supported by a wooden framework and fronted 

 by a plastic screen. To check the uniformity of 

 the field, the potential was measured at regular 

 intervals throughout the tank and the equipotential 

 surfaces were determined. The electric field, 

 which is normal to the equipotential surfaces, 

 was very nearly uniform except for a slight 

 distortion near the ends of the tank. 



Procedure and Results 



The experiments to be described below were 

 made without any attempt at quantitative meas- 

 urennent of the response. Only a few fish were 

 available; namely 1 jack or ulua ( Caranx sp. ), 2 

 dolphin or mahimahi ( Coryphaena hippurus), 2 

 little tunny or kawakawa (Euthynnus yaito), and 

 3 yellowfin or ahi ( Neothunnus macropterus ). 

 The fish were caught by trolling, transported to 

 Coconut Island in the live well of the research 

 vessel Salpa , and placed in the large 35 x 1 1 x 4- 

 foot tank, which was supplied with running salt 

 water (Tester 1952). The experiments are des- 

 cribed in the order in which they were conducted. 



The apparatus was initially tested on a jack 

 about 50 cm. in length, with electrodes spaced 

 a distance of 33 feet, with a source supply of 60 

 volts (2 banks of 10 batteries) and with a fre- 

 quency of interrupters (capacitor discharge) of 

 10 per second. Without application of the field, 

 the jack persisted in keeping to one corner of the 

 tank, despite attempts to make it move around. 

 This end of the tank was made negative. When 

 the current was turned on, the fish was impelled 

 rapidly to the positive electrode where it re- 

 mained, bumping the protective screen, as long 

 as the current persisted. When the current was 

 turned off, the jack returned to its corner. This 

 response would be classed as "perfect. " 



The test was repeated with the sanne source 

 voltage but at a frequency of 1 c.p. s. There 

 was still a good electrotactic response but the 

 swimnning movements were slower. Each pulse 

 produced a strong nnuscular spasm. The same 

 test was repeated a second time after an interval 

 of a few minutes. This time narcosis began to 

 set in and there was a loss of equilibrium. After 

 the current was shut off the fish swam in an 

 inverted position. A day or so later it died. 

 Undoubtedly a satisfactory response could have 

 been obtained with a lower source voltage. 



Figure 5. --The mechanical interrupter. 

 A. General view showing variable speed 

 motor, contactor (one is removed), and 

 drive cams. B. Close-up view of spring- 

 loaded contactor. 



In working with members of the tuna family 

 it was more difficult to determine the effect of 

 the current since these fish were in constant 

 motion, swimming back and forth. The procedure 

 generally followed with tunas was to turn on the 

 current when the fish was swimming towards the 



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