METHOT: SURVIVAL OF LARVAL ENGRAULIS MORDAX 



a longer period. A correction for this difference in age 

 is necessary before differences between the seasonal 

 distribution of larval abundance and the resultant 

 distribution of juvenile birth dates can be interpreted 

 as differences in larval survival. Few juveniles collect- 

 ed in November were <5-mo-old so the relative 

 abundance of older juveniles need only be adjusted 

 by the inverse of survival from age 5 mo to age at cap- 

 ture. Age- specific survival rate was assumed to 

 increase from 64% per month at age 3 mo to 88% at 

 10 mo (calculated from preliminary estimates of 

 juvenile mortality rates in Smith 1981). If one 

 assumes no seasonality in juvenile survival, the resul- 

 tant birth- date distributions are as if all the monthly 

 cohorts had been sampled at the same age rather 

 than at variable ages in November. Because most 

 juveniles were between 6 and 10 mo old, corrected 

 birth- date distributions are similar to the uncor- 

 rected distributions (Table 7). 



Relative Larval Survival 



The juveniles' birth-date distributions, corrected 

 for juvenile mortality, and the seasonal distributions 

 of larval production for the northern anchovy are pre- 

 sented in Figures 2 and 3. The ratio of monthly birth- 

 date frequency to monthly larval production is an 

 index of larval survival relative to survival from other 

 months in the same spawning seasoa Survival tended 

 to increase within the 1978 season and decrease 

 within the 1979 season (Fig. 4). In both years the only 

 anomalies to the trends were low relative survival of 

 larvae born in February. 



DISCUSSION 



This study has documented seasonal changes in 

 survivorship of larval northern anchovy. Both the 

 magnitude and the timing of changes are important. 

 The magnitude of the seasonal changes determines 

 whether annual variation in recruitment could be 

 caused by short seasonal events. The timing of the 

 changes in survival relative to environmental events 

 elucidates the linkage between oceanographic con- 

 ditions and recruitment. 



Temporal Patterns in Larval 

 Survival 



To evaluate the importance of seasonal changes in 

 larval survival of the northern anchovy requires 

 estimates of annual variation in recruitment. The age 

 composition of the central population of northern 

 anchovy is monitored through the fishery and trawl 



a. 

 O 



> 



> 

 oc 

 z> 

 </) 



111 

 oc 



DEC JAN 



FEB MAR APR 

 MONTH 



MAY 



JUN 



FIGURE 4. — Relative survivorship is the ratio of the fraction of north- 

 ern anchovy juvenile's birth dates to the fraction of annual larval pro- 

 duction per 30 d. To enable comparison between years the relative 

 survivorship for 1978 has been scaled by the ratio of recruitment 

 (2.0) and the ratio of annual larval production (0.98) between the 

 2 years. 



surveys. Mais (1981a) analyzed the age composition 

 of the commercial fishery in southern California and 

 suggested the following ranking of recent year 

 classes: 1974 weak, 1975 weak, 1976 mediocre- 

 strong, 1977 weak, and 1978 very strong. During the 

 1978-79 fishing season the 1978 year class of the 

 northern anchovy contributed 65% of the southern 

 California catch and in the following season the 1979 

 year class contributed 35% (Mais 1981a). In the 

 spring 1979 trawl survey the 1978 year class con- 

 tributed 62% and in spring 1980 the 1979 year class con- 

 tributed 35% (Mais 1980, 1981b). Thus the fishery 

 and the survey indicate that the 1978 year class was 

 about twice as large as the 1979 year class. 



This difference in recruitment cannot be explained 

 by the abundance of young northern anchovy larvae. 

 Northern anchovy larval production in 1979 was 

 2.1% greater than in 1978. Thus the larger 1978 year 

 class resulted from higher survival because larval 

 abundance was less in 1978 than in 1979. 



The critical question is whether the recruitment 

 variation described above requires more annual 

 variation in larval survival than is caused by the 

 seasonal changes described in this study. I evaluated 

 this question by scaling the northern anchovy larval 

 abundance estimates (Figs. 2, 3) with the ratio of lar- 



747 



