HOUDE: ABUNDANCE AND POTENTIAL YIELD OF SCALED SARDINE 



20 



10 - 



~ 10 



u o 



< 50 



o 



z 



CO 



< 

 o40 



UJ 



I- 

 < 

 2 



u>30 



20 



10 



1971 



l~~1 



1972 



-Jl3- 



,,,,  ,,,,,,  ,,,,,.,,,,,.,,,,,.,,,,,.,,,,,. ,  ,  ,  ,  , , T 



1973 



N L - (134.8070 x I0")e° 3829L 



l.l- 2.1-3.1- 4.1- 

 2.0 30 4 5.0 



6.1- 8.1- 10.1- 

 7.0 9.0 1 1.0 



LENGTH-CLASS (mm) 



i • I 

 12.1- 14.1- 

 130 150 



16.1- 

 17.0 



18.1- 

 19.0 



FIGURE 8. — Length-frequency distribution of annual larval 

 abundance estimates for scaled sardine larvae collected in the 

 eastern Gulf of Mexico, 1971-73. Frequencies in each 1-mm 

 length class are expressed as estimated annual abundance and 

 have been corrected for daytime avoidance. A fitted exponential 

 function for 1973 data provides an estimate of the instantaneous 

 coefficient of decline in abundance by length. 



set at 0.8 mm and nonfully vulnerable larval stage 

 duration was 1.0 day. I believe that those values 

 are the best estimates for scaled sardine larvae, 

 but other values also were assigned from which 

 both mean ages and duration-corrected abun- 

 dances were generated. Duration-corrected abun- 

 dances (Table 8) were regressed on mean ages in 

 an exponential regression to estimate the instan- 

 taneous mortality coefficient (Z) for age in days. 

 Mortality coefficients were calculated for var- 

 ious combinations of mean daily growth incre- 

 ments and durations of the nonfully vulnerable 

 larval stage for 1973 data (Table 9). Possible 

 values of the mortality coefficient, Z, range from 

 0.1822 to 0.3471, which correspond to daily per- 

 centage losses of 16.7 to 29.3%. For data from 

 Table 8, where mean daily growth increment was 

 0.8 mm and nonfully vulnerable larval stage 



< 



4.0 6.0 8.0 10.0 12 14 16.0 



MIDPOINT OF LENGTH CLASS (mm) 



18.0 



FIGURE 9. — Night to day ratios of sums of catches, standardized 

 to numbers under 10 m 2 of sea surface, for scaled sardine larvae 

 collected in 1971-74 in the eastern Gulf of Mexico. The ratios 

 were calculated for larvae within each 2-mm length class from 

 1.1 to 19.0 mm SL. A fitted exponential regression describes the 

 relationship. Larval abundance estimates for each length class 

 at stations occupied during daylight were corrected by the appro- 

 priate ratio factor for each length class to account for daytime 

 avoidance. 



TABLE 8. — An example of data from 1973 used to obtain stage 

 duration, mean age, and duration-corrected abundance of scaled 

 sardine eggs and larvae. Duration-corrected abundances were 

 subsequently regressed on mean ages to obtain mortality rates 

 (Table 9). Abundance estimates in the second column of the 

 Table were previously corrected for daytime avoidance. In this 

 example, the mean daily growth increment (o) was set at 0.80. 

 The nonfully vulnerable size classes were 1.1 to 3.0 mm. Calcu- 

 lating procedures were given in Houde (1977a), equations (12) 

 to (16). The regression for these data is presented as Figure 10. 



duration was 1.0 day, the estimated mortality 

 coefficient isZ = 0.2835, corresponding to a 24.7' < 

 daily loss rate (Figure 10). The most probable 

 scaled sardine mortality estimate for abundance 

 at age data, Z = 0.2835 ± 0.0754 at the 0.95 

 confidence level, is similar to those for thread 



625 



