Another problem of isolation of the trap 

 entrance develops when current flows from a 

 control device to a point not associated with 

 it. On occasion, the current leaves the down- 

 stream extremity of the intended field and 

 flows to one of the banks. In one stream, this 

 path became comparatively narrow and ex- 

 tended some 50 feet below the control device 

 to a point on the shore opposite the one on 

 which the power supply was located. In other 

 streams, the path remained broad. It is as- 

 sumed that this phenomenon results from the 

 presence of a highly conductive material. 

 This problem became more acute during the 

 latter part of the season, but it caused dif- 

 ficulty only when the unexplained current 

 crossed ahead of the trap entrances. For the 

 most part, this problem was overcome by 

 providing a conductor, or a series of con- 

 ductors, to pick up and carry the current 

 across the area that affected the entrance to 

 the trap. 



The data collected have led to few recom- 

 mendations for changes in the electrical 

 equipment. In a few of the deeper streams 

 equipped with a single suspension, the mortal- 

 ity may be reduced by moving the bottom 

 electrode upstream from the suspended elec- 

 trode. In deep water, the surface field above 

 the bottom electrode may be weak enough to 

 allow fish swimming near the surface to ap- 

 proach the suspended electrodes . When water 

 velocities are low, the fish may become paralyzed 

 and sink close to the bottom electrode, where 

 they may be killed. 



Few effective changes of equipment can be 

 made in streams in which conductivity increases 

 greatly and there is no compensating reduction 

 of the surface area of the immersed electrodes. 

 The voltage gradient can be reduced by the in- 

 stallation of a voltage -dropping device (variable 

 transformer), by increasing the distance be- 

 tween the electrodes, or by decreasing electrode 

 surface area . All of the preceding means have 

 limitations. A disadvantage in a voltage -dropping 

 device is the impossibility of providing for sudden 



fluctuations in water levels . An increase 

 of distance between electrodes which would 

 be large enough to reduce mortality of fish 

 moving upstream and which still would 

 maintain an electrical field strong enough 

 to stop sea lampreys, could produce an 

 energized stretch of stream that would be 

 too long for the downstream migrant fish 

 to pass safely. Decreasing the area of the 

 immersed electrodes would require the 

 more expensive installation of a suspension, 

 but this appears to offer the most effective 

 procedure . 



On the basis of our present knowledge, 

 it is believed that the physical character- 

 istics are so varied and play such an 

 important part in the operation of the 

 control devices that modifications cannot 

 completely solve all existing problems. 

 The solution to the major problem, that of 

 excessive mortality of useful fish, will de- 

 pend on a careful study of physical factors 

 of individual streams so that control devices 

 may be properly designed to meet local 

 conditions . 



Literature Cited 



Applegate, Vernon C . 



1950. Natural history of the sea lam- 

 prey ( Petromyzon marinus) in 

 Michigan . U . S . Fish and Wild- 

 life Serv . , Spec . Sci . Rept . - - 

 Fish., No. 55, 237pp. 



Applegate, Vernon C, Bernard R. Smith, 

 and Matt Patterson. 

 1952. Sea lamprey spawning runs in 

 the Great Lakes, 1951. U.S. 

 Fish and Wildlife Service, Spec. 

 Sci. Rept. --Fish., No. 68, 37 pp. 



Loeb, Howard A . , and Albert E . Hall, Jr . 



1952. Sea lamprey spawning: Michigan 

 streams of Lake Superior. U.S. 

 Fish and Wildlife Serv. , Spec. 

 Sci. Rept. --Fish., No. 70, 67 pp. 



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