HOUDE: ABUNDANCE AND POTENTIAL YIELD OK SCALED SARDINE 



Larval Abundance 



Larval abundance varied annually and season- 

 ally (Table 7; Figure 7); the greatest abundances 

 being observed in 1973 and 1974 cruises. Abun- 

 dance estimates for cruises in which larvae oc- 

 curred, ranged from 0.20 to 16.63 x 10 1( ' larvae. 

 Estimated annual abundances of larvae were low 

 in 1971 and 1972, but increased in 1973 (Figure 8). 

 No annual estimates were available for 1974, but 

 the great abundance of larvae from cruise CL7412 

 (Figure 7) suggests that more larvae were present 

 in that year than in any previous year. The in- 

 creases in larval abundance between 1971 and 

 1974 are further evidence that spawning intensity 

 increased during the period. 



Some scaled sardines as long as 30 mm SL were 

 collected but few larvae longer than 20 mm were 

 taken, and only larvae from 1.1 to 20.0 mm are 

 included in the length-frequency distributions. 

 Most larvae of 1.1 to 3.0 mm were distorted from 

 collection and preservation. Scaled sardine larvae 

 are 2.4 mm at hatching, but within 15 h their 

 length increases to more than 4.0 mm, mostly 

 due to straightening of the body axis rather than 

 true growth (Houde et al. 1974). The most abun- 

 dant larvae were 2.1 to 4.0 mm in 1972-74, but 

 were larger in 1971 (Figure 7) when towing speed 

 was faster (Houde 1977a) and mesh escapement 

 by small larvae may have been greater. 



The ratio of night-caught to day-caught scaled 

 sardine larvae increased slowly as larvae in- 

 creased in length. No larvae longer than 18.0 mm 

 were sampled during daylight hours. An exponen- 

 tial model R = 0.7999e° 0550X was fitted to the data 

 (Figure 9), where R is the ratio of night-caught 

 to day-caught larvae and X is standard length. 

 It provided the correction factor R, by which day- 

 time catches were adjusted to obtain abundance 

 estimates of larvae by 2-mm length classes in 

 each station area (equation 11, Houde 1977a). 



An exponential decrease in abundance of larvae 

 was observed in 1973 (Figure 8) and the larval 

 mortality rate was estimated from these data. 

 Larvae longer than 3.0 mm were assumed to be 

 fully vulnerable to the sampling gear. Abun- 

 dances (Figure 8) were previously corrected for 

 daytime avoidance. An exponential function was 

 fitted to the data, and the instantaneous rate of 

 decline in abundance per millimeter increase in 

 length was estimated for larvae from 3.1 to 

 20.0 mm SL. The instantaneous coefficient, Z = 

 0.3829, is a measure of larval mortality, if gear 



TABLE 7. — Abundance estimates of scaled sardine larvae for 

 each cruise. Estimates include larvae in all size classes and 

 were obtained using equations (2) and (3) (Houde 1977a). 



'Positive area is defined as the area representing stations where either 

 eggs or larvae of scaled sardines were collected 



2 Values are not adjusted for cruises that did not encompass the 

 entire area, nor have estimates been corrected to account for gear 

 avoidance by larvae at stations sampled in daylight. 



avoidance was not too great for larval length 

 classes in the analysis. The 0.95 confidence limits 

 on Z are Z ± 0.0833. The. observed coefficient 

 corresponds to a 31.8% decrease in larval abun- 

 dance per millimeter increase in length. Although 

 mortality was not estimated for 1972 larvae, the 

 high estimated abundance of larvae longer than 

 10 mm (Figure 8) indicates that survival may 

 have been relatively good in that year. 



Mortality relative to age of larvae was deter- 

 mined assuming an exponential model of growth 

 for scaled sardine larvae, based on evidence from 

 laboratory rearing experiments. Mean daily 

 growth increments of scaled sardine larvae reared 

 at temperatures above 26°C exceeded 0.5 mm, and 

 frequently were in the range of 0.7 to 1.0 mm 

 (Houde and Palko 1970; Saksena and Houde 1972; 

 Saksena et al. 1972). Methods to estimate age at 

 length and mortality have been reported (Houde 

 1977a). 



Mean egg stage duration for scaled sardine is 

 about 0.81 day. In 1973 the nonfully vulnerable 

 length classes were 1.1 to 3.0 mm. Duration of 

 that larval stage is from 1.0 to 3.0 days based on 

 laboratory experiments (Saksena and Houde 

 1972; Houde et al. 1974). 



An example of duration-corrected abundance 

 data at estimated mean ages for eggs and larvae 

 up to 20.0 mm in 1973 is given in Table 8. In this 

 example the mean daily growth increment was 



623 



