APPLICATIONS OF SALT IN ELECTROFISHING 



Success in electrofishing is largely de- 

 pendent upon the electrical resistivity of the 

 water. Resistivity, or specific resistance, is 

 defined as the electrical resistance of a cubic 

 centimeter of any material and is measured in 

 ohms. Measurements can be expressed in 

 ohms resistivity or in its reciprocal ohms con- 

 ductivity. In water, resistivity varies inverse- 

 ly to a great extent with the quantity' and quality 

 of dissolved solids and to a lesser but import- 

 ant degree with temperature. It, in turn, 

 influences the strength and range of an electric- 

 al field in water and it must therefore be over- 

 come in electrofishing to reach and narcotize 

 fish. Failure to take the factor of resistivity 

 into account often predisposes the application 

 of electrogear in certain waters to mediocre or 

 poor results. 



The wide variety of AC and DC electrode 

 systems in use today with power inputs of 115 

 to 500 volts, represents more or less success- 

 ful means for collecting fishes in waters of 

 various resistivities. Research and development 

 continue to result in improvements on electrode 

 systems, power sources, and methods of ap- 

 plication to increase the efficiency of electro- 

 fishing in very high and very low resistivity' 

 waters. Little has been done, however, to alter 

 resistivities to improve electrofishing. Such 

 alteration would be unnecessary on most waters 

 and impractical or impossible on some. How- 

 ever, it has proven practical to reduce the 

 extremely high resistivities of trout streams in 

 the southern Appalachian Mountains to levels at 

 which all- season electrofishing can be done 

 efficiently . 



Resistivities measured in 50 streams in 

 Great Smoky Mountains National Park in North 

 Carolina and Tennessee, and in Shenandoah 

 National Park in Virginia, ranged from 28,500 to 

 207,000 ohms with the majority exceeding 50,000 

 ohms. These are among the highest readings 

 obtained in natural waters in North America . 

 Measurements made recently on 15 trout streams 

 in the northern Appalachian Mountains in New 

 Hampshire ranged from 22,000 to 122,000 ohms 

 and indicate that the high resistivit>' condition is 

 perhaps typical of streams draining the entire 



Appalachian chain of mountains. In contrast, 

 spring water in production pools at the Leetown, 

 W.Va., fish -cultural station has resistivities of 

 2,460 to 2,600 ohms at 54° to 62° F. Samples 

 of single-distilled water at the same station 

 ranged from 90,000 to 110,000 ohms at 80° F. 



The use of an alternate -polarir>- electrode 

 system enabled successful electrofishing in streams 

 with resistivities up to 100,000 ohms but efficiency 

 declined sharply above that level . The necessity 

 of sampling fish populations in as many of the 

 streams of the parks as possible, often during 

 cold seasons when low temperatures increased 

 the electrical resistance of waters, led to experi- 

 ments with blocks of cattle salt to reduce resistiv- 

 ities and increase the efficiency of the shocker 

 equipment . 



METHODS 



A portable, batter\' -powered, 1,000-cycle 

 conductivit%- bridge (Model RC- 7, Industrial Instru- 

 ments, Inc.), with K-0.1 probe was used to 

 determine the resisti\'ities of waters in the labor- 

 atory and field. Readings are obtained quickly and 

 directly over a wide range in ohms resisti\"it>-. 



The salt used in laboratory and field trials 

 was 50-pound blocks of white cattle salt, common- 

 ly available at about one dollar each . No appreciable 

 differences were noted in trials made with plain and 

 mineralized blocks of salt that would justify the 

 slight additional cost for the latter. In field tests, 

 one to several blocks of salt were placed 25 to 50 

 yards upstream from seine -blocked sections 100 

 yards long in streams of 5 to 50 cfs flow. 



The portable, gasoline -powered generators 

 used for electrofishing were of 230-volt, AC, 600- 

 and 2500-watt capacities. The electrode systems 

 employed were Petty-type, alternate -polarity units 

 (Petty 1955). Improvements incorporated into this 

 electrode system for park work included trailers 

 to expand the electrical field in high-resistivity 

 waters and the substitution of No. 8, 440-wire, elec- 

 tric welding cable for the braided copper shielding- 

 wood dowel electrodes (Lennon & Parker 1955). 

 The welding cable has proven easier to use on 

 rough streams and is much more durable than 

 shielding. 



