OMORl and GLUCK LIFE HISTORY OF SERGESTES SIMILIS 



different sampling dates and locations. The von 

 Bertalanffy and logistic equations were used to fit 

 these growth data. 



RESULTS 



Daily and Ontogenetic Vertical 



Migrations of Larvae and 



Earl)' Postlarvae 



Coastal upwelling is generally weak in south- 

 ern California during the winter (Bakun 1973). 

 This is consistent with the data on environmental 

 properties at the sampling stations (Figure 2). The 

 thermocline remained at about 75 m at all stations 

 on Line 60 with the mixed layer temperature 

 ranging from 1L5°C inshore to 14.0°C offshore. 

 Salinity was usually <33.30%o in water above 

 75-m depth. On Line 90, except for the two outer- 

 most stations, the thermocline was at 30-50 m and 

 the temperature within the mixed layer was 

 >13.5°C. Salinity was >33.20%oat all depths. On 

 Line 100 the thermocline was at about 50 m at Stn. 

 100.35 and 100.60. Temperature within the mixed 

 layer was about 15°C, and salinity was >33.50%o. 

 The position of the oxycline coincided with that of 

 the thermocline at nearly all stations. Generally, 

 the oxygen level at depths below the mixed layer 

 increased going seaward. 



The main population of S. sirnilis larvae was 

 always between the surface and 100-m levels, and 

 they occurred in greater abundance at stations on 

 the continental slope (Figure 3). The population 

 density was highest at Stn. 90.32 (101 

 individuals/m^). The larvae did not occur at Stn. 

 90.120, 90.140, and 100.120. In these southern 

 offshore stations the temperature above 100 m 

 was >16°C. The temperature-salinity curves 

 characterized the water mass as eastern North 

 Pacific Central water, where S. si mil is has never 

 been found. In this water mass, the J'ortmanni 

 type" larvae (the Sergestes corniculum group, see 

 Yaldwyn 1957 and Omori 1974) were commonly 

 distributed. 



The vertical distributions of larvae and early 

 postlarvae from eight stations where they were 

 abundant shows that the larvae were scattered 



BL = 3.15 + 2.85 CL for females. 

 BL = 2.55 + 3.11 CL for males (Omori etal. 1972). 

 The regression for juveniles with carapace length 5.5 mm or less 



BL 



3.08 CL. 



from 20 to 100 m during the daytime (Figures 4, 5). 

 On Line 90, the distribution pattern did not coin- 

 cide well between the stations closest to shore 

 (Stn. 90.28 and 90.32) and the offshore stations 

 (Stn. 90.60 and 90.70). At Stn. 90.60 in the day- 

 time, the larvae were widely distributed through- 

 out the 0-110 m layer, but larvae occurred only 

 between 44 and 88 m at Stn. 90.32 during the day. 

 The greatest population density observed was 

 within the 66-88 m layer at Stn. 90.32 (about 3,500 

 individuals/1,000 m^). Nighttime larval distribu- 

 tion was between 20 and 90 m at Stn. 90.70, but 

 again, it was below 40 m at the closest inshore 

 station. A similar inshore and offshore as- 

 semblage was observed along Line 100, although 

 the vertical distribution of the larvae was ex- 

 panded more widely. At Stn. 100.35 the larvae 

 were most abundant between 50 and 100 m in the 

 daytime and and 80 m layer at night. On the 

 other hand, at Stn. 100.60 the main population in 

 the daytime occurred between 20 and 120 m, while 

 at night the distribution ranged from the surface 

 to 140 m with considerable numbers in the 0-40 m 

 layer. At both stations, there was a clear daily 

 vertical migration of the main population of zoeal 

 and postlarval stages. 



With the present sampling method, there was 

 some doubt whether the same population was 

 measured by day and night tows. However, as 

 indicated in Figures 4 and 5, the estimates of 

 abundance beneath 1 m^ of sea surface did not 

 differ appreciably between day and night at the 

 two closest stations on Lines 60 and 90 and be- 

 tween day and night tows at the same station on 

 Line 100. It can be said, at the least, that the 

 avoidance of nets by larvae in the daytime was no 

 greater than at night. 



When abundance vs. depth is combined and av- 

 eraged for each larval stage at each station, the 

 extent of daily vertical migration becomes clear. 

 The first protozoeal stage shows at least a re- 

 stricted daily vertical migration (Figure 6). The 

 larvae gradually increase their range of vertical 

 distribution with growth while gradually inhabit- 

 ing deeper water. Thus, the main population of 

 early postlarva (40-45 m at night and 70-75 m in 

 the daytime) shows a deeper distribution than ear- 

 lier larval stages. 



Eggs of S. sirnilis (about 0.3 mm in diameter) 

 were slightly heavier than the density of the ex- 

 perimental water; the difference in sinking rates 

 was not significant at the 59c level between 1 0° and 

 14"C under laboratory conditions (Table 3). 



187 



