CONSIDERATIONS FOR ELECTROFISHING 



The effectiveness of the shocker's output 

 energy is sometimes reduced drastically by 

 environnnental or biological factors. The re- 

 sistive effects of water do not alter the pulse 

 frequency or duration but depress the electric 

 energy input. The power that reaches a fish is 

 then modified by the animal's physiological 

 makeup. The following discussion of these 

 phenomena may help to clarify the wide varia- 

 tion of reactions among fish. Some small 

 adjustments in output power can be made to 

 reduce the erratic actions and escape of fish, 

 but this behavior generally has to be accepted. 



INFLUENCE OF WATER RESISTIVITY 



The resistivity of natural waters depends 

 on the quantity of ionized salts available to 

 carry the electricity. Salts ionize more freely 

 with increase in water temperature, and this 

 is more pronounced in waters of low salt 

 content (McMillan, 1928; figs. 7 and 8). 



How rising water resistivity increases the 

 power required to elicit a given response in 

 fish may be demonstrated by the voltages 

 (assuming a constant power supply) required 

 to paralyze sea lampreys at different resistiv- 

 ities (McCauley, 1 960). Under his experimental 

 procedure only about 0.8 v. was required in 

 water with resistivities from 400 through 3,000 

 ohm cm. 3, after which the voltage require- 

 ments increased, approaching infinity at about 

 30,000 ohm cm. 3. 



Practices which reduce the effect of highly 

 resistive waters by delivering more electrical 

 energy to fish include use of high frequencies 

 and durations and use of square waves with 

 peak voltages of 300 to 400 v.; it helps also to 

 maintain large electrodes close together and to 

 take full advantage of the "surprise effect." 

 In some areas water resistivity is so great 

 that electrofishing is generally impractical. 

 Lennon and Parker (1958), who found extreme 

 resistivities in Appalachian mountain streams, 

 attacked this problem by adding salt to the 

 water to improve electrofishing. 



INTRASPECIFIC AND INTERSPECIFIC 

 VARIATION IN FISHES 



Individual variation is notable among fish 

 even though they are of the same species and 

 have similar lengths. The laboratory experi- 

 ment of Haskell et al. (1954) on brown trout 

 demonstrates this variability. 



The larger the individual of a species, the 

 more sensitive it is to a given electric shock 

 (McMillan, 1928; McLain and Nielsen, 1953; 

 Taylor et al., 1957). Fish absorb power as a 



function of body surface area and particularly 

 length (Holzer, 1931), Also, the greater resist- 

 ance of smaller salmonids (Nakatani, 1954), 

 and possibly small fish of other species as 

 well, further reduces their response to shocks. 



The senior author's observations (on many 

 fishes in the State of Washington) suggest to 

 him that the degree of galvanotaxis may be 

 related to a fish's benthic or pelagic behavior. 

 For example, the substrate-boring petro- 

 nnyzontids exhibit little galvanotaxis, if any. 

 Fresh-water and euryhaline cottids and the 

 brown and black bullheads move a minimal 

 distance, if at all, and only just before electro- 

 narcosis. Starry flounders in brackish water 

 react well in view of their mode of swimming 

 and dependence upon the bottom. In other 

 fishes which do not stringently "adhere" to the 

 bottom, or are pelagic, the galvanotaxis re- 

 sponse is generally progressively stronger 

 through the groups Catostomidae, Cyprinidae, 

 and Clupeiformes . 



The foregoing infornriation agrees with 

 Vibert's (1963) theory that an internal innate 

 behavioral pattern is of such strength that the 

 stimulating effect of electricity is resisted. 

 Thus, a fish closely associated with the bottom 

 resists galvanotaxis and assumes typical 

 cryptic behavior, whereas the pelagic fishes 

 exhibit a locomotory pattern of escape. 



EFFECTS OF TEMPERATURE 



Fish flesh has a certain resistivity that 

 decreases with increasing temperature 

 (Whitney and Pierce, 1957). These authors 

 found that electrofishing in highly resistive 

 waters was mildly enhanced because the 

 rniore conductive fish tend to distort the elec- 

 tric field by the absorption of electricity. 

 Theoretically, success of electrofishing should 

 increase with rise of temperature, but workers 

 in the field have noted differently. Smiith and 

 Elson (1950) believed that salmon parr ex- 

 hibited the best response below 25° C, and 

 suckers at less than 20° C. Webster, Forney, 

 Gibbs, Severns, and Van Woert (1955) had 

 greater success in shocking brown trout at 

 7.7° C. than at 16.6°C., when both a.c. and 

 d.c. were used. Fisher and Elson (1950) 

 experimentally determined that brook trout and 

 Atlantic salmon acclimatized to 5.5° C. made 

 maximum darts from shocks at 10°C. and 

 15°C. The curve of relative maximum dart 

 response had roughly the appearance of a 

 normal distribution when plotted against tem- 

 perature. Other experiments by these authors 

 indicated that maximum shock response was 

 at the temperature preferred by the fish; the 

 work implied that each fish species has a 

 temperature at which it responds most strongly 

 to electrical stimulation. 



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