ELECTRIC FISH SCREEN 103 



applies for water having a resistivity of 10,000 ohms per inch cube, but for other 

 resistivities the constant term 3.70 must be modified, as will be shown later under the 

 discussion of the influence of water resistivity upon paralysis-voltage gradient. 



It has been suggested that different species of fish may require different voltage 

 gradients to produce paralysis. This point should be investigated further. Tests 

 were made on four different species at Bonneville, and all gave results that fell within 

 the range between the minimum and maximum curves. It is probable that no 

 greater variation will be found between the different species involved in protection 

 problems than is found between individuals of the same species. That there is 

 considerable variation between individuals is shown by the separation of the maximum 

 and minimum curves of Figure 5. This variation is due to several factors, among 

 which may be mentioned slight variation in the length of individuals in a test group, 

 variation in the condition and vitality of individuals and the position of the fish 

 with respect to the lines of current flow, and the equipotential surfaces in the electric 

 field. Great care was exercised to obtain healthy, normal fish. Some of the rainbow 

 trout at BonnevUle were found to be infested with an external copepod parasite, 

 which attacks the mouth and gills. For this reason all of the rainbow trout used were 

 examined carefully for this parasite, and those infested to a degree that would affect 

 their vitality were rejected. 



HIGH-FREQUENCY TEST 



A high-frequency oscillator was set up, as shown in Figure 2, and a group of 30 

 chinook-salmon fingerlings, averaging 3.04 inches in length, was subjected to a 

 500,000-cycle electric field. Plate voltages more than 100 times those used at 60 

 cycles were applied. None of the fish gave any indication of feeling the existence of an 

 electric field in the water. Tnis phenomenon probably is due to the fact that virtually 

 all of the current at this high frequency is flowing on the surface of the water between 

 the electrodes, and consequently there is no appreciable electric field in the water. 



CONTINUOUS-CURRENT TEST 



A continuous-current test was made on a group of 30 chinook-salmon fingerlings, 

 using the direct-current exciter in the hatchery hydraulic-power plant as a source of 

 contmuous potential. The voltage gradients for paralysis were 1.33 volts per inch 

 minimum and 2.0 maximum. The average length of the fish was 3.0 inches. These 

 values check the 60-cycle alternating-current tests very well, as shown by the max- 

 imum and minimum curves in Figure 5. 



DURATION OF APPLICATION AND MORTALITY 



Returning to the 60-cycle alternating-current source of supply, a series of tests 

 was made to determme the influence of the time of voltage application on the number 

 of fish killed. These tests were made on chinook-salmon fingerlings having an average 

 length of 3.1 inches. They were made bysubjecting 14 groups of approxknately 30 fish 

 each to a definite voltage gradient for a fixed period of time. The voltage gradients 

 were selected from below the paralysis value to values of approximately twice this 

 voltage gradient, and two arbitrary periods of application (1 muiute and 5 minutes) 

 were chosen. An entirely fresh lot of fish was used for each test to eliminate any 

 cumulative effects from repeated voltage applications. The results of these tests 

 110928—28 2 



