PLANKTON OP THE GULF OF MAINE 
225 
Vertical distribution. — In an earlier report (Bigelow, 1915, p. 293) I have noted 
that west and south of Cape Cod, Centropages typicus is most abundant near the 
surface, citing as noteworthy examples of this one station (10088) 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 O. 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 ofl 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. dnmarchicus 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). Failure 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, O. 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. 
