82 
BULLETIN OF THE BUREAU OF FISHERIES 
about 25 cubic centimeters, graduated as required, serves well for copepods and all 
smaller animals. The chief difficulty is that it is not always easy to make sure that 
the diluted plankton is evenly distributed in the fluid while the sample is being 
taken, because the various animals settle at different rates. Therefore, it is usually 
advisable to take two or sometimes three samples from each haul and average the 
results. 
Animals as large as amphipods, Sagittfe, and euphausiids are seldom so numer- 
ous but that it is easy to count the entire number caught in a vertical haul, and as a 
rule it is necessary to remove them before taking the sample of copepods, etc., lest 
the} 7 clog the mouth of the pipette. Fish eggs, also, can usually be counted directly 
from the entire catch, though they sometimes occur in such numbers that it is neces- 
sary to take a sample for this purpose. The copepods have been counted for most of 
the vertical hauls, the results being discussed in the chapter on that group (p. 167). 
Notes on numerical strength of other animals will be found under the particular 
species. 
The unit of measurement best available for the volume depends upon whether 
horizontal or vertical nets are used. If the former, calculation of the amount per 
hour’s hauling, as employed by Jespersen (1924), can hardly be bettered; but vertical 
hauls lend themselves to a somewhat more exact measure, namely, the amount present 
under some chosen area of the surface of the sea, which is usually expressed in cubic 
centimeters of plankton per square meter. This would be a sufficient index to the 
total productivity of any locality at any given time, and hence is often extremely 
instructive from the biologic viewpoint; but, as I shall have occasion to emphasize 
later (p. 90), it does not necessarily throw any light on the density with which the 
plankton is aggregated, since it neglects the possible stratification of the latter at 
different levels. 
On this basis the animal plankton of the gulf as a whole, like the phytoplankton 
(p. 399) , is apparently at its lowest annual ebb late in February and during the first 
half of March, when it was only in the western basin and over a tongue extending 
from the Eastern Channel and eastern edge of Georges Bank northward along the 
axis of the eastern basin to the 100-meter contour off Grand Manan (fig. 37) that we 
found as much as 75 cubic centimeters per square meter in 1920. Nor did we make 
any rich hauls then even in these comparatively productive zones, judged by mid- 
summer standards (p. 83). In all other parts of the gulf at the time, both inshore and 
over the basin, except as just qualified, and on Georges Bank as a whole, the water 
supported less than 25 cubic centimeters of plankton per square meter of sea surface, 
with several of the catches too small to measure, while on one occasion (off Cape 
Elizabeth, March 4, station 20059) the vertical net yielded nothing whatever. 
If the minimal catches of February and March, 1920 (less than 25 cubic centi- 
meters), be credited with 15 cubic centimeters of zodplankton per square meter 
(probably an excessive estimate), the average for the whole gulf at this season was 
only about 40 cubic centimeters, contrasted with about 100 cubic centimeters in 
midsummer, and the distinction between rich and barren was decidedly more sharply 
marked than we have found it during the more productive seasons of the year. 
