to be about as effective as an uninterrupted 

 pulse cycle. Groups of pulses produced by the 

 Type III shocker are of no proven advantage to 

 date. All of the Bureau of Commercial Fish- 

 eries shockers produce a characteristic volt- 

 age discharge "spike" (fig. 7) which probably 

 has a duration too short to produce a pro- 

 nounced stimulating effect on the fish. 



Output voltage.- - The voltage potential of 

 the d.c. shocker should be appreciable to 

 provide a strong stimulative gradient at a 

 distance from the anode; yet practical limits 

 must be set for the safety of the operators 

 and in consideration of the physics of water 

 conduction of electricity. An output of 150 v. 

 approaches the desirable minimum in more 

 conductive fresh waters that characteristically 

 have resistivities of 1,000 to 15,000 ohm 

 cm. 3. Greater voltages are recommended for 

 general-purpose shockers, especially if work 

 is to be done in highly resistive waters. The 

 curvilinear relation, however, between output 

 voltage and the waterborn voltage gradient at 

 various distances from the anode (fig. 3) sug- 

 gests that there is a practical upper limit. 

 Considering all factors, we feel that output 

 voltages above 400 are hardly justifiable even 

 in the most resistive waters, providing 

 optimum- sized electrodes are used. 



Amperage. - -The amperage in electrofish- 

 ing provides the quantitative effect of a stimu- 

 lating current. Because the power that can 

 be transmitted through water is a function of 

 resistivity, the optimum amperage varies 

 with the water. It is most convenient, how- 

 ever, to use maximum possible amperage. 



Our experience with different dynamotor 

 power sources for the Type IV shocker indi- 

 cates that a current of 60 milliamperes (ma.) 

 is inadequate but that one of 150 to 180 ma, 

 is adequate in waters over 5,000 ohm cm. 3 

 resistivity; a shocker capable of producing 1 

 to 2 a. is adequate in all fresh waters. Gen- 

 eral-purpose shockers should be able to pro- 

 duce 2 to 3 a. for sampling in less resistive 

 waters. 



e> 



< 



_I 

 O 

 > 



500 

 400- 

 300- 

 200 

 100 

 0- 



500-, 



400 



300- 



200- 



100 



o-J 



500n 

 400 

 300- 

 200- 

 100 

 0-" 







15,000 OHMS 



10 20 30 40 50 



5,000 OHMS 



10 20 30 40 50 



T-i 



500 OHMS 



I — I — I — I 1 1 — [ — 



10 20 30 40 50 

 TIME ( MILLISECONDS ) 



Figure 8. — Distortion of wave shape of Type IV shocker 

 caused by a 15-, 5-, and a 0.5-thousand ohm load. 



Output Energy of the Shocker 



The Type IV shocker has a potential output 

 of 450 V. pulsed at frequencies from ZO to 100 

 per second into square waves having durations 

 fixed at 6 msecs. The frequency ranges com- 

 monly used in all waters are 50 to 70 pulses 

 per second, which result in a 30- to 40-percent 

 duty cycle. Decreasing water resistivity dis- 

 torts the wave shape primarily by diminishing 

 the output voltage (fig. 8). 



The Type IV shocker has proven highly 

 successful in attracting fish from distances, 

 and they often forcibly collide with the anode 

 through galvanotaxis. The frequency-duration 

 possibilities are just within the optimum range 

 for moderately resistive waters, but the high 

 voltage potential produces additional requi- 

 site energy for strong galvanotaxis. The addi- 

 tional stimulative energy desirable in highly 

 resistive waters is partially attained through 

 a rise in output voltage. 



