The significance of resistivity in electrofishing 



A measurement of resistivity at once in- 

 forms an operator of electrofishing gear whether 

 AC or DC power can be used, whether 115, 230, 

 or more volts are required, what type of elec- 

 trode systems can be employed efficiently, and 

 whether or not salt is needed to reduce the re- 

 sistivity to a workable level. Since natural 

 waters vary greatly in resistivity from one lake 

 or stream to another, and vary within a given 

 body of water with temperature and water levels, 

 a measurement at the time of electrofishing in- 

 sures tlie proper selection of gear and indicates 

 roughly the efficiency of collecting which can be 

 expected. This use of resistivity measurements 

 not only contributes to a general improvement 

 of electrofishing operations but permits the re- 

 sults of such fishing in different bodies of water 

 or in the same bodies under various conditions 

 to be more validly compared. 



was found that the Petty-type alternate -polarity 

 electrode system with 230 volts AC furnished 

 the best, and in many cases the only results in 

 electrofishing (Petty 1955; Lennon and Parker 

 1957). An experimental DC apparatus with 

 pulsed 230 -volt current produced results at re- 

 sistivities up to 50,000 ohms. Paddle -type 

 electrodes with 230 volts AC and straight DC 

 apparatus with 230 volts failed completely. In 

 contrast, Leetown Run had a resistivity of 2,400 

 ohms and paddle -type electrodes with 115 volts 

 AC, or straight DC apparatus with 115 volts 

 worked very well. Pulsed DC at 230 volts in a 

 soft water, electrode system worked well but 

 was obviously too powerful. The soft water, al- 

 ternate-polarity electrode systems with 115 and 

 230 volts AC not only overloaded the generator 

 but killed a large proportion of the fish taken. 

 Obviously the latter system which wo rked better 

 than others at 50,000 or more ohms was unsuit- 

 able in hard water at 2,400 ohms . 



Resistivity also indicates the type of 

 water as very soft (over 17,000 ohms), soft 

 (8,000 to 16,600 ohms), medium hard (4,300 to 

 7,700 ohms), hard (1, 100 to 4,200 ohms), and 

 very hard (less than 1,000 ohms) (table 5). 

 These types are usually defined in terms of 

 total hardness (U.S.D.A., 1955) but are related 

 here to resistivity and conductivity to assist in 

 evaluating the performance of electrofishing 

 gear in various waters . 



There has been considerable misunder- 

 standing among fishery workers regarding the 

 reported performance of soft water or hard 

 water electrofishing gear due to the fact that the 

 terms soft or hard were used arbitrarily rather 

 than according to the total hardness standard. 

 Thus some types of gear which have been report- 

 ed effective in soft water were actually tested in 

 medium hard water and they fail to function in 

 true soft water. Further confusion can be avoid- 

 ed only by typing water and gear by the standard 

 rather than relatively. 



Resistivities in park streams commonly 

 range over 100,000 ohms in winter and the ef- 

 ficiency of the 230 -volt, alternate -polarity electro- 

 fishing gear decreases as resistivities increase . 

 Blocks of stock salt are employed to reduce the 

 resistivities to a range of 30,000 to 40,000 ohms, 

 a range which is easy to achieve and at which the 

 electrogear functions efficiently (Lennon and 

 Parker, 1959). 



The resistivities measured on selected 

 streams in association with electrofishing opera- 

 tions have ranged from 10,000 to 207,000 ohms 

 in Great Smoky Mountains National Park, from 

 31,000 to 114,000 ohms in Shenandoah National 

 Park, from 2,300 to 4,850 ohms in Jefferson and 

 Berkeley Counties, W. Va., from 22,000 to 122,000 

 ohms in Coos and Carroll Counties, N. H., and 

 from 20,000 to 41,.000 ohms in Kennebec, Lincoln, 

 and Washington Counties, Me. (table 6). Those 

 listed were not corrected to a standard temperature 

 since they represent the prevailing condition under 

 which electrofishing was conducted. 



Examples of ftie importance of resistivity 

 in electrofishing were provided by trials on trout 

 streams in Great Smoky Mountains National Park 

 and at the Leetown (W. Va.) Fish-Cultural sta- 

 tion. The resistivities of many of the park 

 streams tested were over 50,000 ohms and it 



It has been our experience that no one 

 electrode system, nor voltage level, nor type of 

 electric power (AC or DC) performs efficiently 

 over this wide range of resistivities. It is desir- 

 able, therefore, to have two or more electrode 

 systems, two or more types and levels of voltage. 



