NATIONAL FISHERY CONGRESS. 173 



On the whole, one may say that where nets of sufficient sixe 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 t\vo 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 Apstein 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 ' 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, Job. Zur Plankton Methodik, I, Die Planktonpumpe. Bio. Centralblatt, xvn, 1897, pp. 

 190-198. 



