FISHERY BULLETIN: VOL. 81, NO. 4 



val abundance between the 2 years (0.979), and the 

 monthly fractions of juvenile birth dates (Table 7) 

 were scaled by the ratio of recruitments (2.0). The 

 ratio, (scaled birth date fraction)/( scaled larval abun- 

 dance fraction), estimates relative survival of a 

 month's spawn (Fig. 4). Because of the scaling, these 

 ratios can be compared both between and within 

 years. Survival of winter spawn in 1979 was similar to 

 survival of winter spawn in 1978, but survival was 

 much greater in April- May 1978 than April- May 

 1979. Thus, the larger 1978 year class was not 

 necessarily the result of greater survival throughout 

 the spawning season. The increase in survival during 

 the last 2 major months of the 1978 spawning season 

 was sufficient to cause a large increase in 

 recruitment. 



Detection of Changes in Survival 



Detection of a match between spawning and favor- 

 able environmental conditions of the northern 

 anchovy seems more likely than detection of an event 

 which results in poor survival. If a short- duration 

 favorable environmental period results in a doubling 

 of year class abundance, then more than half of the 

 year class would have been born during that period 

 Such a concentration of birth dates could have been, 

 but was not, detected in 1978 and 1979. A particular 

 match apparently was not necessary for these two 

 year classes. Conversely, a short- duration unfavor- 

 able environmental period that destroys all northern 

 anchovy larvae born during the period may cause 

 only a small reduction in year class abundance and a 

 short gap in the birth-date distribution that would be 

 difficult to detect with small sample sizes. 



The effect of other environmental events will be 

 more difficult to detect. Long- duration events or 

 events that affect larvae of a wide age range will have 

 little effect on the birth-date distribution. Secondly, 

 environmental events which do not extend over the 

 geographic range of spawning may not be detected 

 even though they have important local effects. 



Spatial Pattern 



This study was designed to study temporal changes 

 in frequency of juvenile birth dates but a strong spa- 

 tial pattern also was detected. Northern anchovy 

 juveniles collected north of Pt. Dume were larger 

 than southern juveniles because of earlier birth 

 dates. HewittandMethot(1982) showed that spawn- 

 ing contracted towards the San Diego area as the 

 1978 and 1979 spawning seasons progressed. This 

 trend would contribute to an earlier mean birth date 



for northern fish, if currents and eddies did not sub- 

 stantially redistribute the larvae. In addition, if 

 juveniles routinely move northward along the coast, 

 early born fish will be further north by November. 

 There are no data on the distribution of late larvae 

 with which to investigate the cause of latitudinal pat- 

 tern in juvenile birth dates. Geographic pattern in 

 birth date will contribute to geographic pattern in 

 size at age of adults. 



Relation to Environmental 

 Conditions 



Two oceanographic factors, stability of the upper 

 water column (Lasker 1978) and offshore transport 

 (Parrish et al. 1981), have been suggested as factors 

 in recruitment of northern anchovy. These factors 

 should have had different effects on recruitment in 

 1978 and 1979. 



Lasker (1975, 1978) suggested that northern 

 anchovy larvae are likely to encounter adequate prey 

 concentrations only when the upper water column is 

 stable and prey are aggregated into layers. Of par- 

 ticular importance is the inshore chlorophyll max- 

 imum layer which may be composed of dino- 

 flagellates suitable as prey for first feeding larvae. 

 These layers are homogenized as storms or upwelling 

 events destroy the stratification of the upper tens of 

 meters of the water column. The winter of 1977-1978 

 was particularly stormy (Lasker 1981) and the 

 isothermal surface layer was as deep as 50 m until 

 stratification was restored in March. This hypothesis 

 correctly predicts lower larval survival in winter 1978 

 than in spring 1978, but incorrectly predicts a poor 

 year class in 1978. 



Any hypothesis concerning the availability of prey 

 will predict increasing survival through the major 

 spawning season (December to June). Zooplankton 

 biomass increases to its seasonal maximum in June 

 (Smith and Lasker 1978). Stratification (differences 

 between temperature at 10 and 30 m) increases so 

 the prey of larval fish probably are increasingly 

 aggregated into layers. Day length increases so that 

 larvae can feed longer per day (Hunter 1972). A 

 measurable response of northern anchovy larvae to 

 the above factors is an increase in growth rate from 

 0.43 mm/d in January to 0.55 mm/d in June (Methot 

 1981) despite trivial changes in mean temperature 

 (Table 3). If food availability is important to larval 

 survival then survival should consistently increase 

 through the spawning season. 



The second major hypothesis concerns offshore 

 transport and coastal upwelling caused by the pre- 

 dominantly northwest winds (Parrish et al. 1981). 



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