POWER: MODEL OF NORTHERN ANCHOVY DRIFT 



year-round, except during peak spawning in the 

 spring, it is clear that the relationship between the 

 time of northern anchovy spawning and the time 

 that this countercurrent diminishes is critical. The 

 simulations indicated that eggs and larvae from 

 early spawning (i.e., January) are carried upshore 

 into the Santa Barbara Channel and north of Point 

 Conception, while those from later spawning 

 (March) move in the opposite, southeasterly direc- 

 tion. The sizes and birth dates of juveniles collected 

 in the fall of 1978 and 1979 were in accordance with 

 this pattern. Methot (1981) reported that juvenile 

 northern anchovy collected during both fall seasons 

 in the northern portion of the Southern California 

 Bight had birth dates (as determined from daily 

 growth increments in otoliths) in the preceding 

 months of December and January, and these fish 

 were generally larger than those collected farther 

 to the south. The northern anchovy collected in the 

 south had predominantly February and March birth 

 dates. It may be that the northern group, contain- 

 ing fish from early spawning, were advected to the 

 north by the Southern California Countercurrent 

 and that the southern group of fish from late spawn- 

 ing were produced when the surface countercurrent 

 had diminished. Future studies of the transport and 

 distribution of northern anchovy larvae or other 

 planktonic species in the Southern California Bight 

 should incorporate as much information as is avail- 

 able on the presence and magnitude of the South- 

 ern California Countercurrent and the Southern 

 California Eddy. 



Nearshore winds in the Southern California Bight 

 are relatively weak, and downshore wind speeds 

 generally increase farther offshore (Bakun and 

 Nelson 1976; Nelson 1977; Dorman 1982). The 

 implication, in terms of offshore transport, is that 

 larvae closest to shore are affected least by offshore 

 transport, while those farther offshore experience 

 a much greater impact. Thus the areal extent of 

 northern anchovy spawning interacts with offshore 

 Ekman transport; in years when most northern an- 

 chovy spawn close to shore there will be decreased 

 offshore transport, because of weak inshore winds, 

 than in years when northern anchovy spawn farther 

 offshore. The impact on the products of offshore 

 spawning will depend on the magnitude of the winds 

 in the offshore areas in each particular year. North- 

 ern anchovy larvae that began drift farthest north 

 in the Southern California Bight (location A) and at 

 the more offshore location (D) were most affected 

 by increases in offshore directed Ekman transport, 

 indicating southerly and inshore spawning are best 

 for reduced dispersal in March. Hewitt and Methot 



(1982) stated that the area of northern anchovy 

 spawning was more compact and more northerly in 

 1978 than in 1979. Survival of young larvae was 

 about the same in both years, indicating that early 

 mortality from starvation and predation was not 

 substantially different in the two years. Survival 

 through the juvenile stage was greater in 1978 than 

 in 1979, however, and Hewitt and Methot (1982) 

 cited increased offshore transport in 1979 as a possi- 

 ble reason. 



Superimposed on the effects of spawning location 

 is the interaction between the increase in downshore 

 wind speeds (offshore directed Ekman transport) as 

 one progresses offshore and the magnitude of inter- 

 annual variations in the wind speeds. In the simula- 

 tions the effects of the 3 x increase in Ekman trans- 

 port were substantially greater than those of the 

 1.5 x increase. The 1.5 x change was not a great 

 enough increase to carry many northern anchovy 

 larvae into offshore regions of higher, offshore 

 directed Ekman transport. The inshore 3 x increase 

 carried a greater fraction of larvae farther offshore, 

 and the 3x increase in the offshore region subse- 

 quently operated on a greater proportion of the 

 larval population. Thus there was an interaction 

 between enhanced offshore directed Ekman trans- 

 port in the nearshore area and increased Ekman 

 transport farther offshore, the two of these acting 

 together to produce the extensive drift evident in 

 the simulation results. Years in which downshore 

 winds increase in only the inshore or the offshore 

 regions would not produce as much overall offshore 

 dispersal. Bakun and Nelson's (1976) statistical 

 analyses of the "upwelling index" indicates that pro- 

 longed increased Ekman transport is feasible, 

 although the 3 x condition would probably be a par- 

 ticularly bad year. It should also be noted that 

 Ekman transport was incorporated into the model 

 as acting uniformly on the 50 m surface layer, and 

 presumably the model depicts the drift of "average" 

 larvae. Larvae that remain near the surface or at 

 50 m would undergo greater or lesser transport, 

 respectively. Alternatively, it is known that winds 

 in the Southern California Bight have a strong diur- 

 nal periodicity (Bakun and Nelson 1976; Dorman 

 1982), and a diurnal vertical migration coupled with 

 diurnal changes in the winds could significantly alter 

 larval drift. 



In summary, the simulation results indicated that 

 seaward dispersal of northern anchovy larvae is 

 generally small, but that seasonal effects are strong- 

 est in the area of peak spawning (location A) and 

 that March spawning at this point minimizes off- 

 shore dispersal. Spawning at locations or times near 



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