140 



Fishery Bulletin 103(1) 



poor survival lasting shorter than the duration of the 

 spawning season, yet longer than the duration of an 

 individual spawning event. The distributions indicated 

 that variation in survivorship was not only a function 

 of individual spawn success. Rather, periods of good 

 and poor survivorship in 1999 and 2000 were of longer 

 duration than one spawning event. The period of excep- 

 tionally good survival that led to the majority of the 

 strong 2000 year class was approximately one month in 

 duration and incorporated several spawning events. Yet 

 this window of good survival was much shorter than the 

 entire 2000 spawning season. Variation in survivorship 

 between individual spawnings may be less important in 

 shaping the year class than survivorship variation on 

 a longer time scale. 



Visual examination of the spawning-date distribu- 

 tion superimposed upon juvenile age at length indicate 

 that faster growth had a positive effect on recruitment 

 in 2000, and a negligible effect in 1999. For larval 

 growth to affect recruitment, larvae must be available 

 from hatching eggs. Year-class strength variation in 

 Pacific herring could depend upon both egg and larval 

 survival. 



The timing of peak herring spawning in San Fran- 

 cisco Bay may be a tradeoff between maximizing larval 

 growth rates and spawning when hydrographic condi- 

 tions are optimal for embryonic development. In the two 

 years examined, growth rate increased with the progres- 

 sion of the spawning season. It follows that the herring 

 population could maximize recruitment by spawning 

 later so that larvae grow faster. However, because delta 

 outflow is generally high in February and March on 

 account of winter storms, late season spawning may ex- 

 pose eggs to low salinities and thus decreased hatching 

 rates. Peak spawning may occur in January as a trade 

 off between growth-rate and egg-hatching success. 



Acknowledgments 



This research would not have been possible without the 

 extensive cooperation of the California Department of 

 Fish and Game Belmont and Stockton offices. In par- 

 ticular, we would like to thank Diana Watters, Ken Oda, 

 Sara Peterson, Kathy Hieb, Kevin Fleming, Tom Greiner, 

 Suzanne Deleon, and the entire crew of the RV Longfin. 

 Stephen Bollens, Steven Obrebski, Ken Oda, and three 

 anonymous reviewers provided very helpful comments 

 on various drafts of this manuscript. 



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