26 
bulletin of the bureau of fisheries 
on top of the water after dark, notably near Mount Desert Rock on August 16, 
1912 (station 10032), where the 4-foot net, towed for half an hour, yielded nearly 
3 liters of plankton, chiefly copepods, with Calanus finmarchicus dominating, besides 
Euchseta, Centropages typicus, Metridia, Anomalocera, and Pseudocalanus ; also the 
shrimps Meganyctiphanes, Thysancessa inermis, TJi. longicaudata, TJi. gregaria, and 
Nematoscelis; the pteropods Limacina and Clione; Euthemisto of both species; 
the two common chastognaths Sagitta elegans and S. serratodentata; Tomopteris; 
Stephanomia; and larval redfish in lesser number; in short, a typical Calanus com- 
munity. A second instance of this sort came to our notice off southern Cape Cod 
on July 22, 1916 (station 10346), when the surface net yielded about as much Calanus 
(nearly a liter), with a sprinkling of Pseudocalanus and Metridia, an odd Euthemisto, 
Sagitta elegans, and Clione, as did the 30-meter net, although the mouth area of the 
latter was four times the greater, and it was towed for an equal period. As a rule, 
however, this vertical migration does not bring nearly so large a proportion of the 
zooplankton to the top of the water at any time during the night, for our catches have 
almost always been far richer (more varied, as well) at some little depth than im- 
mediately on the surface. This is illustrated by a station off Cape Cod on August 
23, 1914 (station 10256), where the catch of Calanus, Euchseta, Meganyctiphanes, 
Euthemisto, S. elegans, and Stephanomia was several times larger in the 130-0 
meter haul than in the surface haul, even after allowing for the use of nets of different 
diameters. 
"Whatever the precise physiological stimulus may be which causes so many of 
the copepods and other pelagic animals to rise at sunset and to sink again soon after 
midnight — and this is still an open question (p. 204) — its results are certainly confined 
to a far shoaler stratum in the Gulf of Maine, where it is never necessary to lower the 
net deeper than 40-100 meters to find the Calanus community at full strength at 
any time of day, than in the San Diego region off southern California, where Calanus 
in particular congregates as deep as 200 fathoms by day, to swim upward nearly or 
quite to the surface in the darkening hours (Esterly, 1911). Nor is it probable that 
the daily vertical migration in the Gulf of Maine often covers more than 100 fathoms 
even for Euchseta, which sinks considerably deeper in the daytime than does Calanus 
but less often reaches the surface at night. Until more extensive data are available 
it is idle to do more than tbuch on this interesting question. 
Apart from these vertical diurnal migrations our hauls have afforded glimpses of 
vertical stratifications of three other sorts (sometimes all three of them are exem- 
plified at a given station) : (1) As between young and adult communities as a whole; 
(2) between the adults of the several groups, genera, or species, even within the 
rather narrow depth limits (say, 10 to 100 meters) where the Calanus community as 
a whole attains its most abundant development; and (3) between the planktonic 
communities of the upper 100 meters or so, on the one hand, and of the deepest water 
of the gulf, on the other. Perhaps as illustrative a case as any that has come under 
our notice, and one typical of the western side of the gulf as a whole in early summer, 
is afforded by a station off Cape Cod on July 8, 1913 (station 10057), where it was the 
surface hauls alone that yielded any considerable number of copepod nauplii and 
eggs; the haul at 15-0 fathoms (27-0 meters) caught swarms of Calanus and many 
