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Fishery Bulletin 103(1) 



in 2000. Assuming linear growth between these time 

 periods, the advancement of these length-frequency 

 modes resulted in growth rates of 0.75 mm/d (Fig. 6, 

 arrow in A), 0.83 mm/d (arrow in B), and 0.64 mm/d 

 (arrow in C). McGurk (1984b) demonstrated daily in- 

 crement deposition in herring if the larval growth rate 

 exceeded 0.36 mm/d. Our data did not allow us to es- 

 timate growth rates of larvae; however, the estimated 

 juvenile growth rates presented above are much greater 

 than necessary for daily increment deposition. 



Discussion 



Catches of juvenile herring were much greater in 2000 

 than in 1999. Between the months of March and June 

 2000, cumulative CPUE was more that seven times 

 greater than during the same period in 1999, yet an 

 estimated 12% more eggs were deposited during the 



10 May 2000 



20 



30 40 50 60 



Standard length (mm) 



Figure 6 



Length frequencies for juvenile herring (Clupea pallasi) 

 captured on 10 May 2000, 22 May 2000, and 6 June 2000. 

 Arrows represent the estimated propagation of length modes 

 through time. Linear growth rates, calculated from each 

 trajectory, are as follows: A=0.75 mm/d; trajectory B = 0.83 

 mm/d); and C = 0.64 mm/d. 



1988-99 spawning season. Because observed differences 

 in recruitment between 1999 and 2000 far exceeded dif- 

 ferences in the total eggs spawned, differential survivor- 

 ship during the egg or larval stages (or both) must be 

 responsible for disparate year-class strengths. 



The spawning-date distributions presented for 1999 

 and 2000 did not contain all herring caught by the mid- 

 water trawl survey between the months of March and 

 June. Because they could not be accurately assigned 

 ages with an age-length key (Table 1), 178 herring were 

 omitted from the distribution in 1999. Most specimens 

 omitted from this distribution were caught in the early 

 April 1999 and late June 1999 survey legs. As a result, 

 the spawning-date distribution likely underestimated 

 the recruitment from very early and very late season 

 spawnings. In 2000, 208 specimens, from a variety of 

 survey legs, were omitted from the spawning-date dis- 

 tribution (Table 1). Because a large number of herring 

 were caught in 2000, it is unlikely that these omissions 

 would significantly change the shape of the spawn- 

 ing-date distribution. The loss of data in this case 

 does not change the overall result of large year-class- 

 strength variation. 



The noncontinuous sampling schedule for juve- 

 niles may have resulted in either an underestima- 

 tion or overestimation of CPUE and thus year-class 

 strength. In several months, the mid-water trawl sur- 

 vey was conducted over two legs separated by several 

 days (Table 1, Fig. 3). This noncontinuous sampling 

 could have produced error in our estimates because 

 aggregations of juveniles, through movement between 

 areas, could conceivably have escaped detection by 

 trawls (resulting in CPUE underestimation) or have 

 been sampled twice in the same month (resulting in 

 CPUE overestimation). However, O'Farrell (2001) 

 showed that dispersal of herring from a successful 

 spawning event could occur through much of San 

 Francisco Bay. Therefore, we do not believe that ag- 

 gregations of juveniles were completely missed by 

 the mid-water trawl survey. The degree to which ag- 

 gregations of juveniles were sampled more than once 

 in a sampling month is not known. 



Variation in age estimates undoubtedly produced 

 back-calculated spawning-dates that did not match 

 exactly with true spawning dates. Yet, for some 

 spawning events, very good matches between back- 

 calculated and reported spawning events indicate 

 that the age estimations were accurate for many of 

 the cohorts examined (O'Farrell, 2001). Other cohorts 

 that did not match as well with reported spawnings 

 may be the result of 1) a spawning event undetected 

 by the spawn-deposition study, 2) a small, "spot" 

 spawning that did not qualify as a true spawning 

 event for the spawn-deposition study, or 3) very slow 

 or fast growth through a portion of the larval life 

 history that interrupted daily increment deposition 

 (McGurk, 1984b, 1987). 



Increased survival did not occur throughout the 

 entire 2000 spawning season. Instead, periods of 

 good survival and poor survival were present, yet the 



