NATIONAL FISHERY CONGRESS. 171 



customary in order to prevent clogging to wash the net at the end of each haul with a 

 stream from a hose. It was further suggested by Hcuscn, 1 who recognized the eflect 

 of clogging on the net coefficient, that the net be more thoroughly washed at the 

 end of each day's work. Hensen 2 and Frenzel 3 have more recently suggested other 

 methods of cleaning the net. 



The change in the net due to shrinkage of the cloth and consequent narrowing 

 of the pores does not seem to have been noted by Hensen. It was first pointed out 

 by lieighard. 4 Both causes of change in the net coefficient have been since studied 

 by Kofoid. 5 He finds that owing to clogging of the net " the coefficient of tbe net 

 varies with the amount and constitution of the plankton from 1.5 to 5.7," and that 

 " from 84 per cent to 90 per cent of the 30-meter catch is taken in the first 15 meters of 

 the (horizontal) haul." Kofoid finds further that from the shrinkage of the net " the 

 total area of the openings in a square centimeter . . . decreases over 50 per cent." 



The first difficulty in using Hensen's method, that arising from change in net 

 coefficient, owing to clogging and shrinkage, seems at first sight to be sufficiently 

 serious. The second difficulty is that the openings in the cloth, although very minute, 

 are still so large that some of the organisms of the plankton pass through them and 

 are lost. After correcting the "catch" by multiplying by the net coefficient, the result 

 still does not express the total amount of plankton present in the column of water 

 through which the net was drawn. This source of error was known to Hensen, 6 but 

 he does not appear to have determined the extent to which the smaller plankton 

 organisms pass through the net. Kofoid 5 has now called attention to this subject and 

 has determined for certain forms the percentage of loss from this source. He finds 

 that "of Codonella as many as twenty-one individuals may escape to one retained" 

 and that there is a great loss of other small organisms. Kofoid adds, referring to his 

 predecessors, that, "the leakage of the plankton through the silk has been minimized 

 or ignored and without tests of the extent to which it occurs." An active purpose on 

 the part of plankton workers, such as is implied in the phrase "minimized or ignored," 

 is nowhere evident in the literature. The truth is rather that Kofoid's predecessors 

 have omitted to investigate this source of error quantitatively. 



Though neither the variation in the coefficient of the plankton net nor its pene- 

 trability to the smaller plankton organisms were discovered by Kofoid, he has 

 rendered important service in pointing out their extent. 



It remains to consider to what degree the errors due to the above causes detract 

 from the value of the results hitherto obtained by the Hensen method. The plankton 

 catches thus far made by this method (as by others) have been utilized principally 

 in two directions: 



I. They have been measured in order to determine the volume of plankton present 

 in the water. For this purpose the plankton is concentrated, either by allowing it to 

 settle in a graduated cylinder or by the use of the centrifuge, and the volume is then 

 read off'. This method is not accurate; it is merely the best method hitherto devised 

 for the purpose. The plankton, which is thus measured, consists of large and small 

 organisms, and as it settles the smaller organisms are mostly packed between the 



1 Heusen. Bestimnunig des Planktons, p. 13. 



2 Hensen. Bernerkunen zur Plankton Methodik. Bio. Centralblatt, XVH, 1897, p. 510-512. 

 s Frenzel. Zur Plankton Methodik. Bio. Centralblatt, xvn, 1897, p. 364-371. 

 'ReiKhard. Loc. cit., p. 59. 



5 Kofoid. On some important sources of error in the Plankton Method. Science, Dec. 3, 1897. 

 (i Hensen. Die Bestinunnng, *'tc.. p. 10, sec. 3, and p. 75. 



