FISHERY BULLETIN: VOL. 75, NO. 1 



1000- 



500 - 



r-100 



b 



X 



(jj 50 

 o 

 z 

 < 



Q 

 Z 



m 



< 



a 



UJ 



>" 10 



UJ 



<r 

 cc 



o 5 

 o 



< 

 q: 



o 



0.5 



Etrumeus teres survival 

 • - 1971 - 1972 

 x • 1972- 1973 



1971-1972 



N t -(23l.455x I0")e" 0l3l7t 



® 



® 



1972-1973 S "«* 



N t - (125.123 xlo")e" 0,286t > 



x < 



 i 



_L 



2 4 6 6 10 12 14 16 16 20 24 28 

 ESTIMATED MEAN AGE (DAYS) 



32 



FIGURE 12. — Estimated abundance of egg and larval stages of 

 round herring in the eastern Gulf of Mexico in 1971-72 and 

 1972-73. Abundance is expressed as a function of estimated age. 

 Fitted exponential functions give estimates of the instantaneous 

 rates of decline in abundance for eggs and larvae up to 31 days 

 of age. The two symbols enclosed in circles represent nonfully 

 vulnerable length classes and were not included in the re- 

 gression estimates of instantaneous decline. 



would have been lower. In 1971-72, Z = 0.0563 for 

 fully vulnerable larval stages and Z = 0.1123 for 

 those stages in 1972-73. The results suggest that 

 egg and nonfully vulnerable larvae mortality 

 were higher in 1971-72 than in 1972-73. Mortality 

 of vulnerable larval stages appears to have been 

 higher in 1972-73 when the population declined by 

 10. 6% /day as opposed to 1971-72 when it declined 

 only 5.5%/day. The higher mortality rate of 

 larvae in 1972-73 also was apparent in the mor- 



tality estimates based on larval lengths (Fig- 

 ure 11). 



High mortality of eggs or newly hatched larvae 

 may be characteristic of many clupeids, including 

 round herring. Smith (1973) recently reported 

 that Pacific sardine eggs experience high mortal- 

 ity, the instantaneous rate being Z = 0.31 during 

 that stage. Pilchard, Sardina pilchardus, eggs 

 undergo high mortality during early embryonic 

 stages (Southward and Demir 1974) and embryos 

 ofClupeonella delicatula suffered high mortality, 

 especially under unfavorable temperature re- 

 gimes (Pinus 1974). 



The best probable estimates of mortality from 

 the egg to 16.0-mm larval size are near the middle 

 of the ranges given in Table 11, at instantaneous 

 growth rates of 0.0498. In 1971-72, Z = 0.1317 is 

 the most probable estimate while Z = 0.1286 

 seems most probable in 1972-73. These estimates 

 correspond to average daily losses of 12.3% in 

 1971-72 and 12.1% in 1972-73. Estimates of the 

 instantaneous mortality coefficients based on the 

 two examples given in Table 10 and Figure 12 

 coincide with what I believe may be the best esti- 

 mates of mortality. Confidence limits, at the 0.95 

 probability level, were placed on the instantane- 

 ous mortality coefficients derived from these 

 examples. They were wide, ranging from Z = 

 0.0635-0.1999 in 1971-72 andZ = 0.0823-0.1749 

 in 1972-73. The coefficients Z = 0.1317 in 1971-72 

 and Z = 0.1286 in 1972-73 did not differ sig- 

 nificantly between years U-test; P>0.50). 



The estimates of mortality rates could be too 

 high if avoidance by larvae was increasing sig- 

 nificantly as they grew, reducing their probability 

 of capture. If growth was not exponential, but 

 linear, during the larval phase, then the mortality 

 estimates may be too low, because duration- 

 corrected abundances gave relatively high values 

 to older larvae that presumably were growing 

 through length classes at an increasing rate. 



Because of the difficulty in ageing eggs or larvae 

 of marine fishes, few estimates of mortality rates 

 in relation to age have been reported. Ahlstrom 

 (1954) reported that about one Pacific sardine 

 larva survived to 21.25 mm/100,000 eggs spawned 

 during the first 40-45 days of life, which corre- 

 sponds to an instantaneous daily loss rate of 0.16- 

 0.17. Japanese sardine was investigated by 

 Nakai and Hattori (1962). They reported survival 

 from egg to the 15.0 mm stage as 0.10% in 54 days, 

 corresponding to an instantaneous rate of Z = 

 0.1279. This rate is nearly identical to that which 



84 



