26 BULLETIN OF THE BUEEAU 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, Thysanassa inermis, Th. longicaudata, Th. gregaria, and 

 Nematoscelis ; the pteropods Limacina and Clione; Euthemisto of both species; 

 the two common chsetognaths 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, Euchasta, Meganyctiphanes, 

 Euthemisto, S. elegans, and Stephanomia was several times larger in the 130-0 

 meter haid 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 Euchasta, which sinks considerably deeper in the daytime than does Calanus 

 but less often reaches the surface at night. Until more extensive data are avadable 

 it is idle to do more than touch 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 haids 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 



