recovery of a fish even when it was held 

 down against the positive electrode by 

 pulsing direct current. Location of the 

 fish with respect to the electrodes prior 

 to shocking was quite critical in the effi- 

 ciency of electrical control and capture of 

 the fish. Frequently, as schools passed 

 the installation the particular fish selected 

 in test operations would not pass through 

 the optimum location between electrodes. 

 Two men were necessary and three were 

 desirable for operating the gear. 



At this location, the efficiency achieved 

 in these tests was about 30 percent, or 3 

 out of 10 attempts to capture the tagged 

 fish were successful. It was decided that 

 this was inadequate to war rant further use 

 of the electrofishing device at this site. 



Operation at the second site was better 

 in all respects. Reduction of necessary 

 manpower, water-current characteristics 

 (depth, speed, "edge" between fast and 

 slow water), and migration path (depth, 

 width, distance offshore) all contributed 

 to a more efficient operation. The only 

 shortcoming was the lack of advance ob- 

 servation of the migrating fish, which 

 could have been easily offset by the con- 

 struction of an elevated observation tower 

 50 to 75 yards downstream. When the op- 

 erator sighted a tagged fish, he could 

 proceed to the shocker site, start the 

 generator, and be in position to shock and 

 collect the fish as it came between the 

 electrodes. Thus, with the shocker in- 

 stalled in this location, one man could 

 operate the whole collection system. 



No high-seas tagged salmon were ob- 

 served to pass the electrofishing installa- 

 tion during operating hours. The tower 

 counts were conducted for 20 minutes of 

 each hour. Additional hours were spent 

 in observation for tagged fish during the 

 height of the migration, but no high- seas 

 tagged salmon were observed to pass the 

 towers. Subsequent spawning-ground sur- 

 veys by the Fisheries Research Institute 

 personnel resulted in the recovery of two 

 spaghetti-type high-seas tags from the 

 niamna Lake spawning population of 

 680,000 red salmon. 



Optimum Pulse Frequencies and Duty 

 Cycles 



Figure 3 illustrates the general reac- 

 tions of the adult red salmon to various 

 pulse -frequency rates and duty cycles. 



slve eleclronorcosif 



Figure 3.--Responsesofadult red salmon (Oncorhynchus nerka) 

 to various combinations of square-wave pulse frequencies 

 and duty cycles. "Optimum Response" explained in text. 



After preliminary tests in which opti- 

 mum frequencies and duty cycles were 

 determined (see below), efforts to collect 

 65 specific fish were made. Fifty-four, 

 or 83 percent, of the specific fish sought 

 were captured. Efforts were made to 

 capture fish from the outer fringes of the 

 migration path as well as the center, 

 since a tagged fish might appear at any 

 number of locations within the general 

 migration path. Most failures occurred 

 when the selected fish was on the outer 

 edge of the migration path and over the 

 offshore edge of the negative electrode. 

 Alnnost 100 percent success was achieved 

 in the test series if the selected fish in 

 each test was located between the ener- 

 gized electrodes. 



Referring to figure 3, it is seen that 

 use of square-wave pulses of more than 

 140 per second caused immediate paralysis 

 of the fish with no electrotaxis toward the 

 positive electrode. The immobilized fish, 

 still in the river current, were carried 

 away from the operator's net. Duty cycles 

 of less than 35 percent at all frequencies 

 resulted in insufficient control of the fish, 

 causing only flight from the electrodes, 

 usually at a very high speed. Usually, 

 pulse frequencies below 60 per second, 

 except at higher duty cycles, also caused 

 fish to escape. In this low -frequency range 

 at duty cycles above about 65 percent, 

 some short reactions of electrotaxis were 

 illustrated. However, the fish seemed to 

 adjust to the pulsating electrical current. 



