202 
BULLETIN OF THE BUREAU OF FISHERIES 
first half of October, 1915, considerable differences were noted in the percentages from 
station to station, but during the last week of the latter month the percentage of 
Calanus (50 per cent) was nearly uniform at the several stations off Cape Cod and 
in Massachusetts Bay. In December, 1920, and January, 1921, considerable regional 
differences obtained in the horizontal hauls, with extremes of 90 per cent of C. jin- 
marchicus in the western basin (station 10490, table, p. 304) but only 10 per cent 
in the eastern basin (station 10502). 
The only definite regional subdivision that can be drawn in summer, from the 
standpoint of the percentages of C. jinmarchicus, is between the gulf proper (including 
its offshore banks) and the waters outside the continental edge. Calanus is usually 
dominant in the copepod community of the former, but is only a very minor element 
in that of the latter. Experience suggests that the farther out in the Atlantic'basin 
abreast the gulf, the less important relatively and the less plentiful absolutely is this 
copepod. It is probable that this is equally true throughout the year, but it is 
certain that the line of demarcation lies farther out from the continental edge in late 
winter and spring than in the warm season, when the high salinities and temperatures 
of the inner edge of the Gulf Stream are closest in to the banks — witness the notable 
dominance of Calanus off the southeastern slope of Georges Bank in March and 
April, 1920, and the increase in its percentage in the catches off the western end of 
the bank from February (5 and 6 per cent) to May (80 per cent, station 20129; table, 
p. 303). 
The data so far gathered show that this species may attain a very high percent- 
age anywhere in the inner parts of the gulf. When the local copepod plankton is 
more intensively studied, characteristic regional differences maybe developed there, too. 
Vertical distribution . — The vertical distribution of C. jinmarchicus in the Gulf 
of Maine varies somewhat with the season of the year. In spring, as exemplified 
by the February to May cruises of 1920, it was taken in' all but one of the surface 
hauls, irrespective of the time of day. The numbers of specimens per haul do not 
suggest any diurnal migration upward by day and downward by night, such as this 
copepod carries out in summer (p. 204), the average being somewhat greater for hauls 
made between 7 a. m. and 6 p. m. (average, 521 Calanus per haul in February and 
March; 1,458 in April and May) than for those made between 6 p. m. and 7 a. m. 
(average 263 for February and March; 838 for April and May). Whether Calanus 
actually is as plentiful at the surface during the spring months as it is at the lower 
levels can hardly be determined from the data available. 
Further evidence that the surface stratum is as productive of Calanus in spring 
as are the underlying waters is afforded by the average percentages of occurrence, 
which for the surface hauls 2 are about the same as for the verticals for the several 
months, and show a corresponding increase with the advance of the season (p. 201), 
as follows. Note, also, that the only spring hauls yielding 100 per cent Calanus in 
1920 were at the surface (stations 20100, 20112, and 20113). 
1 Taken in hauls uniform in duration and in the diameter of the net employed. 
