204 
BULLETIN OF THE BUREAU OF FISHERIES 
Most of the other surface catches of Calanus in the gulf that can be classed as 
“rich” have been made during the hours after the sun has declined below an altitude 
of about 8 to 10°, or before it has risen that high in the morning. More specifically, 
five of these rich surface catches were at 6 to 7 p. m., two at 8 to 10 p. m., five at 
10.30 p. m. to midnight, two at 1 to 2 a. m., and two at 6 a. m. Cases in point are 
statons 10024, 10027, 10038, and 10042 in July and August, 1912; a swarm off 
Gloucester on July 7, 1913; stations 10093, 10097, and 10100 in August, 1913; and 
stations 10246, 10247, and 10254 during August, 1914. Thus in the Gulf of Maine 
Calanus shows some tendency in summer to diurnal migration upward toward the 
surface at the approach of sunset, which it deserts after sunrise in the morning. 
Esterly (1911a, p. 142), in his study of the diurnal migrations of C. finmarchicus at 
San Diego, Calif., where the surface was practically barren of it during the day, 
found it “overwhelmingly more abundant at the surface about twilight or imme- 
diately after” than at any other hour, with its plurimum at about 7 to 8 o’clock in 
the evening; but the fact that we made as many rich catches about midnight as 
about sunset suggests that in the Gulf of Maine it is as likely to swim upward at 
one hour of night as another. It has been as scarce at the surface at most of our 
night stations, even when plentiful deeper down, as it usually is in the daytime, 
evidence that the vertical movement is only carried out at particular times and 
places, or that it usually fails to bring any large percentage of the Calanus right up 
to the top of the water. For example, “ Calanus certainly did not come to the 
surface off Cape Cod during the night of August 5 [1913], for surface hauls taken at 
2 a. m. and at practically the same locality at 8 a. m. (station 10086) yielded very 
few Calanus, although the deep haul caught thousands” (Bigelow, 1915, p. 290). 
Other instances of the same sort for other hours between sunset and sunrise might be 
mentioned. 
Our few stations (10399 to 10404) in the western part of the gulf for October 
31 to November 8, 1916, indicate a similar tendency on the part of Calanus to shun 
the surface by day but to ascend by night during the autumn as during the summer, 
for the one surface haul moderately productive of large Calanus was at 4 a. m. (sta- 
tion 10402), while juveniles were taken in numbers on the surface at 6 a. m. (station 
10400) . At the other stations (10 a. m., 2 p. m., and 3 p. m.) the surface hauls yielded 
few, though it was moderately plentiful at 50 to 180 meters. 
During the winter, as the water continues to cool and the sun is low, the surface 
must gradually offer a more favorable environment to Calanus, resulting in its 
occurring as regularly and probably as plentifully there by March as deeper down, 
irrespective of the time of day. How early in the winter this takes place remains 
to be learned, however. 
These observations corroborate Esterly’s conclusion that when Calanus does 
carry out a vertical diurnal migration it is not induced thereto solely by the time 
of day, but that the direction of its vertical swimming (or sinking) is governed 
by geotropism, which changes with physiological changes in the animal itself. Es- 
terly’s experiments pointed to varying degrees of solar illumination as governing 
these changes, thus bringing its reactions into line with those of other copepods. 
(See, for example, Parker, 1902, on Labidocera.) This explanation, however, does 
