FiGi'KE 10. — Revolving fish screens in irrigation canal. Waterflow from right to left. 



Oreo-on, and California have in- 

 stalled or have re({iiired the instal- 

 lation of hundreds of screens to pre- 

 vent entrance of the j^oung fish into 

 the irrigation ditches. On some of 

 the very large Federal irrigation de- 

 velopments such as the Delta-Men- 

 dota diversion, at Tracy, Calif., 

 complicated fish-screen installations 

 are required. Such screens save 

 many thousands of young fish an- 

 nually. In many streams, however, 

 the diversion of water for irrigation 

 has resulted in the complete loss of 

 spawning areas, as the water re- 

 maining has been insufficient to sup- 

 port fish life. Often artificial prop- 

 agation must be relied upon to re- 

 ])lace these losses. 



Probably the greatest threat to 

 the resource is in the contemplated 

 construction of power dams in the 

 streams of Alaska and British Co- 

 lumbia, where tremendous hydro- 



power potentials exist. Piecemeal 

 reduction of available spawning 

 areas has been most harmful. The 

 construction of a dam on a relatively 

 small stream usually is not consid- 

 ered particularly harmful to the 

 overall salmon production, but in 

 the aggregate these projects destroy 

 major portions of the resource. It 

 is anticipated that large segments of 

 the remaining salmon populations 

 will be destroyed by the construc- 

 tion of dams, and that the salmon 

 hatchery will be necessary for par- 

 tial preservation of some of the af- 

 fected populations. 



A comprehensive research pro- 

 gram is expected to determine 

 means by which adult salmon may 

 be passed over dams without delay 

 and means by which the young sal- 

 mon may proceed downstream with- 

 out suffering substantial mortalities 

 in passage through turbines or over 



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