The largest concentration of eggs occurred in San Joaquin 

 River from Antioch to the mouth of Old River. Next in importance 

 were Old River and Middle River, which will be the two principal 

 channels carrying transferred Sacramento water to the Tracy Pump- 

 ing Plant. San Joaquin River in the vicinity of Mossdale was also 

 found to be a spawning area of significant importance. Very few eggs 

 were taken during either year from the Kokelumne River area and 

 Sacramento River downstream from Walnut Grove. In 19U7 and 19U8 

 Calhoun (19U8) recovered considerable numbers of eggs in Sacramento 

 and Feather Rivers and at their confluence^ establishing them as 

 .spawning grounds of importance. 



The area of densest larval bass concentration was in San Joaquin 

 River between Three Mile Slough and Antioch. False River and San 

 Joaquin River below the mouth of Mokelumne River yielded repre- 

 sentative samples of larvae. Significant numbers of larval bass were 

 taken from Sacramento River downstream from Walnut Grove, confirm- 

 ing the importance of the upper river spawning grounds. Comparatively 

 small numbers of larvae were taken in the southern Delta regions. 



Sufficient information has been collected to emphasize the impor- 

 tance of the several Delta areas as nursery grounds for juvenile striped 

 bass. The distribution of youn^ bass was general in the western and 

 central sections of the Delta by the middle of July in 19u9j the 

 importance of these regions, including Old River and Middle River up- 

 stream to the Santa Fe Bridge, is revealed by the abundance of juvenile 

 striped bass collected in those areas. The southern Delta region, which 

 is a spawning ground of importance, did not approach the areas mentioned 

 above in numbers of juveniles present j however, young bass were present 

 in sufficient numbers to indicate that it was also a nursery ground of 

 consequence. 



Water temperature appears to have exerted a very important influence 

 both in determining the time of spawning and the rate of development of 

 larval and post-larval striped bass in the Delta. Spawning activity as 

 reflected by collections of eggs was seen to have been related to water 

 temperatures of $8 degrees and higher, with peak spawning occurring 

 when temperatures ranged between 60 degrees and 67 degrees. The influ- 

 ence of warmer waters in speeding the growth of larval and juvenile 

 bass can be seen by reference to Fig. 10, which brings out some dif- 

 ferences between 19h8 and 19u9 populations. The spring water tempera- 

 ture changes of 19U9 were about four weeks earlier than those of 19^8. The 

 date on which average Delta water temperatures attained 65 degrees in 

 19h8 was June 19 j in 19u9 this occurred on May 23 » The first appearance 

 of juvenile bass in tow-net collections in 19U8 was near July 2, approx- 

 imately two weeks following the rise of water temperature above 65 degrees. 

 In 19h9) juveniles were taken on June 6, also two weeks after temperatures 

 reached 65 degrees in that year. This suggests that water temperatures 

 have determined to a great extent the time of spawning and the speed of 

 development of immature bass in the Delta. 



29 



