The Coefficient of the Dredge. 



281 



Table I. — Results of determination of coefficient of net. 



Date 



Pairs 



of 



catches. 



No. of 



resulting 



ratios. 



Counted 

 fraction 

 of catch. 



Catch of Tube. 



Catch of Net. 





Max. 



Min. 



Max. 



Min. 



1895, May 14 



Oct. 12 



Oct. 25 



1896, Feb. 25 

 May 18 , 

 July. 11 



4 

 4 

 6 

 5 

 5 

 5 



16 

 16 

 36 

 25 

 25 

 25 



1-10 

 1-5 



1-10 

 1-4 



1-10 



1-15 



2,910 



1,482 

 8,490 

 1,420 

 5,940 

 4,215 



2,400 

 1,170 

 4,290 

 760 

 4,310 

 2,430 



4,760 



2,292 

 14,520 



3,500 

 12,100 



8,370 



2,920 



1,770 

 10,560 



1,750 

 10,480 



5,680 



Total 



29 



143 





Minimum Ratio; Tube : net : : 21 : 100. 

 Maximum Ratio; Tube : net : : 100 : 100. 

 Average Ratio; Tube : net : : 49.85 ± 1 : 100. 

 Area of opening of tube : area of mouth of net : : 1 : 4. 

 Hence coefficient of net = 2, approximately. 

 Area of opening of net == 314.1 sq. cm. 



Hence to state catch of net in terms of sq. meter of surface, multiply catch by 

 10,000, 



314.1 



X2 = catch X 63.6, which factor was used. 



In determining the number of Crustacea caught by tube or 

 net, each species was counted separately. The individual species 

 show just about the same amount of variation as does the total 

 catch; although in the case of less abundant species the maxi- 

 mum number caught was not infrequently three times the mini- 

 mum. In the case of the tube no difference could be detected 

 in the range of variation of the numbers of species which are 

 active, like Diaptomus, and those which, like Chydorus, or Cy- 

 clops, are relatively slow in their movements. During the sum- 

 mer of 1896 an attempt was made to determine the coefficient 

 of the dredge from the number of spherules of Gloiotrichia, but 

 as this plant is found mainly in the uppermost strata of the 

 water on calm days, it proved an unsuitable object, and its 

 variations in number in successive catches were greater than 

 those of the Crustacea. 



It may be added that there was no constant position of maxi- 

 mum or minimum catch in any series which was made, but the 

 numbers varied in a wholly irregular fashion. 



