ELECTRIC FISH SCREEN 111 



Seasonal variations will occur in the resistivity of the water in a stream. In some 

 streams this variation will undoubtedly be much larger than in others. One stream 

 that has been investigated has a seasonal variation in resistivity of 13 per cent at a 

 constant reference temperature. The maximum resistivity occurs during the period 

 of maximum run-off. Fortunately, electric fish screens do not need to be designed 

 within very close limits, and no trouble is anticipated from variations in the water 

 resistivity of a stream. 



DO FISH SENSE THE SOURCE OF AN ELECTRIC FIELD AND AVOID IT? 



Another fundamental fact that must be known is whether or not fish, swimming 

 into an electrified area, sense the direction of danger. In an effort to determine this 

 point several tests were made in the outside pools at the Bonneville hatchery. The 

 first four tests were made in a rectangular concrete pool 8 feet wide, 4 feet deep, and 

 48 feet long, including the spillway. Twelve feet of the lower end of this pool next 

 to the spillway were divided into two 4-foot channels by a tight partition of 1 by 12 

 inch boards. Facing downstream, the right-hand channel was selected to be pro- 

 tected by the electric screen. The arrangement of this pool and the second screen 

 used are shown in Figure 3. ■•'> < 



The first screen was made of four J/^-inch standard-pipe electrodes spaced 18 

 inches apart and in a single row, making an angle of 60 degrees in the direction of 

 stream flow. Alternate electrodes were connected and made the same electrical 

 polarity; this arrangement made adjacent electrodes opposite in polarity. About 

 200 chinook-salmon fingerlings 10 months old were liberated at the upper end of the 

 pool. When undisturbed they avoided the electrified area around the screen fairly 

 well, but when frightened they would swim through it into the protected channel. 

 This screen was defective in three ways, as will be shown later in the discussion of 

 the proper relation of the lines of current flow and equipotential surfaces with respect 

 to the direction of water flow and the plane of the protected opening and the iroper 

 size of electrodes. First, the lines of electric-current flow were in the wrong direction 

 with respect to the water flow; second, they were in the wrong direction with respect 

 to the protected opening; and third, the electrodes were too small in diameter. 



The second screen test was made in the same pool, and all of the conditions 

 were kept the same, except that the number of electrodes was increased to six and 

 the angle of the hue of electrodes with the direction of water flow was reduced from 

 60 to 30 degrees. Approximately 2,000 chinook-sahnon fingerlings 10 months old were 

 liberated in the open end of the pool and observed. They seemed to avoid the screen 

 fairly weU, although some would swim through into the protected channel. At 4.30 

 p. m. the protected area was cleared of fish and 30 volts left on the electrodes for the 

 night. The next morning at 8 o'clock 50 fingerhngs were in the protected area and 

 6 had been electrocuted during the 15.5 hours the screen had been on. The fish 

 were then driven back and forth past the screen, and they avoided the electrified area 

 fairly well but would dash through at times. This screen was still defective in regard 

 to the size of the electrodes. The direction of the fines of current flow, however, 

 had been improved by making them more nearly parallel with the direction of stream 

 flow and more nearly perpendicular to the plane of the protected opening. Leaving 

 all conditions except the screen as they were before, a third screen was constructed, 

 110928—28 3 



