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



137 



160 

 140 - 



S 120 - 



E 



o 

 5 



15 162 



160 

 140 - 

 120 - 

 100 



80 



60 



40 

 3/1/99 4/1/99 5/1/99 6/1/99 7/1/99 



B 



\ 



36 Jb 



40 

 3/1/00 4/1/00 5/1/00 6/1/00 7/1/00 



40 



D 



i —  1 



NL 



9/1/98 11/1/98 1/1/99 3/1/99 5/1/99 



9/1/99 11/1/99 1/1/00 3/1/00 5/1/00 



Spawning date 



Figure 5 



The upper panels display otolith increments present in 40 mm-50 mm juvenile 

 herring {Clupea pallasi) arranged by capture date for (A) 1999 and (B) 2000. 

 Boxes represent median number of otolith increments, bars indicate ±1 SD, 

 and the number above each point is the sample size. The lower panel displays 

 growth histories for juvenile herring originating from various periods within 

 the spawning season for (C) 1999 and (D) 2000. Boxes represent median spawn- 

 ing-dates and bars represent range of spawning dates at that growth rate. The 

 spawning-date distribution (uncorrected for juvenile mortality) is superimposed 

 upon C and D to ascertain how changes in growth are reflected in survival to 

 the juvenile stage. 



specimens recruiting from earlier spawning periods. 

 The period of greatest recruitment occurred during the 

 slower growth period in 1999 (Fig. 5C). 



In 2000, 40 mm-50 mm juvenile herring were caught 

 in five survey legs conducted during three months (April, 

 May, and June). The data are displayed by survey leg; 

 pooling the data by month, however, does not change the 

 result. Median increment counts differed significantly 

 for the 2000 surveys (Fig. 5B; #=76.39, P<0.0001). Oto- 

 lith increment counts for 40 mm-50 mm specimens did 

 not differ for the 5 April 2000 and 10-11 April 2000 

 surveys. However, the age at length for these surveys 

 was significantly greater than for the three later survey 

 legs (10 May 2000, 22-24 May 2000, and 6 June 2000), 

 which did not significantly differ from each other. Her- 

 ring caught in the three later surveys grew significantly 

 faster than herring caught in the two earlier surveys. 

 The significant decrease in age at length indicates that 

 juvenile herring that were a product of spawning be- 



tween 15 January 2000 and 18 March 2000 grew faster 

 than specimens recruiting from earlier spawning events. 

 The majority of juvenile recruits in 2000 were a product 

 of the fast growth period (Fig. 5D). 



Accuracy of growth-rate estimates determined from 

 growth increments on otoliths 



The above analyses depended upon the assumption that 

 increments were deposited daily in the otoliths exam- 

 ined. Two lines of evidence point to the validity of this 

 assumption. First, back-calculated spawning-dates gen- 

 erally agreed with the known spawning season of San 

 Francisco Bay herring, and several peaks in back-cal- 

 culated spawning dates match known spawning events 

 quite closely (Fig. 4, A and B). 



Second, juvenile growth rates appear to be high 

 enough for daily growth (McGurk, 1984b). Clear length- 

 frequency modes were visible for three sampling events 



