O'Farrell and Larson: Year-class formation in Clupea palllasi 



139 



periods of good survival in 2000 led to a much stronger 

 year class than that of 1999. Detecting a "match" of 

 favorable conditions that led to recruitment success was 

 not possible in our study because of the myriad factors 

 that can determine recruitment success. Rather than 

 attempting to explain the observed survival differences 

 with specific mechanisms, we suggest what may pos- 

 sibly contribute to the observed patterns. 



Larval survival 



The degree to which larval survival depends upon biotic 

 or abiotic factors is difficult to estimate. Fox (2001) 

 presented data showing that year-class strength in the 

 Blackwater stock of Atlantic herring {Clupea harengus 

 L.) was determined by survival after the egg stage. How- 

 ever, it is not clear whether variation in survival was due 

 to density-dependent or environmental factors. A recent 

 study has shown that salinity can affect larval survival 

 after hatching in San Francisco Bay herring (Griffin et 

 al., 20041. Here, the salinity during embryonic develop- 

 ment was a factor in yolksac larval survival in different 

 salinity treatments. Regardless of the form of mortality 

 operating on larvae, small changes in larval growth 

 rate can lead to large changes in levels of recruitment 

 (Houde, 1987). Faster larval growth results in shorter 

 larval stage duration and thus decreased exposure to 

 the characteristically high mortality of the larval stage. 

 Age at size for herring in this study decreased signifi- 

 cantly as the spawning season progressed both in 1999 

 and 2000. From this finding, we infer that positive 

 changes in growth rate occurred during the spring and 

 summer. Seasonal positive shifts in growth have also 

 been observed in Pacific herring populations in Prince 

 William Sound, Alaska, between the months of June and 

 October (Stokesbury et al., 1999). 



In 1999, the greatest number of recruits came from 

 mid to late-season spawning events. The late February 

 to early March spike in recruitment (Fig. 4 A) may be 

 partially explained by within-year growth variation. This 

 group of survivors appeared to be derived from a rela- 

 tively small number of eggs. Recruits from that spawn- 

 ing period grew significantly faster than recruits from 

 earlier spawning events. The largest spawning events 

 of the 1998-99 spawning season produced recruits that 

 grew slower than the recruits spawned in early March 

 and thus may have experienced lower relative survival. 



Within-year growth rate variation also partially ex- 

 plains the 2000 year class. The 2000 year class was 

 dominated by late season recruitment, primarily from 

 spawning in February 2000. Herring from spawning 

 events occurring between late October 1999 and mid- 

 January 2000 had a significantly higher median age at 

 length than herring produced from subsequent spawn- 

 ing times. This slow growth may in part explain the 

 near lack of recruitment from the two highest magni- 

 tude spawns occurring from 1 to 3 Jan 2000 and from 

 19 to 24 Jan 2000. However, age at length decreased 

 (and thus growth rate increased) for spawning events 

 occurring from late January 2000 to early March 2000. 



The timing of the growth rate switch (from slow to fast) 

 coincided closely with the spawning period producing 

 the greatest amount of recruitment. The general trend 

 of high levels of recruitment from late season spawning 

 events indicates that increased growth rate played a 

 role in the good survival during this period. However, 

 recruitment from very early spawning events and the 

 small number of recruits resulting from late March 

 2000 spawning was not explained solely by this within- 

 year growth variation. 



Egg mortality 



Variation in mortality during the egg stage may also 

 affect recruitment in San Francisco Bay herring. Fertil- 

 ization, embryonic development, and hatching success of 

 Pacific herring are strongly tied to environmental condi- 

 tions (Alderdice and Velsen, 1971, Griffin et al., 1998). 

 The optimal range for fertilization and development of 

 the San Francisco Bay population is between 12 ppt 

 and 24 ppt, and both percent fertilization and percent 

 hatching is maximized at 16 ppt (Griffin et al., 1998). 

 The herring spawning season in San Francisco Bay is 

 a time of rapidly changing salinities. High salinities 

 generally persist through the fall months. In winter, 

 rapid decreases in salinity due to freshwater from the 

 San Joaquin-Sacramento Delta, storm drain runoff and 

 local creek purges (Oda 3 ) are common, yet the magnitude 

 varies between years (Conomos et al., 1985). In the two 

 years examined, salinity during the winter spawning 

 season varied both above and below the optimum range 

 determined by Griffin et al. (1998). These salinity fluc- 

 tuations could have a large effect on the supply of larvae 

 into the San Francisco Bay system. 



Mortality during the egg stage can be exceedingly 

 high in Pacific herring due to predation and other biotic 

 interactions (Alderdice and Velsen, 1971; McGurk, 1986; 

 Rooper et al., 1999, Bishop and Green, 2001). As a re- 

 sult, egg incubation time may have a significant effect 

 upon eventual recruitment. The length of times of egg 

 incubation and the yolksac larval stage were combined 

 in our study and the combined period was given a con- 

 stant value of 14 days. In actuality, egg incubation time 

 (Taylor, 1971; McGurk, 1987) and embryonic develop- 

 ment (Alderdice and Velsen, 1971; Griffin et al., 1998) 

 are strongly linked to environmental factors and likely 

 have a significant effect upon recruitment before growth 

 rates can determine survival. Analysis of egg incubation 

 and yolksac larval duration for separate cohorts was not 

 performed in our study. It may, however, play a large 

 role in larval abundance. 



Conclusion 



The 1999 and 2000 spawning-date distributions indicate 

 that year classes can be shaped by periods of good and 



3 Oda, K. 2000. Personal commun. Calif. Dep. Fish and 

 Game, 411 Burgess Dr., Menlo Park, CA 94025. 



