PLANKTON OF THE GULP OP MAINE 225 



Vertical distribution. — In an earlier report (Bigelow, 1915, p. 293) I have noted 

 that west and south of Cape Cod, Centre-pages typicus is most abundant near the 

 surface, citing as noteworthy examples of this one station (1008S) where the surface 

 haul yielded ten times as many specimens as the haul from 80 fathoms, though made 

 with a net of only one-sixth the mouth area, and another (10083) where the surface 

 haul brought in several hundred C. typicus and the haul from 20 fathoms only one 

 specimen. Our largest catches of the species have also been on the surface, where it 

 swarmed oft Marthas Vineyard on July 10, 1913 (station 10062), and at 15 fathoms off 

 New York on July 12 (station 10066). 



Observations of this same tenor were made in the Gulf of Maine during August, 

 1912, C. typicus amounting to about 40 per cent of the copepods at the surface at 

 station 10041 but not over 2 per cent at 40 meters; about 60 per cent at the surface 

 and not found at all at 30 meters at station 10042. At a third station for that month 

 (in Massachusetts Bay, station 10044) it and C. Jinmarchicus each constituted 50 

 per cent of the copepods on the surface. Our few records for it north of Cape Cod in 

 August, 1914, are also from surface hauls; and while it has figured in a considerable 

 number of hauls at various depths in one year or another, it has never been more 

 than a trifling percentage of the copepod catch in the deeper horizontals, and 

 rarely in the verticals (p. 225). FaUure to take it in the surface hauls during the 

 spring of 1920 (table, p. 303) is not necessarily significant in this connection, the species 

 being so rare at that season that it might have been missed by the nets. Consequently 

 it may be classed as typically a surface form in the gulf, most plentiful above 20 

 meters and perhaps never sinking as deep as 100 meters. It is likewise most numerous 

 near the surface in north European seas. 



Relation to physical conditions. — In different seas C. typicus occurs over a wide 

 range of temperature and salinity. Along the Atlantic seaboard of North America 

 its presence is established in water as warm as 24.4° (Bigelow, 1915, p. 293) and 

 as cold as 3.05° (station 20104, April 15, 1920). It did not occur in the coldest 

 waters of the gulf, for example in the inner part of Massachusetts Bay, at the season 

 of minimum temperature, and the locations of the few early spring records suggest 

 either that it tends to withdraw from the coastal waters as the latter chill or that 

 the specimens living there perish, leaving only those that are in the parts of the 

 gulf less subject to winter cooling to survive the cold season. The fact that the 

 species did not appear in the surface hauls for March or April suggests that C. typicus 

 may sink in the deeper parts of the gulf as the surface chills. In the western basin, 

 for instance, where this copepod was comparatively numerous on February 23, 1920 

 (station 20049), it might have been in temperatures anywhere between 5.6° and 

 2.8°, according to the precise depth at which it was living. 



However this may be, C. typicus increases notably in abundance about when 

 the upper 20 meters or so have warmed to the maximum annual temperature, and 

 the tendency of the species to keep near the surface makes it safe to set 8° to 10° as 

 the lower limit to its active multiplication in the gulf. In autumn it is probable 

 that its numbers fall off after the upper 20 meters have chilled appreciably below 

 this figure, which, speaking broadly and for the gulf as a whole, takes place some 

 time during November. 



