FISHERY BULLETIN: VOL. 72, NO. 3 



measurement, because the density of an electrical 

 field in seawater is not uniform. For ease of 

 measurement, the electrode-to-electrode voltage 

 was measured at the output of the pulse generator 

 and did not take into account cable and connection 

 losses. Also, due to the hookup restriction in the 

 research vessel's instrumentation room, short 

 lengths of parallel conductors were utilized, 

 resulting in a 409^ total cable loss. Therefore the 

 true electrode-to-electrode voltages and average 

 field strengths are related to measured values as 

 follows: 



A. 150 V = 90 V electrode to electrode = 22.5 



V/m. 



B. 120 V = 72 V electrode to electrode = 18.0 



V/m. 



C. 90 V = 54 V electrode to electrode = 13.5 



V/m. 



D. 60 V = 36 V electrode to electrode = 9.0 



V/m. 

 The configuration of the electrical field at a pulse 

 generator output of 120 V along with actual 

 measured field strengths (expressed as voltage 

 drops measured across 10 cm) at various posi- 

 tions within the field are shown in Figure 2A. 

 The measurements are fairly close in value but 



not exact. The pickup probe was attached to a long 

 pole and the measurements taken from the side of 

 the RV George M. Bowers. Because of water 

 current and boat movement, it was difficult to 

 hold the probe parallel to the electric field in 

 exactly each position shown. 



Laboratory tests indicated a field strength of 

 about 15 V/m was required to properly produce 

 electrotaxis in fish 10 cm long. Field strengths 

 throughout the volume of water within the 

 electrode envelope could be maintained equal to or 

 greater than the 15 V/m requirement. 



Based on initial field tests, the general zones of 

 fish response produced by the electrode array are 

 1) effective control, 2) possible control, depending 

 on fish size and its orientation, and 3) fright zone 

 (Figure 2B). The zone of control also extends to 

 the back side of the positive electrode. 



RESULTS 

 Captured Fish 



Voltage, pulse width, and rate are equally im- 

 portant for controlling the species tested (Figures 

 3-5, Table 1). Comparison between the pulse 

 widths indicates that a higher percentage of 

 experimental animals were controlled at the 

 wider pulse widths (0.8 ms). The lower and inter- 

 mediate stimulation voltages (60 and 90 V) were 

 not as effective in controlling the animals as the 

 higher voltage (120 V). Furthermore, the com- 

 bination of 0.8 ms pulse width with 120 V appeared 

 to be adequate for inducing electrotaxis at the 

 widest range of pulse rates (20 to 35/s). 



The ideal pulse rates for inducing electrotaxis 

 varied for each species. Spanish sardines and 

 scaled sardines were under good control at 20 

 to 35 pulses/s and round scad at 25 to 35 



PUISE WIDTH MS 



B FRIGHT ZONE 



...f ; ^»ii CON I «oi ioi,, 



Figure 2. — A. Field strength configuration at 72 V electrode 

 to electrode. B. General zones of fish response. 



Figure 3. — Percent positive response of scaled sardines to 

 various combinations of voltage, pulse rate, and pulse width. 



660 



