type cf traveling water screen can hold its clearance to .159 inches , 

 the theoretical opening of the 5x5 to-the-inch mesh of No. 19 wire. 

 There would be no advantage in reducing the screen size to less than t he 

 limits of these clearances. Sufficient information has been gained from 

 other projects where traveling water screens are used to prove that sea- 

 ward migrating salmon are screenable with ^S to-the-inch mesh. It' is 

 recognized that the eggs and immature forms of striped bass and larval 

 shad cannot be successfully screened by this size of mesh. 



In order to determine the size at which juvenile striped bass could 

 be screened, a weekly series of tows testing the effectiveness of 5x5, 

 No. 19 hardware cloth in the screening of striped bass juveniles were 

 completed betweer June 20 and July 22, 19U9- The method used in deter- 

 mining the screenable size of the young bass was to secure a 12-1/2 

 square foot circular section of screen, four feet from the mouth and 

 inside the cone of a five-foot standard tow net. This apparatus was 

 towed through the water at 2.5 feet per second, which is approximately 

 the calculated velocity of the current in the approach canal to the Tracy 

 Pumping Plant. The results revealed that the maximum length of striped 

 bass passing through the screen was 26mm (1 inch)j however, only one 

 individual of this size was found in the net behind the screen. Bass 

 20mm (0.8 inch) and less in length passed through the screen consistently, 

 while many individuals 20-25mm (0.8 to 1 inch) were stopped by the screen. 

 The following table shows the calculated screening efficiency of $x$ 3 No. 

 19 hardware cloth as derived from a regression of mean length of striped 

 bass upon percentage screened? 



MEAN LENGTH PER CENT SCREENABLE 





The same tests were planned for learning the screenable size of 

 shad, but lack of sufficient time prevented fulfillment of plans. However, 

 preserved specimens of shad juveniles 32mm and over could not be forced 

 through the S^S No. 19 screen meshes. 



Figure 12 presents growth curves of mean and minimum lengths of 

 salmon, shad, and striped bass, plotted in relation to mean size screen- 

 able determined as described above. Examination of this figure shows 

 that with few exceptions, king salmon juveniles are screenable at all 

 times during their period of residence in the Delta. It also demonstrates 

 the particular vulnerability of shad, and to a lesser degree, striped bass. 



It is evident that large portions of the populations of larval shad 

 and striped bass may fall within the influence of the pumps &r I pass 

 through S'x.'-) mesh screen. This suggests that intensive study be made to 

 determine if there are means of affording them protection. Additional 



12 



