in July-August as in October-January, February- 

 March, April, or May-June (the copepods have been listed at 39 stations from 

 that region) ; but while it was recognized at three of the four December-May stations 

 over the shallows west and southwest of Nova Scotia, out to the 100-meter contour, 

 it failed at two out of five summer-autumn stations there. It appears in the lists for 

 only eight out of 17 July-August stations in the eastern half of the basin, east of 

 longitude 68° 30' W. (including the Eastern and Northern Channels), where it was 

 taken at every station for September, January, March, and April, and at four out 

 of five May-June stations. 



On Georges Bank and over the shelf off Marthas Vineyard it likewise occurred 

 in all the vertical hauls for the spring of 1920 but failed at four out of eight July- 

 August stations in 1913 and 1914, though present at all three stations off Marthas 

 Vineyard on October 21 and 22, 1915 (stations 10331 to 10333; table, p. 298). Our 

 few hauls outside the continental edge abreast the gulf also point to a definite and 

 similar seasonal cycle for Pseudocalanus, it being present at six out of seven of the 

 December-May stations but at only two of the five for May-October. Thus, while 

 Pseudocalanus is uniformly frequent throughout the year in the western half of the 

 gulf, irrespective of depth, and along the northern coast, it occurs somewhat less 

 frequently and regularly in the southeastern and eastern part during the two-month 

 period, July-August, than at any other time of year. Apparently it follows the 

 same seasonal cycle, but with a decidedly greater impoverishment in summer, on 

 the offshore banks and in the more oceanic water outside the continental edge, 

 though more tows are needed in this region before a final pronouncement can be 

 made. 



It must be borne in mind that any planktonic animal may or may not be taken 

 most frequently when most abundant (may even be most frequent when least 

 numerous), the relationship between the two measures of occurrence depending on 

 the uniformity of distribution. In the case of P. elongatus the data afforded by 



*> In contrast, compare the seasonal fluctuations in the regional distribution of such an immigrant species as Sagittix ssnalo- 

 dentata (p. 320). 



ftl Eighty-five per cent for December-May, 90 per cent for June-October; total number of hauls, 51. 



