OMORl and GLUCK LIFE HISTORY OF SERGESTES SIMILIS 



I5n 



Figure lO— Average growth of Sergestes 

 stmtlis off the California and Oregon coasts. 

 Solid lines (A) are growth curves fit by the von 

 Bertalanffy equation and (Bl are those fit by 

 the logistic equation. Dashed lines are growth 

 curve of early developmental stages of 1 0° and 

 14°C determined in the laboratorj- lOmori 

 1979). 



^,0 



X 

 H 

 O 



LlI 



O 



< 

 < 



FEMALE (B)^^__- 



o---^ FEMALE 



/o-:^5^-»-MALE(B) 

 " -• MALE (A) 



o FEMALE 



• MALE 



i^ UNSEXED OR COMBINED 



-1 — I — \ — I — I — r 



n — I — I — I — I — i — I — r 



JFMAMJJASONDJFMAMJJASONDJFMAMJJA 

 MONTHS 



usually peaks in the April-July period in southern 

 California waters (Mullin and Brooks 1970; Brin- 

 ton 1976). Shrimp which encounter the best feed- 

 ing conditions probably grow rapidly with low 

 mortality rates and form distinctive modal groups 

 such as those traced in Figure 9. The growth rate 

 gradually decreases after 10-mo old. and the 

 females add only 5 mm CL in 20 more months 

 before dying. The difference in growth rates be- 

 tween the sexes becomes apparent after the 

 shrimp attain a length of about 8 mm CL. The 

 males grow slower than the females, but attain 

 sexual maturity '2-I mo earlier than females be- 

 cause the females become mature at 10.5-11.0 mm 

 CL, whereas the males mature at 9.5-10.0 mm CL 

 (see Genthe 1969; Omori 1979). Since five data 

 points for females on the upper right-hand side of 

 Figure 10 are on the asymptote, it is highly 

 speculative whether these shrimps represent that 

 age-group, or possibly an age-group spawned 5-6 

 mo later; in which case they would be placed on a 

 different curve. It is apparent, however, that the 

 longevity of the females of S. siinilis is more than 2 

 yr and that they spawn in two successive spawn- 

 ing seasons during their lives. These observations 

 agree well with those of Matthews and Pinnoi 

 (1973) on Sergestes arciticus Kroyer, which is the 



most closely related species to S. similis ( Judkins 

 19721, in Kursfjordan, western Norway. 



ACKNOWLEDGMENTS 



We acknowledge the helpful reviews by M. M. 

 Mullin and J. G. Morin. Thanks also to the staff 

 and graduate students of the Food Chain Research 

 Group of the Institute of Marine Resources for 

 their assistance in sampling at sea. Omori is in- 

 debted to the Institute of Marine Resources and 

 the Marine Life Reseaixh Group of Scripps In- 

 stitution of Oceanography for the financial sup- 

 port and hosting of his visit. 



LITERATURE CITED 



AHLSTROM, E. H. 



1948. A record ofpilchard eggs and larvae collected during 

 surveys made in 1939 to 1941. U.S. Fish Wildl. Serv., 

 Spec. Sci. Rep. Fish. 54, 76 p 



Bakun, a. 



1973. Coastal upwelling indices, west coast of North 

 America. 1946-71. U.S. Dep. Commer., NOAA Tech. 

 Rep ^fMFS SSRF-671, 103 p. 

 B.^RHAM. E. G. 



1957. The ecology of sonic scattering layers in the Mon- 

 terey Bay area. California. Hopkins Mar. Stn.. Stanford 

 Univ. Tech. Rep. 1. 182 p. 



197 



