130 



Abstract — Inter and intra-annual var- 

 iation in year-class strength was ana- 

 lyzed for San Francisco Bay Pacific 

 herring (Clupea pallasi) by using oto- 

 liths of juveniles. Juvenile herring 

 were collected from March through 

 June in 1999 and 2000 and otoliths 

 from subsamples of these collections 

 were aged by daily otolith increment 

 analysis. The composition of the year 

 classes in 1999 and 2000 were deter- 

 mined by back-calculating the birth 

 date distribution for surviving juve- 

 nile herring. In 2000, 729% more 

 juveniles were captured than in 1999, 

 even though an estimated 12% fewer 

 eggs were spawned in 2000. Spawn- 

 ing-date distributions show that 

 survival for the 2000 year class was 

 exceptionally good for a short (approx- 

 imately 1 month) period of spawn- 

 ing, resulting in a large abundance 

 of juvenile recruits. Analysis of age at 

 size shows that growth rate increased 

 significantly as the spawning season 

 progressed both in 1999 and 2000. 

 However, only in 2000 were the bulk 

 of surviving juveniles a product of 

 the fast growth period. In the two 

 years examined, year-class strength 

 was not predicted by the estimated 

 number of eggs spawned, but rather 

 appeared to depend on survival of 

 eggs or larvae (or both) through the 

 juvenile stage. Fast growth through 

 the larval stage may have little effect 

 on year-class strength if mortality 

 during the egg stage is high and few 

 larvae are available. 



Year-class formation in Pacific herring 

 (Clupea pallasi) estimated from 

 spawning-date distributions of juveniles 

 in San Francisco Bay, California 



Michael R. O Fan ell 

 Ralph J. Larson 



Department of Biology 



San Francisco State University 



1600 Holloway Avenue 



San Francisco, CA 94132 



Present address (for M. R. O'Farrell, contact author): Department of Wildlife. 



Fish and Conservation Biology 

 University of California, Davis 

 One Shields Avenue 

 Davis, California 95616 



E-mail address (for M R O'Farrell)' mrofarrellffi'ucdavis edu 



Manuscript submitted 27 February 2003 

 to the Scientific Editor's Office. 



Manuscript approved for publication 

 2 August 2004 by the Scientific Editor. 



Fish. Bull. 103:130-141 (2005). 



Both biological and physical sources 

 of mortality have been suggested as 

 important in determining year-class 

 strength in fish populations. Food lim- 

 itation at first feeding (Hjort, 1914; 

 Cushing, 1975; Lasker, 1975; Cushing, 

 1996), larval retention (lies and Sin- 

 clair, 1982; Sinclair and lies, 1985), 

 a juvenile critical period (Bollens et 

 al„ 1992; Thorisson, 1994), as well as 

 predation and environmental condi- 

 tions may ultimately affect recruit- 

 ment. Egg development time and 

 larval growth rate have the capacity 

 to adjust the relative impacts of these 

 mortality sources on individual prop- 

 agules by modifying stage duration 

 (Houde, 1989; Yoklavich and Bailey, 

 1990). 



Juvenile fishes can be used to as- 

 sess both inter- and intra-annual 

 variation in egg and larval survival. 

 Interannual variation in year-class 

 strength is often inferred from mea- 

 sures of juvenile abundance (e.g., 

 Baxter et al., 1999). In addition, when 

 the total number of eggs spawned is 

 known, juvenile abundance can be 

 used to assess overall variation in 

 egg and larval survival. Intra-annual 

 variation in egg and larval survival 

 can be estimated from the birth-date 

 distribution of surviving juveniles, 

 as determined from otolith daily in- 

 crement analysis. Particularly when 

 data on actual spawning-date distri- 



butions are available, the birth date 

 distribution of survivors can be used 

 to identify periods of spawning that 

 contributed differentially to juvenile 

 recruitment (Methot, 1983; Rice et 

 al., 1987; Yoklavich and Bailey, 1990; 

 Moksness and Fossum, 1992; Fox, 

 1997; Takahashi et al., 1999). 



Recruitment of juvenile Pacific her- 

 ring (Clupea pallasi) varies interan- 

 nually by over an order of magnitude 

 in San Francisco Bay (Baxter et al., 

 1999) and is the culmination of sever- 

 al processes. Schools of adult herring 

 enter San Francisco Bay in discrete 

 batches during the fall and winter. 

 These schools shoal and deposit eggs 

 and milt during spawning events that 

 often correspond to the quarter moon 

 phase. Spawning events can vary in 

 duration from approximately one day 

 to one week, and simultaneous events 

 may occur at different spawning sites 

 throughout the bay. Herring lay adhe- 

 sive eggs intertidally and subtidally 

 on rocks, algae, aquatic plants, pier 

 pilings, and other substrates (Alderd- 

 ice and Velsen, 1971; Hay, 1985). Eggs 

 can experience extremely high mortal- 

 ity due to predation (McGurk, 1986; 

 Bishop and Green, 2001), suboptimal 

 temperature and salinity conditions 

 (Alderice and Velsen, 1971; Griffin et 

 al., 1998), as well as reduced hatch- 

 ing and developmental abnormalities 

 associated with certain substrate se- 



