FISHERY BULLETIN: VOL. 81. NO. 4 



TABLE 6. — Size/birth-date nomograms stratified by 10 mm size interval and 

 30-d date interval (labeled by approximate month). 



TABLE 7.— Birth- date distributions calculated from size/birth-date nomograms (Table 6) and regional 

 juvenile size distributions (Table 4). Distributions are presented as %/30-d date intervals (labelled as 

 approximate months). Combined birth- date distribution is mean of regional distributions with weight- 

 ing factors proportional to nearshore shallow area (Table 4). Mortality correction factor accounts for 

 the greater duration that the early born fish are exposed to juvenile mortality (see text). After multiply- 

 ing the combined distributions by the mortality correction factors, the distributions are presented only 

 for those months with larval abundance data. 



Combining the regional results to produce an 

 overall juvenile birth- date distribution is prob- 

 lematic, especially in 1978 when no samples were 

 collected south of San Diego. Each region's weight- 

 ing factor should be proportional to the abundance of 

 juveniles in the region. Because local abundance of a 

 pelagic schooling fish is measured crudely by a trawl 

 survey, the areas of the primary juvenile habitat 

 (Table 4) were used as weighting factors. The north 

 region has only 16.6% of the total area nearshore of 

 the 90 m (50 fathom) isobath, so contributes little to 

 the total. Although the north region contributed 

 nearly 50% of the total area from which samples were 

 obtained in 1978 (Table 4), I assume that the un- 

 sampled fish from Baja California had birth dates 

 similar to those of San Diego-Pt. Dume fish so I use 



16.6% for the north's weighting factor in 1978. The 

 combined birth-date distributions are in Table 7. 



Correction for Juvenile Mortality 



The birth- date distributions for the northern 

 anchovy presented above represent the birth dates of 

 those fish which survived until November. A monthly 

 cohort's contribution to the birth- date distribution of 

 its year class is a function of the spawning rate during 

 that month, the mortality rates experienced by that 

 cohort, and the age of that cohort when sampled. 

 Northern anchovy juveniles which had been born 

 during January are expected to be less abundant in 

 November than juveniles born in May because the 

 older juveniles experienced mortality as juveniles for 



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