FISHERY BULLETIN VOL. 77, NO. 1 



that the ovary may contain ova at different de- 

 velopmental stages and that not all ova are neces- 

 sarily released at once (King 1948; Mauchline 

 1968; Roger 1973; Omori 1974). We observed that 

 S. simili.s off southern California always retained 

 considerable numbers of immature ova after 

 spawning. Because the volume of a pair of mature 

 ovaries from this shrimp represents about lOT of 

 the body volume, we can estimate from the volume 

 of each egg that one female has at least 1 ,500 but 

 probably closer to 2,000-2,500 eggs. Nevertheless, 

 the number of eggs released by a female in the 

 laboratory was always <1,140 (Omori 1979). 

 Thus, as has been pointed out for Euphausia pa- 

 cifica off southern California (Brinton 1976), it 

 appears possible that under optimal environmen- 

 tal conditions small ova of S. similis may develop 

 later and produce a second spawning. If a female, 

 which produced the first clutch in late December, 

 released the second clutch 3-4 mo later, two modal 

 size-groups might be seen sometimes in the same 

 age-group. It is probable that unfavorable en- 

 vironmental conditions would prevent the spent 

 ovary from maturing again until the following 

 year. Further evidence of this phenomenon is pro- 

 vided by the increase in the number of spent 

 females and the decrease in the number of fully 

 grown ova in ovaries of S. similis off southern 

 California and Oregon during the summer 

 (Genthe 1969; Pearcy and Forss 1969). The 

 length-frequency histograms in October 1972. 

 November 1975. and from August 1976 to March 

 1977 (Figure 8) indicate either the occurrence of 

 multiple spawnings for S. similis or individuals 

 from farther north being mixed into the southern 

 California population. 



Growth, Sexual Maturity, and Longevity 



If S. similis population is composed of age- 

 group shrimp only and all attain sexual maturity 

 after about 1 yi", the size structure of the popula- 

 tion sampled shows the presence of only one or two 

 size groups. However, the obvious occurrence of 

 three size groups of females during certain periods 

 of the year in this study indicates that the females 

 ofS. similis live 2 yr or more. Large females, 13-16 

 mm CL, carrying developed ovaries are sometimes 

 collected, indicating that S. similis can reproduce 

 at least twice during its lifetime. The absence of 

 male individuals >14 mm CL resulted in a strong 

 imbalance of sex ratio, indicating that the males 

 die out at an age of < 20 mo. Genthe ( 1969 ) showed 



evidence of sex reversal (protandrous herma- 

 phroditism) from male to female in S. similis. 

 Similar phenomena have been observed in other 

 sergestids of the genus Acetes (Omori 1975). A 

 detailed study is needed to determine the meaning 

 of these findings, although at the present time we 

 believe that the variance may be explained by 

 abnormalities, since the frequency of occurrence is 

 small. The bias in sex ratio favoring females above 

 a certain size indicates the possibility of multiple 

 fertilizations by males. Thus some females of the 2 

 age-group may mate with males of the 1 age- 

 group. 



We obtained an average gi'owth trend of S. 

 similis throughout its life by shifting the average 

 or modal lengths of populations off the California 

 and Oregon coasts horizontally in accordance with 

 the month of their sampling. The growth of S. 

 similis >6 mm CL was best fitted by the von Ber- 

 talanffy equation (Figure 10): 



/, = 14.7(1 - e<"'03™') for females, and 

 /, = 12.0(1 - e-o "MS") for males, 



where /, is the carapace length in millimeters at t 

 days. Because of the large mesh sizes of the IKMT 

 nets, however, any average or modal length calcu- 

 lated from these samples over considerable size 

 ranges on either side of 7 mm CL is probably 

 greater than the length of the natural population. 

 Therefore, the initial modal length of the 3-mo old 

 population was fit by eye and connected with those 

 growth curves of larval and early postlarval stages 

 at 10° and 14°C which were obtained under 

 laboratory conditions. It took 52 days for S. similis 

 to reach the first postlarval stage at 14°C (Omori 

 1979). Under these conditions the logistic equa- 

 tion (Figure lOB) seemed to give the better fit to 

 the growth curves: 



/ 



14.7 



1-e- 



12.0 



' J _p-0.01254((-188.4) 



for females, 



for males. 



Growth is very rapid during the postlarval 

 stages. The juveniles at 4-8 mo old grow, at 0.91 

 mm CL/mo, during the period from April to Au- 

 gust (logistic equation). The biomass of total zoo- 

 plankton, as well as young Calanus and 

 Euphausia , which are considered to be the most 

 important food for juvenile and adult S. similis. 



196 



