OMORI and GLUCK: LIFE HISTORY OF SERGESTES SIMILIS 



RELATIVE ABUNDANCE (%) 



20 40 



20 40 



20 40 



20 40 



20 40 



20 40 60 



J I I 



FIGLIRE 6. — Vertical distribution of larvae andpostlarvae of Sergestessimilisoff southern California. 

 Abundance vs. depth at all sampling stations was combined and averaged. Horizontal line indicates the 

 depth at which the cumulative catch represented 509c of the total catch. PZ, protozoeal stages; Z. zoeal 

 stages; PL, postlarval stages. 



say whether this 1976 cohort represents the 

 large-sized group or not. In the 10-ft IKMT sam- 

 plings the most conspicuous female cohort of 

 10.5-13.0 mm CL in April reached 13.5-15.5 mm 

 CL in October (Figure 8B). The males grew from 

 an average 10.8 to 11.7 mm CL between April and 

 August. In many cases, the size structure of the 

 population showed the presence of only one or two 

 obvious size groups, but in three cases (April 21, 

 June 21, and July 29) the histograms of females 

 indicated three size groups. Development of the 

 smallest cohort of age-group was traceable until 

 August in both females and males, but in October 

 and November, two cohorts of age were appar- 

 ent. 



Some estimates of growth were attempted using 

 changes in the average or modal lengths in vari- 

 ous months. In order to show the growth trend 

 more definitely, the results of all previous length 

 measurements of S. fiimilis from various waters 

 were reanalyzed and the average or modal lengths 

 for each size group were plotted together with the 

 present data (Figure 9). Except for the points de- 

 rived from the offshore population in the subarctic 

 North Pacific, where the environment is quite dif- 

 ferent from that of southern California, the major- 

 ity of cohorts had average or modal lengths which 



fell within the growth curves of three year classes 

 fitted by eye. 



These data indicate the following: 1 ) as expected 

 from spawning season data, in most cases the 

 modal progressions are evident starting in winter 

 or early spring, 2) growth trends of S. siriiilisoff 

 southern California appear similar to the popula- 

 tion off Oregon (Pearcy and Forss 1969), and 3) 

 growth rates do not vary greatly among many 

 different populations, although there is evidence 

 that a few modal groups grew about twice as 

 rapidly as the ordinary one. 



The ratio of females to males in all collections 

 was 1.3:1 (553:422). Sex ratio in a cohort was not 

 skewed greatly towards females until the modal 

 length of the female population reached about 13 

 mm CL. At that point, the males of the cohort 

 rapidly disappeared from the collection, account- 

 ing for the observed imbalance in sex ratio (69:2). 



DISCUSSION 



Ontogenetic Migration 



Omori (1979) found experimentally that: 1) 

 ovigerous females of S. similis shed their eggs at 

 night, 2 1 the eggs took 105 h to hatch into nauplii 



191 



