closer to the electrode, oscillotaxis (Van 

 Harreveld, 1938) occurs; and the strong volt- 

 age gradient surrounding the electrode pro- 

 duces electronarcosis. 



The typical set of fish reactions to increas- 

 ing strength of d.c. (Vibert, 1963) differs 

 from those caused by a.c, primarily in the 

 replacement of oscillotaxis by galvanotaxis 

 (Van Harreveld, 1938). The alternating direc- 

 tional response to the a.c. is replaced by a 

 unidirectional one that causes the fish to move 

 progressively toward the anode. This direc- 

 tional response is of primary importance in 

 electrofishing as the fish can be attracted out 

 of swift or turbid water or from heavy under- 

 water cover to a predictable point. Although 

 d.c. produces the desired galvanotropic effect, 

 it still leaves much to be desired because of 

 refractoriness of the fish. 



Extensive, strong, directional swimming 

 does not occur unless the d.c. is pulsed. The 

 reactions of fish to pulsed d.c. of increasing 

 strength are similar to those reported for 

 d.c- -except that before directional swimming 

 commences, the pulsations set up a series of 

 movements which are probably galvanotropic 

 reflexes of a spinal origin (Van Harreveld, 

 1938). We believe that locomotion may be a 

 voluntary escape reaction at this time but that 

 galvanotropic reflexes produced by pulsed d.c. 

 inhibit the fish from turning away from the 

 anode. The result is violent milling move- 

 ments; some fish escape and others succumb 

 to galvanotaxis. 



Optimum Pulsed D. C. for Electrofishing 



Complexity of the problem becomes mani- 

 fold in determining the optimum pulsed d.c. 

 for electrofishing. The voltage, frequency, 

 duration, wave shape, and amperage all nnust 

 now be considered and combined in such a 

 way that they supplement each other to pro- 

 duce a strong swimming motion of fish toward 

 the anode. The least electrical energy required 

 to induce repetitive galvanotropic reflexes is 

 of prime importance in electrofishing be- 

 cause the electric power in water disperses 

 as the reciprocal of the square of distance 

 from the anode. Within a given distance from 

 the anode, the electric energy is at the 

 threshold strength for continuous galvano- 

 tropic reflexes. This is the area of greatest 

 concern, for it determines the effective range 

 of a shocking unit. The following sections 

 summarize the type of pulsed d.c. which has 

 the greatest threshold for galvanotaxic stimu- 

 lation of fish. 



Effect of voltage on fishes .- -Adelman and 

 Haskell (1957) and Vibert (1963) have demon- 

 strated separate stimulating intensities for 

 muscle and nerve tissue in fish. They found 

 that direct stimulation of muscle requires 



considerably greater voltage than is needed 

 for neural reaction. Neural stin-iulation, there- 

 fore, is unquestionably the most useful stimu- 

 lus in electrofishing because it occurs at 

 lower electric intensities than n-iuscle stimu- 

 lation but achieves similar results. 



If a fish neuron is subjected to voltage that 

 increases gradually from zero, an initial reac- 

 tion occurs at a certain intensity, termed the 

 threshold voltage (Haskell, MacDougal, and 

 Geduldig, 1954), Voltage increments above 

 the threshold do not increase the strength of 

 the neural reaction. Thus, the reaction is of 

 the all-or-nothing type (Prosser and Brown, 

 1961). 



Effect of repetition rate and duration of a 

 pulse .- -A single pulse of electric energy of 

 subthreshold strength does not leave the ner- 

 vous system unaltered since a series of 

 pulses of the same value may elicit response 

 (Cooper and Eccles, 1930; Prosser and Brown, 

 1961). This phenomenon, ternned summation 

 (of inadequate stimuli), has been studied by 

 Haskell and Adelman (1955) in hatchery brown 

 trout. In their experiments the optimum fre- 

 quency for summation was close to 180 pulses 

 per second. Their comparisons of the thresh- 

 old response of pulsed and unpulsed d.c. 

 showed that the threshold stimulus occurred 

 at 20 percent less voltage with pulsed d.c. 

 Pulsed electricity of "summation strength" 

 is probably insufficient to produce a significant 

 galvanotropic response and is assumed to 

 cause a fright reaction. 



When electric energy of constant voltage 

 (sonnewhat greater than that required for 

 summation) is pulsed with increasing fre- 

 quency, the neurally induced muscle flexures 

 strengthen from isolated twitches to sporadic 

 volleys and finally to strong unified contrac- 

 tions. This phenomenon is termed facilitation 

 (Prosser and Brown, 1961). For exan-iple. 

 Gray (1936) noted that a cycle of 50 electrical 

 pulses per second was required to elicit one 

 undulation per second from eels with the spinal 

 column transected behind the medulla. This 

 finding nullifies implications (Burnet, 1959), 

 probably born of the German theory (Halsband, 

 1956), that frequencies should approximate the 

 natural undulation rate of fish. 



Optimum frequencies for electrofishing have 

 not been demonstrated adequately in fishes, but 

 our experience with varied sizes and species 

 has indicated a general "favorable" range. 

 Pulsations below 50 per second attract fish 

 poorly. The most desirable fish reactions 

 are at 50 to 90 pulses per second except in 

 resistive waters. Here, frequencies as high 

 as 100 pulses per second are necessary to 

 produce similar reactions. Rates from 90 to 

 140 pulses per second cause very rapid 

 swimming undulations, but frequencies above 

 140 tend to narcotize. Excessively high fre- 

 quencies have no apparent effect upon fish. 



