Fish and Fishermen 235 



In the San Joaquin River unit of California's Central Valley 

 Project, to take an outstanding example, so much of the 

 water is withheld from the natural channel for these two 

 purposes that it becomes dangerously low and as a result 

 dangerously warm. Furthermore the young fish tend to fol- 

 low the main flow, which leads them out into the diversion 

 canals and eventually to the farmers' fields. Fish screens to 

 prevent just such mishaps were invented long ago and have 

 been in use for years. Cylinders of fine wire mesh which re- 

 volve in such a manner as to pass debris over the top, let the 

 water through, and hold the fish, can be built, but on large 

 diversions they are extremely costly to install and to main- 

 tain. Electrical "screens" can be built, devices which produce 

 a shock strong enough to repel fish without killing themj 

 but when little fish are seized with the desire to go to sea 

 they are apt to keep making attempts at such a screen until 

 they become paralyzed, so that they drift with the current 

 through the electric field and into the diversion ditch. 

 Supersonics have yet to be tried. Maybe these high-frequency 

 waves, which are said to induce in human beings almost un- 

 bearable discomfort and fear, will have a more powerful 

 effect on the fish than the call of the sea. 



Whatever the type of "screen" and however effective it 

 may be, an indispensable requisite is a place for fish to go 

 other than through the screen — a flow of water leading 

 toward safety and a life in the ocean rather than toward 

 danger and a death in the alfalfa fields. Even when an alter- 

 nate path is provided the little fish may accumulate in front 

 of the screen if most of the water is flowing through it. To 

 overcome this attraction of the major current and to guide 

 them to where they should go even though it be a compara- 

 tive trickle, moving lights are being tried. While this may 

 sound at first like the product of an unbalanced mind, It is 

 based on the following reasoning: 



