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Fishery Bulletin 103(1) 



ground with 2000 grit sandpaper. Otoliths were al- 

 ternately ground and examined under the microscope 

 at 100 x to ensure that the section was thin enough to 

 allow sufficient light transmission, yet not over-ground 

 so that the edges of the otolith were lost. 



Daily increment deposition in herring begins at 

 yolksac absorption, corresponding with the first heavy 

 ring near the nucleus (Geffen, 1982; McGurk, 1984a; 

 McGurk, 1987; Moksness and Wespestad, 1989). This 

 heavy ring was located in all herring examined and 

 increment counts were initiated there. Increment counts 

 were made at 1000 x (with an oil immersion objective) 

 and 400x (without oil immersion) magnification along 

 the axis of maximum resolution. All increments were 

 counted from the first heavy ring until the last ring on 

 the edge of the otolith. 



Several days after the first reading, the same reader 

 performed a reading on the second otolith. If the two 

 increment counts differed by more than a value of 7, 

 a third reading was conducted at a later date on the 

 highest quality otolith. If the three increment counts 

 differed from each other by more than a value of 7, 

 otolith data from that fish were not used in further 

 analyses. Where two readings differed by 7 or fewer in- 

 crements, the final increment number for each fish was 

 determined by averaging the two increment counts. 



Daily otolith increment deposition has been demon- 

 strated in Pacific herring larvae reared in captivity 

 (McGurk, 1984a; Moksness and Wespestad, 1989) and 

 in the field (McGurk, 1987). In our study, otolith in- 

 crements were assumed to be deposited daily and the 

 validity of this assumption is treated in the "Results" 

 and "Discussion" sections. Precision of otolith incre- 

 ment counts was determined by computing the average 

 percent error for each otolith examined (Beamish and 

 Fournier, 1981). 



Spawning-date distributions 



Spawning-date distributions were constructed from 

 specimens retained for otolith analysis in 1999 and 

 2000. Distributions were calculated 1) by adding a con- 

 stant of 14 days to the otolith increment count and 2) 

 by subtracting that value (otolith increments+14) from 

 the Julian date of capture. Because Pacific herring begin 

 daily increment deposition at yolksac absorption, the 

 constant of 14 days was added to the increment value to 

 account for egg incubation and the yolksac larval period. 

 Taylor (1971) reported a 9-day egg incubation period 

 for a British Columbia Pacific herring stock between 

 13.4°C and 13.8°C. For San Francisco Bay spawned 

 herring, Griffin et al. (1998) found developmental rate 

 to be influenced by salinity; the greatest hatching rate 

 occurred 10 days after fertilization at a salinity of 14 

 ppt. Yolksac absorption occurs in Pacific herring 4-7 

 days after hatching (McGurk, 1987; Griffin et al., 2004, 

 and references therein). The final value of 14 days for egg 

 incubation and yolksac absorption used in our study was 

 determined 1) from laboratory-derived values reported 

 for British Columbia (Taylor, 1971; McGurk, 1987) and 



San Francisco Bay (Griffin et al., 1998) herring popu- 

 lations and 2) by visually matching back-calculated 

 spawning-date distributions with the observed spawn- 

 ing-date distribution from the CDFG spawn-deposition 

 survey. 



The back-calculated spawning-date distributions 

 determined from specimens used for otolith analy- 

 sis were extrapolated to include as many herring as 

 possible caught in the juvenile surveys of 1999 and 

 2000. Length-frequency distributions were converted 

 to spawning-date distributions by using age-length 

 keys. Separate age-length keys were constructed for 

 each survey in both 1999 and 2000. In some cases, 

 the monthly survey was split into two legs separated 

 by several days. When the monthly survey was split 

 into legs, separate age-length keys were constructed 

 for each leg. 



It was not possible to fit all herring caught between 

 the months of March and June into age-length keys 

 because some samples were inadvertently discarded 

 after measurement in the field. If the range of lengths 

 in the discarded samples extended beyond the sizes of 

 samples aged, a complete age-length key could not be 

 constructed. To avoid ascribing a possibly inaccurate 

 age to a fish outside the size range of the age-length 

 key, those fish were not included in the spawning-date 

 distribution. Table 1 displays the number of herring 

 caught in each leg, the number of otoliths used to con- 

 struct the age-length key for that survey leg, and the 

 total number and proportion of juveniles caught that 

 are represented in the spawning-date distribution. The 

 number of juveniles caught was greater than the num- 

 ber of juveniles in the spawning-date distribution for 

 all but one survey leg. This discrepancy was due to 

 discarded fish (in the field) with lengths not within 

 the range of the age-length key constructed from the 

 subsampled individuals. 



Mortality estimate corrections are often superimposed 

 upon spawning-date or hatching-date distributions to 

 account for different size juveniles captured (Methot, 

 1983). Presumably a larger juvenile is older, and thus 

 has been exposed to mortality factors for a longer pe- 

 riod of time than has a smaller juvenile. The lack of a 

 correction for juvenile mortality can lead to an under- 

 representation of larger juveniles in the distribution. 

 Because of the noncontinuous mid-water trawl sampling 

 schedule, mortality rates could not be estimated from 

 the data used in our study. As a result, mortality cor- 

 rections were calculated by using an instantaneous 

 mortality rate value of 0.016/d, corresponding to the 

 greater of two mortality rates calculated from juve- 

 nile Pacific herring in Prince William Sound, Alaska 

 (Stokesbury et al., 2002). 



Spawning-date distributions were corrected for mor- 

 tality by calculating abundance at age 100 days (N 100 ). 

 For fishes aged at less than 100 days: 



M - M e-0.0161100 -al 

 JV 100 _ JV n e ' 



(1) 



where a is the age of the fish in days. 



