FISHERY BULLETIN: VOL. 75, NO. 3 



TABLE 9. — Summary of mortality estimates for scaled sardine larvae from the eastern Gulf of Mexico, 1973. Estimates were obtained 

 from the exponential regression of egg and larvae abundances on mean age. Instantaneous growth and mortality coefficients were 

 calculated for various possible combinations of mean daily growth increment and duration of the nonfully vulnerable larval stages. 

 Egg stage duration was assigned the value 0.81 days. Nonfully vulnerable larval lengths were 1.1 to 3.0 mm SL. Explanation of 

 the estimating method is given in equations (12) to (16) of Houde (1977a). 



herring (Z = 0.2124 in 1971 and Z = 0.2564 in 

 1973), but higher than those for round herring: 

 Z = 0.1317 in 1971-72 andZ = 0.1286 in 1972-73 

 (Houde 1977a, b). 



The y-axis intercepts (7V ) of the regressions 

 in Table 9 also estimate the number of eggs 

 spawned in 1973. Their values are lower than 

 those calculated by the Sette and Ahlstrom ( 1948) 

 method for 1973 (Table 4), which is considered 

 the best estimate of annual spawning. A higher 

 than expected mortality rate of eggs or nonfully 

 vulnerable larvae may have caused the discrep- 

 ancy (Figure 10). Larval mortality, considering 

 only fully vulnerable stages, may be lower than 

 that for the entire egg-larval stage. For data from 

 Table 8 and Figure 10, the mortality coefficient 

 for fully vulnerable 3.1- to 20.0-mm larvae is 

 Z --= 0.2458, a daily loss rate of 21.8%. 



The numbers of probable survivors at hatching, 

 5.5 mm, and 15.5 mm were estimated in 1973 

 for three instantaneous growth rates that likely 

 encompass the true rate for scaled sardine larvae 

 (Table 10). Initial egg abundance was the 1973 

 estimate from Table 4. The estimated number 

 alive at each stage was calculated from the param- 

 eters of the exponential functions (Table 9) and 

 from the estimated age in days at each stage 

 (Table 8). 



Mortality was high during the egg and larval 

 stages. An apparent mortality of 85 to 91% oc- 

 curred between spawning and hatching (Table 

 10). Less than 2% survived to 5.5 mm, when larvae 

 would have been feeding for 2 days at 26° to 28°C 

 (Houde et al. 1974). More than 99.9% mortality 

 had occurred by 15.5 mm, when larvae were be- 

 ginning to transform to juveniles. At the most 

 probable growth rate, g = 0.0883, only 5 larvae/ 



6 8 10 12 14 16 



ESTIMATED MEAN AGE (DAYS) 



20 22 



FIGURE 10. — Estimated abundance of egg and larval stages of 

 scaled sardines in the eastern Gulf of Mexico in 1973. Abundance 

 is expressed as a function of estimated age. A fitted exponential 

 function gives an estimate of the instantaneous rate of decline 

 in abundance for eggs and larvae up to 23 days of age. The 

 symbol enclosed in the circle represents the nonfully vulnerable 

 1.1- to 3.0-mm length classes and was not included in the regres- 

 sion estimate of instantaneous decline. 



10,000 spawned eggs were estimated to have sur- 

 vived to 15.5 mm and 20 days of age in 1973. 



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