ELECTRIC FISH SCREEN 



107 



INFLUENCE OF WATER RESISTIVITY ON THE PARALYSIS-VOLTAGE GRADIENT 



A series of paralysis-voltage gradient tests were made with chinook-salmon 

 fingerlings, using water varying in resistivity from 11.6 to 10,030 ohms per inch cube 

 to determine the influence of water resistivity upon the paralysis-voltage gradient. 

 The results of these tests (Nos. 31 to 37, inclusive) are summarized in Table 3 

 and shown graphically in Figure 7. The water resistivity was adjusted to the desired 

 value in these tests by first making a salt (NaCl) water solution that had the resis- 

 tivity of sea water; fresh water was then added to obtain the desired increments in 

 resistivity. These data show a rapid change in the voltage gradient required to 

 paralyze fish at low values of water resistivity, a slower change at intermediate 

 resistivities, and a rapid change again at high resistivities. Tlie range of resistivities 

 covered by this investigation, it should be noted, is from sea water to high-resistivity 

 mountain-stream water, and in this range the minimum paralysis-voltage gradient 

 changes from 0.27 to 1.23 volts per inch, or 4.55 times. It is necessary then to mtro- 

 duce a water-resistivity correction factor in the previous equation. It may now be 



written 



„ 3.70 TF 

 9 = — f 



where g = voltage gradient per inch to produce paralysis, L = length of fish in inches, 

 and TF= correction factor for water resistivity. (See Table 4 for values of W for 

 different water resistivities from 10 to 10,000 ohms per inch cube.) 



Table 3. — Influence of water resistivity on the voltage gradient required to paralyze chinook-salmon 

 fingerlings S.l inches long when subjected to a uniform electric field in water, the resistivity of which 

 was adjusted to the desired value by adding fresh water to a salt (NaCl) solution having an initial 

 resistivity of 11.6 ohms per inch cube and at a temperature of 55.5° F. 



Table 4. — Correction factor, " W," for various water resistivities for the minimum voltage-gradient 



equation g= ' 



Li 



