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 Julj T 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. " 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, 191 4- 

 |The depth is the level at which the major part of the haul was made "J 



Locality 



Southwest Basin 



Georges Bank, northwest part 



Southeast Deep 



Eastern Basin 



Northeast Deep 



Off Mount Desert Rock 



Western Basin _ 



Station 



10214 

 10215 

 10225 

 10249 

 10246 

 10248 

 10254 



Date 



Depth 



in 

 meters 



30 

 150 



30 



60 



60 

 240 



50 

 175 



50 

 150 



50 

 150 



75 

 225 



Volume 

 in cubic 

 centi- 

 meters 



3,550 

 250 

 150 

 375 

 160 

 125 



2,180 

 500 

 150 



1,000 

 150 



1,250 

 150 

 625 



a Assumed to have fished through three quarters of a mile. 



** For discussion of these hauls, with necessary corrections, and for the tabulated results, see Bigelow, 1915, p. 327. 



