554 



FILTER-PAPER CATCHES, ILLINOIS RIVER — Continued. 



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undetermined extent invalidated by the errors above noted, I 

 wish to call attention to the fact that they may still serve to 

 indicate in some degree the extent of the leakage and its sea- 

 sonal distribution — conclusions which are in some measure 

 corroborated by the results of enumeration. The ratios of the 

 volumetric determinations of the plankton by the silk and fil- 

 ter-paper methods in August-December, 1896, in 1897, in 1898, 

 and in January-March, 1899, as shown by the averages, are 

 respectively 1 to 4.1, 1 to 3.6, 1 to 2.6, and 1 to 7.7, or, averaging 

 all collections, 1 to 3.3. If these figures approach the actual loss 

 by leakage it becomes a matter of some volumetric importance. 



An examination of the table reveals the fact that in a third 

 of the cases the estimated plankton in the filter catch is exceed- 

 ed by that of the silk net. It will be seen that most of these 

 cases occur in instances of small plankton, where the tendency 

 above noted, to overestimate the silt is most effective in caus- 

 ing this apparent deficiency. In all cases the total filter catch 

 yreatly exceeds the total silk catch per m.^ (cf. Table III. and 

 the one under discussion). 



In general the preponderance of the filter catches is greatest 

 in the warm season of May-September, the growing period of 



