302 



Illinois Natural Hisr()R\ Survlv Hullictin 



\'ol. 28, Art. 2 





Fig. 2. — Seining a shallow, rocky riffle in the Sangamon River near Mahomet. Agitation 

 upstream from the seine dislodges darters, madtoms, and certain minnows from the gravel. 



dures, and that even a slij^ht chani^e in 

 technique could stronj^ly influence any 

 quantitative value of the sample. We 

 therefore chose a standard procedure of 

 blockinji; ofi a section of stream and col- 

 lectinjz; the fishes within the section with 

 an electric fish shocker. In efforts to ex- 

 tend our list of species in the area, we then 

 seined various habitats close to the blocked- 

 oft section. 



Selection of Site. — Because of the 

 uneven distribution of fishes in a stream, 

 the selection of a sampling station was of 

 considerable importance. Inasmuch as 

 riionipson (^ Hunt had mapped the loca- 

 tion of each of their sites before going into 

 the field, and their maps, on file at the 

 Natural History Survey, were available 

 to us, we chose to revisit their stations. 

 However, we did not necessarily sample 

 at exactly the same places. Their notes 

 indicated that they often seined unusual 

 situations such as deep pools generally 

 found under bridges, whereas we at- 

 tempted to select areas judged representa- 

 tive of particular parts of streams. Al- 

 though our procedure possibly omitted 

 some unusual populations, it produced 



samples that we believe were more nearly 

 representative of each drainage system. 



Sampling Technique. — At each se- 

 lected site, a stretch of stream approxi- 

 mately 150 feet long was blocked off with 

 one-fourth-inch-mesh minnow seines. A 

 crew of three or four men started at the 

 downstream block with an electric seine 

 and proceeded upstream, collecting all fish 

 that could be seen and captured with dip 

 nets of three-sixteenths-inch mesh. On 

 reaching the upstream block, the crew 

 reversed direction, electrofishing back 

 through the sample area. \V'hen the water 

 was exceptionally turbid, a minnow seine 

 was pulled through the enclosed area to 

 recover stunned fish that had been over- 

 looked by crew members using dip nets. 

 Care was exercised to pick up all of the 

 dead and stunned fish that had drifted 

 into the net at the downstream block. 



The electric seine used . in the small 

 streams was 30 feet long; it had 15-inch 

 drop electrodes spaced at 30-inch inter- 

 vals. It was powered by a gasoline-driven 

 electric generator having a maximum ca- 

 pacity of 8.7 amperes of 60-cycle alternat- 

 ing current at 115 \olts. In wider 



