HOUDE; ABUNDANCE AND POTENTIAL YIELD OF ROUND HERRING 



TABLE 11. — Summary of mortality estimates for round herring larvae from the eastern Gulf of Mexico, 1971-72 and 1972-73. 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 assumed to be 2.0 days. Nonfully vulnerable larval stages were 2.1-5.0 mm SL in 1971-72 and 

 2.1-4.0 mm SL in 1972-73. Explanation of the estimating method is given in Equations (12)-(16). 



Season 



Mean daily 



growth increment 



(mm) 



Instantaneous 

 growth coefficient 



(g) 



Nonfully vulnerable 



larvae duration 



(days) 



Instantaneous 



mortality coefficient 



(2) 



/-axis 

 intercept, N a 

 (no. x 10") 



Daily mortality 



rate, 



1 - exp(-Z) 



1971-72 



1972-73 



is most probable for round herring larvae. Hard- 

 ing and Talbot (1973) and Bannister et al. (1974) 

 reviewed the results of several years' investiga- 

 tions on plaice, Pleuronectes platessa. They found 

 that instantaneous mortality coefficients varied 

 from only 0.0209 to 0.0685 from egg stage 1 to 

 larval stage 4 during the long larval life of more 

 than 150 days. Mortality of haddock eggs and lar- 

 vae was reported by Saville (1956), who gave a 

 series of estimates that ranged from 4 to 16%/day 

 (Z = 0.04-0.17) during a 4-yr survey of egg 

 and larvae abundance at Faroe. Jack mackerel, 

 Trachurus symmetricus, larvae have a high rate of 

 mortality (Lenarz 1973), losses ranging from 57 to 

 67% per millimeter of growth. Farris (1961) re- 

 ported mortality of jack mackerel larvae in rela- 

 tion to age. The instantaneous mortality rate, cal- 

 culated from his data, was 0.23 during the first 30 

 days of life. Mortality of Japanese mackerel, 

 Scomber japonicus, larvae was very high 

 (Watanabe 1970), 99.95% mortality having occur- 

 red between the egg and 15-mm larval stage in 

 about 23 days. This corresponds to an instantane- 

 ous rate of Z — 0.3295. Round herring larval mor- 

 tality rates apparently are similar to those of other 

 clupeoids from temperate or subtropical marine 

 waters (Ahlstrom 1954; Nakai and Hattori 1962; 

 Lenarz 1973). On average they are slightly higher 



than those reported for haddock (Saville 1956). 

 Round herring larvae have mortality rates that 

 are much higher than those reported for North 

 Sea plaice larvae and lower than those reported 

 for jack mackerel or Japanese mackerel larvae. 



If any period can be considered critical in the 

 early life of round herring, it must occur between 

 the time that eggs are spawned and when larvae 

 reach 5.5 mm long. Greatest losses occurred at 

 that time in 1971-72 and 1972-73 (Figure 12). 

 Abundance estimates declined by more than 92% 

 between the egg and 5.5-mm larvae in 1971-72. A 

 decline of more than 78% in abundance was esti- 

 mated between egg and 5.5-mm larvae in 1972-73 

 (Table 12, Figure 12). For larvae longer than 5.5 

 mm mortality decreased, the decrease in rate 

 being especially great in 1971-72. 



The number of survivors and percentage survi- 

 val of round herring larvae at various stages were 

 estimated (Table 12) from the number of spawned 

 eggs obtained by Method I and the information on 

 growth and mortality that is summarized in Table 

 1 1 . The Method I spawning estimate was assumed 

 to be a better estimate of initial number of eggs 

 than they- intercept estimates in Table 11. There 

 was an apparent high mortality between spawn- 

 ing and hatching which exceeded 75% in 1971-72 

 (Table 12). The larval populations were reduced by 



85 



