electrical field and its threshold voltage. These 

 authors have defined the basic physical and 

 biological factors which affect the response of 

 selected marine animals to electricity. 



To understand more clearly the basic charac- 

 teristics of electrical fields which control marine 

 fishes, threshold response of selected Gulf of Mex- 

 ico species was investigated. I determined the 

 minimum threshold voltage (in a field with other 

 specific characteristics) for Atlantic croaker, 

 Micropogon undulatus; spot, Leiostomus 

 xanthurus; \ongsp\ne porgy, Stenotomus caprinus; 

 chub mackerel, Scomber Japonicus; and scaled 

 sardine, Harengula pensacolae. I further attempt- 

 ed to determine the minimal effective pulse width 

 by estimating the threshold voltage at selected 

 pulse widths. 



Procedure 



Spot, Atlantic croaker, and longspine porgy 

 were trawled in Mississippi Sound; minimizing 

 injury to fish was accomplished by towing for only 

 10 min. Chub mackerel and scaled sardines were 

 caught by night-lighting off the Mississippi Coast 

 (Wickham, 1970). The experimental animals were 

 held in shipboard tanks of circulating seawater 

 while being transported to the Laboratory. Only 

 fish acclimated for more than 3 days and in good 

 physical condition were used. Threshold voltages 

 were determined for 140 individuals. Each speci- 

 men was subjected to only one test and discarded. 



Studies were conducted in a 72 x 45 x 45 cm, 

 190-liter plexiglass aquarium at temperatures be- 

 tween 15° and 17.5°C; salinities ranged from 19.6 

 to 26AVco. 



An electrical system providing a uniform elec- 

 trical field was used. It had a capacitor-discharge 

 stimulation pulse that could be monitored from 

 the center of the aquarium. Pulse shapes which 

 exhibit a rapid rise in amplitude and slow rate of 

 decay, such as capacitor-discharge pulses, are the 

 most effective for controlling fish (Taylor, Cole, 

 and Sigler, 1956; Vibert, 1967; Klima, 1972). 



A pulse generator was used for the stimulation 

 pulses. Pulses were applied to two monel elec- 

 trodes mounted at opposite ends of the aquarium. 

 Pulse characteristics were measured with a pair of 

 pickup probes. These were constructed from two 

 3-mm diameter bronze rods 10 cm apart and insu- 

 lated so that only the bottom 10 mm of each rod 

 was exposed. Pulse characteristics were displayed 



on an oscilloscope as a graph of voltage against 

 time. 



Threshold voltages were determined with the 

 fish held immobile parallel to the electric field in a 

 plastic mesh tube in the center of the aquarium 

 facing either the positive or negative electrode. 

 Voltage was slowly increased until the fish re- 

 sponded by fluttering of the body. This value was 

 then recorded from the oscilloscope and assumed 

 to be the threshold voltage. 



Results and Discussion 



Kessler (1965) found that variations in pulse 

 width alter the threshold voltage for shrimp. 

 Pulse widths longer than 150 /js are satisfactory 

 for stimulating shrimp, whereas below that width 

 the power required for stimulation would be 

 significantly greater. Longspine porgies stimu- 

 lated with less than 100 /us pulse widths required 

 at least four times more voltage to respond than 

 fish stimulated with a wider pulse (Table 1). At 

 narrow pulse widths threshold voltage was high; 

 at the longer widths it was low, forming an inverse 

 relationship. 



Scaled sardine required higher voltages to elicit 

 a response at narrower pulse widths than at wider 

 pulse widths. At 45 /js it took almost 1.9 V/lOcmto 

 elicit a minimum response, but at 100 yus it took 

 only 1.3 V/10 cm, and at 250 and 1,000 a^s it took 

 only 1.0 and 0.9 V across 10 cm. A comparison of 

 threshold voltages of scaled sardines at different 

 pulse widths shows a significant difference be- 

 tween the threshold values at pulse widths tested 

 (Table 1). Student's ^-test was used in making 

 these comparisons: 



«= 6.316, < 975, ig) =2.101) 45 and 100 ^s, 



(t = 3.815, t Q g^g , jgj = 2.101) 100 and 250 /js. 



(t = 5.000, t 0.975 (18) = 2.101) 250 and 1,000 ^s. 



Although there was a difference in the 

 minimum voltage within the 250 fus to 1.0 ms 

 range, as shown by the reactions of scaled sardine, 

 the difference in actual voltage was minimal. For 

 scaled sardine the most efficient pulse width in 

 terms of electrical power would be not less than 

 250 IJ.S. Generally, threshold voltages at pulse 



852 



