'*yr:% 



FIG. 4-5 A Fyke fish trap (courtesy Illinois State Natural History Survey). 



consin plankton net is especially designed for this 

 purpose. Closing nets or traps can be made so that 

 they may be lowered to any desired depth, then closed 

 and brought to the surface. This enables the investi- 

 gator to determine at what depths the organisms oc- 

 cur. The Kemmerer sampler is used extensively for 

 bringing up known volumes of water from measured 

 depths for plankton or for chemical analyses. Nan- 

 noplankton, which passes through the finest tow net, 

 needs to be filtered out or centrifuged out for quanti- 

 tative measurement (Ballantine 1953). 



Net plankton is ordinarily counted with the use 

 of a Sedgwick-Rafter cell that holds exactly one cubic 

 centimeter at a time, and the number present calcu- 

 lated per unit volume or surface area of the pond or 

 lake. The volume of water filtered is equal to the area 

 of the net opening, times the distance pulled, times 

 a correction factor. No plankton net filters out all the 

 organisms from the column of water through which 

 it is dragged. The efficiency of such nets depends on 

 fineness of mesh, rapidity with which it is pulled, and 

 the abundance of organisms present. Fine-mesh nets 

 offer resistance to water flow, which is further in- 

 hibited as the pores become clogged with organisms. 



so that a part of the water column is diverted around 

 the net as it is pulled. A correction coefficient must 

 be determined for each net and for each different rate 

 at which it is pulled. This may be done by comparing 

 the quantity of catch obtained in the tow net with the 

 density obtained through use of plankton-traps or the 

 Kemmerer sampler. Detailed instructions for con- 

 structing different kinds of nets, and statements con- 

 cerning the advantages, disadvantages, and possible 

 errors in the use of different methods are given by 

 Sverdrup ct al. (1942) and Welch (1948). 



Bottom organisms 



Dip-nets are commonly used for obtaining mac- 

 roscopic bottom organisms and those attached to sub- 

 merged vegetation. In shallow water, bottom or- 

 ganisms may be scooped out from a bottomless 

 cylinder covering a known area. We find that four 

 good scoops with a dip-net are necessary to get most 

 of the organisms from a cylinder covering 0.2 m^, so 

 we sometimes consider two scoopsfull with a dip-net 

 as equivalent to 0.1 m^ when the cylinder is not used. 

 The Surber swift-water net is standard equipment 



40 Background 



