202 BULLETIN OF THE BI7KEATJ 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 Baj'. In December, 1920, and January, 1921, considerable regional 

 differences obtained in the horizontal hauls, with extremes of 90 per cent of C. fin- 

 marcMcus 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.Jinmarchicits, 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 may be developed there, too. 



Vertical distribution. — The vertical distribution of C. finmarchicus 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. 



