FISHERY BULLETIN: VOL. 70, NO. 3 



Figure 4. — The size frequency of sardine and anchovy 

 larvae. The black dot-dash line is sardine larvae 

 caught in the standard CalCOFI silk net between 1951 

 and 1960 in all regions from January to June inclusive. 

 The clear dot solid line is anchovy larvae caught in the 

 standard CalCOFI silk net between 1951 and 1960, in 

 all regions from January to June inclusive. The tri- 

 angle dot-dash line is anchovy larvae caught in the new 

 CalCOFI nylon net (0.505-mm nylon rather than 

 0.55-mm silk) from January to June 1969 in all regions. 



and 1941 sardine catches are included for later 

 reference. In Figure 4, the 1951-60 size com- 

 position of sardine is compared to the anchovy 

 for the same period. Anchovy retention in a 

 new net with smaller, more regular meshes 

 (0.505-mm nylon) and more mesh area is in- 

 cluded for comparison. All samples reported in 

 the size frequency graphs are from the first 

 half of the year to facilitate comparison with 

 earlier samples and eliminate the effects of poor 

 sampling coverage in the latter half of the year. 

 Ahlstrom (1954) acknowledged that larger 

 larvae may avoid capture at night as in 

 the daytime. Avoidance is more pronounced in 



the daylight. In the sardine the night-to-day 

 ratio of catches increases 0.6971 per mm of 

 growth after 4.75 mm (Ahlstrom, 1954, p. 129). 

 Similarly, the anchovy night-to-day catch ratio 

 increases 0.64 for each millimeter growth after 

 3.5 mm (Ahlstrom, 1959, p. 136). Lenarz (in 

 press) described the annual and diurnal varia- 

 tion in size specific catch rate for sardine, an- 

 chovy, hake, and jack mackerel. An important 

 source of variability in avoidance bias is a shift 

 of spawning season. For example, length of day 

 varies from 9.6 hr in winter to 14.8 hr in sum- 

 mer at the latitude of San Francisco near the 

 northern boundary of the survey grid and from 

 10.6 to 13.7 hr off south Baja California at the 

 southern boundary of the sample grid (54% and 

 29% respectively). For the 10-year period, 

 1951-60, the ratio of night-caught to day-caught 

 larvae was 1.72 for sardines and 2.45:1 for an- 

 chovy. When the average catch per positive tow 

 by month is corrected for day length at San Fran- 

 cisco the anchovy: sardine ratio changes from 

 2,96: 1 to 3.75: 1 for the same decade. A propor- 

 tionate shift of anchovy spawning toward June 

 would for example, accentuate this difference. 

 The importance of avoidance and escapement 



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Figure 5. — The correlation coefficient between the an- 

 nual total regional census estimates of sardine eggs and 

 larvae by size class with the Murphy (1966) sardine 

 biomass estimates. The first point is for total eggs, 

 1951-59; the second point is for total larvae, 1951-59; 

 the third point is for total larvae with 1953 and 1959 

 censored. The succeeding points are for standard-size 

 classes for all years 1951-59, 



858 



