FISHERY BULLETIN, VOL 77, NO 1 



NO OF INDIVIDUALS / 1000 m^ 



Figure 4. — Vertical distribution of larvae and postlarvae of 

 Sergestes similis on CalCOFI Lines 60 and 90 off southern 

 California. PZ, protozoeal stages: Z, zoeal stages; PL. postlar- 

 val stages. Estimated total number of larvae beneath 1 m^ of 

 sea surface indicated by n. 



chemical environmental data indicates that there 

 is a relationship between temperature and spawn- 

 ing season (Figure 7). Spawning activity was 

 highest during the period when the vertical 



stratum of optimum temperatures for larvae was 

 thickest. It decreased before colder water was 

 brought in by coastal upwelling which was nor- 

 mally most intense from May to August (see 

 Bakun 1973). A seasonal minimum, or cessation, 

 of spawning occurred during the summer and au- 

 tumn when the upper layer was covered by un- 

 favorably warm temperatures (>15°C). 



Growth 



Because of the smaller mesh size, the 6-ft I KMT 

 retained a larger proportion of small shrimp than 

 did the 10-ft IKMT (Figure 8). While specimens of 

 4 mm CL occurred in the smaller net, few < 7 mm 

 were retained in the larger net. 



Well-defined progressions of size-frequency 

 modes gave indications of average growth rates for 

 certain cohorts, although we sometimes encoun- 

 tered difficulties in interpreting these trends due 

 to inadequate sampling, and possibly to extended 

 spawning of the species. One 1975 cohort (12.0- 

 14.5 mm CL) and two conspicuous 1976 cohorts 

 (5.0-11.0 mm CL) were seen in females collected in 

 August 1976 (Figure 8A). The former cohort was 

 not found in the following two samplings. The 

 large-sized 1976 cohort (mean modal length, 8.4 

 mm CL in August) reached 9.9 mm CL in October, 

 10.5 mm CL in January, and 11.8mmCLin March 

 1977. Growth of the small-sized cohort was trace- 

 able until April 1977, when the shrimp attained 

 an average carapace length of 10.4 mm. Recruit- 

 ment of postlarvae <6.0 mm CL ( 1977 cohort) was 

 intense in April. The histogram for March showed 

 only a single mode of males, and it isnot possible to 



Figure 5.— Vertical distribution of lar- 

 vae and postlarvae of Sergestes similis at 

 CalCOFI Stn. 100.35 and 100 60 off 

 southern California. Estimated total 

 number of larvae beneath 1 m^ of sea sur- 

 face indicated by n. 



100 



NO OF INDIVIDUALS / 1000 m^ 

 



IO(X) \ 10 100 1000 ^ 10 100 ICXXD \ 10 



100 



1000 



J 



190 



