NATIONAL FISHERY CONGRESS. 175 
and the plankton taken would be that found in this volume of water. Ko further 
calculations of any sort would then be necessary. 
If it is possible to thus meet the difficulty arising from clogging and shrinkage 
there still remains the further difficulty due to the leakage of small organisms through 
the net. The net will have collected the larger organisms from a representative 
column of water. In order to obtain these large organisms it is desirable that the net 
should filter a very large volume of water, in some cases many cubic meters. In order 
to obtain the smaller organisms it is, however, not necessary to filter so large a volume 
of water; a few liters would probably suffice. Water for this purpose might be 
obtained by the pumping method or perhaps quite as satisfactorily by the well-known 
method of using flasks so arranged that they can be filled after being lowered to 
desired depths. It would be necessary to take small samples of water from several 
different depths and to remove the plankton from them by some one of the methods 
described by Kofoid as retaining the smaller organisms. The objection to this double 
method is that while it is entirely accurate for the large organisms taken by the net 
from a vertical column of water, it does not give us the smaller organisms from the 
whole of this vertical column of water, but rather from isolated samples of water from 
different levels. It seems to me, however, that if we know the large organisms in a 
vertical column of water, and if we know also the ratio of the larger to the smaller 
for certain parts of the column, we may readily calculate the volume or number of 
small organisms in the whole column. This volume may then be added to that 
obtained by the net and the total volume thus obtained. 
In conclusion, it seems to me that the errors of the Hensen method, the extent of 
which Kofoid has pointed out, are probably greatly exaggerated by the condition under 
which he has used the method. This Kofoid himself suggests. The originator of the 
method probably never intended that it should be used among water-plants and in 
silt-laden waters. For such waters, which are shallow, the pumping and filtering 
methods described by Kofoid are undoubtedly best adapted. On the other hand, these 
methods are by no means so well adapted to deeper and larger bodies of water. For 
these it seems to me the Hensen method must still be retained, and if it can be modified 
as suggested above, it maybe of value in such waters as those of central Illinois. 
Whether or not it can be modified in the way suggested, it can at least be supplemented 
by a method by which the smaller organisms may be more perfectly obtained. 
Even in its present form the method is probably sufficiently accurate under most 
circumstances for the purpose of making rough determinations of the relative 
productive capacities of different bodies of water. It must be remembered that the 
method as used for this purpose is at best rough, but it must also be remembered that 
the variations in volume of plankton are considerable, so that the errors in method 
are probably within the variations in the material upon which it is used. 
Zoological Laboratory of the University of Michigan, 
Ann Arbor , Michigan , January 16 , 1898. 
