Water resistivity changes with temperature (McMillan, 1928) 

 and with changes in amounts of dissolved or suspended materials. The 

 maximum drop in electric current (21 percent) in the laboratory test 

 tank during the course of experimentation vfas observed following a 

 storm which caused the intake water to become very turbid. Changes in 

 water resistivity of this type in the field would not necessarily change 

 the value or uniformity of a voltage gradient. They could conceivably 

 bring about sufficient change in the conditions that control the response 

 of the fish as to require readjustment of the voltage gradient by a 

 change in the input voltage. For example, McMillan (1928) has shown 

 that a voltage gradient of 0.2? volt per inch will paralyze a 3.1-inch 

 Chinook salmon fry ( Oncorhynchus tshawytscha ) in sea water having a low 

 resistivity while 1.23 volts were necessary to produce the same effect 

 in fresh water of high resistivity. It is believed the change in the 

 response of the fish is due to a change in the ratio of water resistiv- 

 ity to body resistivity of the fish and a consequent alteration of the 

 voltage developed across the length of the fish. However, this factor 

 would be a matter of concern only when changes of water resistivity are 

 large and rapid because the leading response on the part of the fish is 

 attainable over a fairly wide range in voltage gradient. 



More important than water resistivity per se is the ratio of 

 water resistivity to bottom resistivity. The bottoms of the two exper- 

 imental streams were roughly 2 to 3 times more resistant to the flow of 

 electrical current than was the water. The higher the ratio of bottom 

 to water resistivity the better conditions are for the establishment of 

 a uniform voltage gradient. Where the bottom is more conductive than 

 the water, it is doubtful whether a voltage gradient of usable character 

 between electrodes spaced more than a few feet apart could be established 

 without the use of artificial insulating materials on the stream bottom. 



This study has revealed several factors which appear to be 

 obstacles to the practical use of pulsed direct current as an effective 

 means of leading desirable fish away from sea lamprey control struc- 

 tures. Size selectivity is without doubt the major limiting factor. 

 The highly diverse physical conditions encountered in streams over a 

 large area also offer a number of obstacles. Still another problem is 

 presented by the possibility that a large number of fish may turn away 

 upon encountering the fringe of the electric field as was indicated by 

 the tests in the Little Ocqueoc River. It may be possible to overcome 

 this latter problem by intermittent operation on a critically times basis, 

 or it may be that the persistence of upstream migrants is great enough 

 to result in penetration of the electrical field. These and comparable 

 problems may be solved by further study. 



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