NATIONAL FISHERY CONGRESS. 
173 
On the whole, one may say that where nets of sufficient size have been used 
under favorable conditions there is no good reason for assuming that the volumetric 
results obtained by Hensen’s method are vitiated by the first two sources of error 
noted above. To what extent they are vitiated by the third source of error (leakage) 
remains to be determined. Since the organisms which escape are the smallest in the 
plankton, they may be volumetrically of little importance. Their importance depends 
upon their abundance, and this must be investigated by other methods. When the 
considerable variations in the volume of the plankton itself are taken into account it 
seems improbable that the error arising from leakage is sufficient to seriously vitiate 
volumetric determinations by the Hensen method or their use for practical purposes. 
II. The catches made by the Hensen net have also been used for enumerating the 
number of organisms contained in them. Of the three sources of error above enumer- 
ated the first two affect this method to the same extent that they affect the volumetric 
method, so that by using suitable nets properly shrunken these two sources of error 
may be avoided here also. The third source of error, that arising from permeability 
of the net, is, however, fatal to the method of enumeration, in so far as it is applied to 
smaller organisms. In the tables of Apsteiu and Hensen, then, the enumerations of 
smaller organisms can not be accepted as final until it is shown that these organisms 
can not escape through the net in considerable numbers. 
For determining the productive capacity of a body of water use has been made of 
the volumetric method only. Where the net used has sufficient filtering surface, and 
where it is not attempted to use the net in situations to which it is unsuited — i. e., 
among water-plants and in silt-laden waters — it seems to me that this method is not 
only practicable, but it is the only practicable method hitherto devised, since it is the 
only method by which the plankton may be obtained from a representative sample of 
the entire body of water. It should be noted in this connection that the variations in 
the plankton itself are far greater than the errors of the method. 
We may now consider the substitutes that have been offered for the Hensen 
method. By this method the plankton is removed from a measured quantity of water 
which remains in position in the lake. We may analyze this procedure into two 
processes — the measuring of the water and the obtaining of the plankton from the 
water. For each of these processes, as carried out by the Hensen method, one or more 
substitutes have been proposed. 
Owing to the inconstancy of the net coefficient due to clogging and shrinkage, it 
may be a matter of uncertainty as to how much water the net actually strains. To 
obviate this difficulty it has been proposed by Kofoid (loc. cit.) and by Frenzel 1 that 
the water to be examined should be pumped through a hose. Water from any desired 
depth may thus be brought aboard the boat and plankton then removed from it by 
the Hensen net or other means. It is obvious that by this method the quantity of 
water obtained may be known with exactness, so the difficulty connected with net 
coefficient vanishes. By the Hensen method the column of water from which the 
plankton is obtained extends vertically from the bottom to the surface. This column 
includes equal volumes of water from all depths and is representative of the whole 
lake. It does not seem to me possible to obtain a representative sample of the 
water of the lake in any other form than that of a vertical column extending from 
1 Frenzel, Joh. Zur Plankton Methodik, I, Die Planktonpumpe. Bio. Centralblatt, xvii, 1897, pp. 
190-198. 
