McMillan (1928) observed no reaction when 

 salmon were subjected to 500,000 cycle a.c. 



Although pulsations can set up the strong 

 rhythmical flexures required for galvanotaxis, 

 a certain minimal amount of electrical energy 

 must reach the fish. If the time (duration) of 

 the electrical impulses is extended, the fish 

 experiences prolonged exposure at given volt- 

 ages and hence is subjected to increased 

 power. An example of the profound effect of 

 duration of exposure of electricity on fish for 

 the threshold summation stimulus has been 

 demonstrated by Adelman and Haskell (1957). 

 As the frequency and duration are electron- 

 ically set and remain stable, their optimum 

 combination is one of the most important con- 

 siderations of electrofishing. The "preferred" 

 frequency- duration combinations that have 

 been determined by others for a variety of 

 fish species and conditions with different types 

 of shocks have varied widely (fig. 4). 



Evaluations made in the laboratory and field 

 by us and our associates have indicated the 

 optimum frequency- duration relations shown 



in figure 5 for waters of 5,000 to 30,000 ohm 

 cm. 3 resistivity. 



As the frequency increases from 50 to 9C 

 pulses per second, the maximum duration 

 can decrease from 12 to 6 msecs. Greater 

 durations would produce fish "fatigue" and 

 constrained movement which may represent 

 refractoriness in the nerves and muscles. 

 Lesser durations do not generally provide the 

 needed energy. We recommend that the mini- 

 mal duration for 50 to 90 pulses per second be 

 8 msecs. at the lower frequencies and 6 

 msecs. at frequencies of 70 and over. Shorter 

 durations may be effective, especially in more 

 conductive waters, but in most situations they 

 do not produce the desired effect, and the rate 

 of escape of the fish may be high. The greater 

 power requirements to compensate more re- 

 sistive waters can be met by extending the 

 duration and increasing the frequency. Thomp- 

 son's (I960) area of optimum response (fig. 4), 

 determined in waters of 41,000 ohm cm.3 

 resistivity, is excellent for describing fre- 

 quency-duration relations in resistive waters. 



90t 



iiJ 



I- 

 < 



3 



o 



80- -'^ 



4 I Momentary 

 * elecfrotoxis — 



70- • 



■3 



LJ 



O 60 



cc 



LJ 

 Q. 



50 



40- ■ 



i 



then flight 



•8 



Flight 



•Excessive electronarcosis 



30 - 



i 



Fligtit 



_L 



20 40 60 80 100 120 140 



FREQUENCY (PULSES PER SECOND) 



160 



180 



200 



Figure -1. — Optimum frequency-duration combinations for electrofishing, as reported by various authors, shown with 

 Thompson's (1^60) "area of optimum response" (shaded). Numbers preceding each citation refer to numbers (1 to 8) 

 shown in graph. 1 — California Marino Research Committee (1950); 2 — Groody, Loukashkin, and Grant (1952); 3 — McLain 

 and Nielsen (1953); 4— Haskell and Adelman (1955); 5 — Taylor, Cole, and Sigler (1957); o--Rollefson (1958); 7— Burnet 

 (1959); 8— Smith. Franklin, and Kramer (1959). 



