NATIONAL FISHERY CONGRESS. 175 



and the plankton taken would be that found in this volume of water. No 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 niter 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 may be 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 l(j, 1898. 



