PLANKTON OF THE GULF OF MAINE 
81 
The choice of a unit and of a method of measurement by which to express the 
quantitative abundance of the zooplanktonic community as a whole, as distinguished 
from its several component groups, is a matter of real difficulty. The 
easiest thing to do is simply to let the whole catch settle in suitable 
jars or graduates until visible shrinkage ceases and to record the 
volume of the resulting mass. Unfortunately, however, this does 
not give a true measure of the actual content of the net, much less 
(owing to the sources of error just mentioned) of the total column 
of water fished through, because it likewise includes the gaps between 
the individual animals composing it, together with any detritus that 
may have been in suspension in the water. This introduces a serious 
error, for plankton settles more or less closely according to the shapes 
of the individual animals composing it, smooth, round, fish eggs, for 
example, packing far more closely and regularly than do copepods 
with their long appendages. Nevertheless, even such simple measure- 
ments as this yield rough pictures of the abundance of the animal 
plankton, hence they have been made for all our vertical tows and for 
many of the horizontal ones. Jespersen (1924) measured the volume 
of the catch after draining the water from it. The process may be 
rendered more accurate if after draining a known amount of water is 
added, when the resultant increase in the volume will correspond to 
that of the catch plus the small amount of liquid which still adhered 
to the plankton after the draining. I have employed this method in 
a few cases where it seemed likely that the direct measurement of 
volume would be seriously misleading because of the character of the 
organisms concerned. The use of the centrifuge would be still better, 
but this has not been attempted for the Gulf of Maine hauls. 38 
Counting is the most instructive method of estimating the catch 
from most points of view, though it entails much labor and time, 
and this is the only method by which the actual numerical strength 
of the several groups of animals composing the zooplankton can be 
learned. Various types of apparatus have been devised for this 
purpose, most of them by the Kiel School of Biologists, the process 
followed for the Gulf of Maine hauls being as follows: The catch 
of the vertical net (its volume having been measured as above) is 
first diluted to a volume of 150 cubic centimeters, well mixed, and 
then, while the plankton is still in suspension, 3 cubic centimeters 
are taken with a suitable pipette and the copepods, fish eggs, etc., 
counted. The ordinary pipette, familiar to every biologist, will 
seldom serve for taking this sample; but it is not necessary to em- 
ploy the complicated “Stempel” pipette, for one of the shape shown 
in the accompanying sketch (fig. 36), with large rubber bulb, 
tube opening about 3 millimeters in diameter, and total volume of 
Fio. 3fi.— Volumet- 
ric pipette used 
for sampling cope- 
pods for counting 
!# For an excellent account of these and of other methods of plankton estimation see Johnstone, 1908, p. 129. 
