FISHERY BULLETIN: VOL. 81, NO. 4 



The birth dates calculated in this study are actually 

 dates of onset of increment formation. The northern 

 anchovy larvae deposit the first increment at about 

 the end of yolk absorption, the fifth day after hatch- 

 ing at 16°C (Brothers et al. 1976). This is close to the 

 mean age of larvae used to estimate larval abun- 

 dances so no constant was added to the juveniles' 

 ages when calculating birth dates. 



The fish in the present study usually had 150-400 

 increments but the daily deposition of increments in 

 northern anchovy otoliths has been confirmed only to 

 100 d in the laboratory (Brothers et al. 1976). The 

 accuracy of my interpretation of daily increments in 

 juveniles was tested by comparing birth- date dis- 

 tributions calculated from early samples with dis- 

 tributions calculated from late samples. The 

 distributions should be indistinguishable if the sam- 

 ples were of the same cohort and mortality during the 

 period was not age selective. In addition to the 

 December 1978 samples used in this study, samples 

 were collected at San Diego in September 1978 and 

 February 1979 (Table 2). The three birth-date dis- 

 tributions were compared by the Smirnov test for dif- 

 ferences in cumulative probabilities (Conover 1971, 

 p. 309). The September and December distributions 

 were very similar (maximum difference = 0.105, 

 P < 0.2) and the February distribution was also not 

 significantly different from September's (0.243, P 

 < 0.02). This test is sensitive to aging errors of the 

 same magnitude as the precision of the ages. If ages 

 of February's fish had been overestimated by 15 d 

 (one- half of fish in each month shifted to the following 

 month) the difference between September and Feb- 

 ruary would have increased to 0.376, P< 0.1. Al-mo 

 error in aging the February juveniles would have 

 made the September 1978 to February 1979 com- 

 parison highly significant (P< 0.01). I conclude that 

 any bias in aging must be less than about 15 d. 



were selected to span a wide size range for an analysis 

 of seasonal patterns of juvenile growth (Methot 

 1981). A less biased estimate of the birth- date dis- 

 tribution was obtained from the size- frequency dis- 

 tribution of a large sample of juveniles and a 

 size/birth-date nomograph (Fridriksson 1934; 

 Kimura 1977). In each year's nomograph, birth-date 

 frequencies (by month) were calculated for fish in 

 each 10 mm size interval All samples within each 

 year were combined in that year's nomograph. 



RESULTS 

 Larval Abundance 



The temporal distributions of northern anchovy lar- 

 vae differed between the two years (Table 3). The 

 maximum abundance occurred in February-March of 

 each year but the peak was greater in 1978. Larvae 

 were much more abundant during May 1979 than 

 during May 1978. The average larva in 1979 was in 

 water 1°C colder than the average larva in 1978 and 

 was further offshore (Table 3). Larval production per 

 30-d date interval was calculated by numerical 

 integration of the area under the dashed lines in 

 Figures 2 and 3. Total larval production during 

 January-May 1979 was 2.1% greater than during the 

 same period in 1978. 



Size and Birth-Date Distributions 



In 1978 and 1979 northern anchovy juveniles, 

 collected north of Pt. Dume, were typically larger 

 (Table 4) and had been born earlier (Table 5) than 

 juveniles collected to the south. The size/birth-date 

 nomograms (Table 6) applied to the juvenile size dis- 

 tributions produced birth-date distributions (Table 

 7) with peaks in March-April for southern fish and 



Table 2. — Birth- date frequency of juvenile northern anchovy col- 

 lected at San Diego between September 1978 and February 

 1979. 



N samples: 



/Vfish: 



Length (mm): 



Sept-Oct. 1978 

 2 

 28 



Dec. 1978 



2 



19 



Feb. 1979 

 1 

 15 



Birth-Date Distribution 



The selected specimens produce a biased estimate 

 of the juvenile birth-date distribution because they 



TABLE 3. — Abundance of northern anchovy larvae. Value in 

 parentheses is the fraction of the abundance that was inter- 

 polated. N samples exclude offshore samples with no larvae. Date, 

 distance offshore, and temperature at 1 m were weighted by larval 

 catch at each station. 



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