92 
BULLETIN OF THE BUREAU OF FISHERIES 
layers of water practically deserted except in regions where active vertical currents 
keep the water thoroughly mixed. Therefore, it is usually safe to assume that 
the plankton is far more densely aggregated at some level, though perhaps only 
through a very narrow vertical zone, than the calculation of volume per cubic meter 
would indicate; but since we have occasionally found it rather uniformly distributed 
from the surface downward, even in the more stagnant parts of the gulf, no hard 
and fast rule can be laid down in this respect. 
Vertical stratification may result from a definite vertical migration of various 
animals toward the surface during the hours of darkness and downward again at 
sunrise, but quite apart from this phototropic phenomenon, which has often been 
described in other seas and which I have touched on above (p. 24), the tendency 
frequently shown by animals of different systematic groups (one of which may be 
and often is far more plentiful than the others) to segregate at different levels during 
the warm half of the year — copepods, for instance, at one depth and Sagittse at 
another — often causes a very uneven quantitative distribution of the plankton 
vertically in summer and early autumn. 
In July and August, 1913, for instance, it was invariably the shoaler subsurface 
haul that yielded the largest catch at stations where two such were made with the 
horizontal nets at different levels, even after making allowance for the use of nets of 
different types, although the reverse might have been expected because of the greater 
volume of water strained by the deeper hauls. 44 Evidently, then, the zooplankton 
was usually densest in the upper strata of water during that particular summer, say 
from 20 meters down to 50 at the localities of record, which were generally distributed 
over the offshore parts of the northern half of the gulf, and it was decidedly less 
abundant below 75 meters on the one hand or in the surface stratum on the other. 
This rule did not hold during the summer of 1914, however, when it was sometimes 
the deeper haul (stations 10215, 10246, 10248, and 10254), sometimes the shallower 
(stations 10214 and 10249), that yielded the largest catches, but usually one was 
much more productive than the other, as illustrated by the following table: 
Comparative catches of horizontal hauls of half an hour's duration ( reduced to a column 1 square meter 
in cross section ) during July and August, 1914 
[The depth is the level at which the major part of the haul was made «] 
Southwest Basin 
Georges Bank, northwest part. 
Southeast Deep 
Eastern Basin 
Northeast Deep 
Off Mount Desert Rock 
Western Basin 
Locality 
Station 
Date 
Depth 
in 
meters 
Volume 
in cubic 
centi- 
meters 
10214 
July 
19. 
/ 30 
\ 150 
3, 550 
250 
10215 
July 
20. 
/ 30 
\ 60 
150 
375 
10225 
July 
23. 
/ 60 
\ 240 
150 
125 
10249 
Aug. 
13. 
/ 50 
l 175 
2,180 
500 
10246 
Aug. 
12. 
/ 60 
\ 150 
150 
1,000 
10248 
Aug. 
13. 
1 50 
\ 150 
150 
1,250 
10254 
Aug. 
22. 
/ 75 
\ 225 
150 
625 
• Assumed to have fished through three quarters of a mile. 
44 For discussion of these hauls, with necessary corrections, and for the tabulated results, see Bigelow, 1915, p. 327. 
