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



131 



lection (Vines et al., 2000). Larvae hatch from eggs 

 after an incubation period, and the San Francisco Bay 

 estuary can serve as a larval nursery area until after 

 metamorphosis into the juvenile stage (Hay, 1985). 



Our objectives were 1) to identify periods in the 

 spawning season that lead to successful (or unsuc- 

 cessful) juvenile recruitment and 2) to evaluate larval 

 and juvenile growth variation for two herring year 

 classes. We used otoliths of juvenile herring from the 

 1999 and 2000 year classes to back-calculate spawn- 

 ing-date distributions and determine spawning times 

 that lead to successful recruitment. Distributions of 

 spawning were obtained from management surveys. 

 Growth was then evaluated to determine its role in 

 year-class formation. 



Methods 



Surveys 



All information on adult herring spawning events and 

 juvenile herring specimens were obtained from ongoing 

 monitoring and management surveys conducted by the 

 California Department of Fish and Game (CDFG). 



Data on timing, location, and magnitude of her- 

 ring spawning events for the 1998-99 and 1999-2000 

 spawning seasons were obtained from the herring 

 spawn survey conducted by the California Department 

 of Fish and Game (CDFG). The survey is conducted 

 from November through March throughout central San 

 Francisco Bay, the area of most herring spawning (Wat- 

 ters et al., 2004). The central bay region is searched for 

 herring spawning on a daily basis from a small boat, 

 and the entire spawning region is covered at least once 

 per week. Eggs are located visually at low tide and 

 by rake in shallow subtidal areas. When a spawning 

 area is located, the number of eggs per square meter 

 is measured from a subsample of the spawning area 

 and is expanded to an estimate of total eggs spawned 

 (for spawning survey method details, see Spratt, 1981; 

 Watters et al., 2004). At the end of the 1998-99 and 

 1999-2000 spawning seasons, information on date, 

 location, spawning area, average eggs/m 2 , total eggs, 

 and the spawning biomass estimate was provided for 

 the purpose of this study (Watters 1 ). 



Juvenile (age-0) herring were sampled monthly from 

 30 stations in San Francisco Bay aboard the RV Long- 

 fin as part of CDFG's Bay/Delta Division's Bay study 

 (Fig. 1). Each station was visited once a month and 

 juvenile herring were retained from catches during 

 the months of April- June 1999 and March- June 2000. 

 Stations were sampled by mid-water trawl with a 3.7-m 2 

 mouth and 1.3-cm mesh codend, towed against the cur- 

 rent, for 12 minutes. Volume of water filtered was cal- 

 culated by using a flowmeter and was used to calculate 



-122°30'W 



38'00'N 



37°30'N 



1 Watters, D. 2000. Personal commun. Calif. Dep. Fish 

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



Figure 1 



Midwater trawl sampling stations in 

 San Francisco Bay. 



catch per unit of effort (CPUE) for each station. Juve- 

 nile herring were measured onboard, sorted from the 

 catch, kept on ice, and transported to the laboratory, 

 where they were frozen. Relative recruitment in each 

 year was calculated by summing the CPUE at each sta- 

 tion for the months of March-June in 1999 and 2000. 



Otolith preparation and analysis 



Frozen juvenile herring, separated by date and station, 

 were thawed in batches and all fish were re-measured 

 for standard length to the nearest mm. If the catch was 

 small at a particular station (less than approximately 

 10 individuals), all specimens from that station were 

 reserved for otolith analysis. If the catch was large, a 

 subsample of the measured catch was reserved for otolith 

 analysis. Subsampling consisted of randomly selecting 

 at least two specimens from each 1-mm length bin in 

 the catch. 



Both sagittal otoliths were extracted from each fish, 

 cleaned with fresh water, and transferred to a micro- 

 scope slide where they were allowed to dry. When com- 

 pletely dry, both otoliths were mounted on the slide, 

 convex side up, with clear nail polish. 



Otoliths were read with a compound microscope. Be- 

 cause otoliths were too thick to allow sufficient light 

 transmission for increment reading, all otoliths were 



