POWER: MODEL OF NORTHERN ANCHOVY DRIFT 



occurring during March current conditions (Figs. 

 3A, 8A). January and October were intermediate 

 between these two extremes. In all cases the cross- 

 shore gradients of larvae were strong. 



In contrast, the alongshore distributions of larvae 

 differed markedly when the simulations were done 

 with currents from the four seasonal periods. Vir- 

 tually all larvae were carried upshore of starting 

 location A by the California Countercurrent in the 

 January simulation (Figs. 7A, 8A), but when the 

 model was run using March currents, a majority of 

 the larvae were downshore of point A after 30 d of 

 drift (Fig. 3A). The July and October simulation 

 results for point A seemed to indicate an annual pro- 

 gression between the March and January extremes 

 (Fig. 8A). 



The seasonal differences in the overall alongshore 

 distributions were even more dramatic for northern 

 anchovy larvae begun at locations B and C. The 

 uniform downshore distribution produced by March 

 currents differed from the distributions formed in 

 all other seasons. Larvae begun at location B were 

 all transported upshore of the starting location dur- 

 ing October current conditions (Fig. 6B). When 

 January currents were used (Fig. 7B), the upshore 

 movement had lessened, so that only 62% of the 

 larvae were at or upshore of location B, and the lar- 

 vae were more evenly distributed along the coast 

 (Fig. 8B). March currents yielded the greatest down- 

 shore movement, and the July distribution (Fig. 5B) 

 was intermediate between that produced by March 

 and October conditions, with the alongshore 

 gradient of larvae again steepening. The changes 

 on an annual basis between upshore, then down- 

 shore transport were similar for larvae begun at 

 location C, except that the July current conditions 

 produced the greatest upshore transport (Figs. 5C, 

 8C); March again produced the maximum down- 

 shore transport for larvae begun at point C (Fig. 

 3C). Larvae begun at location C formed a relative- 

 ly compact distribution after 30 d of drift in the 

 January currents (Fig. 7C). 



The overall alongshore distributions of northern 

 anchovy larvae that started drift at point D ap- 

 peared to be least influenced by seasonal changes 

 in the currents, although March conditions again 

 produced the greatest transport downshore of the 

 starting point (Fig. 3D), with July currents again 

 yielding the greatest upshore transport (Fig. 5D). 

 January currents also produced a very compact 

 distribution of larvae started at location D, similar 

 to that of larvae begun at point C. 



In summary, only northern anchovy larvae begun 

 at location A appeared to have notable differences 



in their model-wide, cross-shore distributions after 

 30 d of drift. Larvae begun at all four locations did 

 have substantial seasonal differences in their along- 

 shore distributions, with March currents consistent- 

 ly producing the greatest downshore dispersal. The 

 least downshore dispersal occurred during January, 

 October, July, and July current conditions for larvae 

 started at locations A, B, C, and D respectively. 

 January currents generally seemed to produce the 

 most compact 30-d distributions of larvae (least 

 dispersal). 



Effects of Increased Offshore 

 Ekman Transport, March Currents 



Increasing the March cross-shore Ekman trans- 

 port by a factor of 1.5 had little effect on the 30-d 

 distributions of northern anchovy larvae begun at 

 locations B and C (Fig. 9); these curves are also 

 closely spaced on the CalCOFI station abscissae. In- 

 creasing the average or "normal" offshore Ekman 

 component by a factor of three produced more 

 noticeable changes in the cross-shore distributions 

 of larvae begun at points B and C, but this effect 

 was not substantial; the contours representing the 

 lower concentrations extended far offshore (Fig. 

 10B, C), but the higher concentration contours, 

 which delimit the majority of the larvae, were not 

 greatly displaced from those of normal March cur- 

 rents (Fig. 3B, C). This is also evident in the cum- 

 ulative percentage, curves. 



In comparison, northern anchovy larvae begun at 

 locations A and D underwent about the same in- 

 crease in offshore dispersal with a 1.5 x offshore 

 directed Ekman component increase as those begun 

 at points B and C did with the 3 x offshore Ekman 

 increase (Fig. 9). When the offshore directed Ekman 

 transport was increased to three times its normal 

 mean value, the effects on larvae begun at points 

 A and D were substantial. A majority of the larvae 

 were carried offshore of starting locations A and D, 

 and a large fraction were transported a significant 

 distance (Fig. 10 A, D), well seaward of the South- 

 ern California Bight. The increase in offshore 

 Ekman transport also noticeably affected the along- 

 shore distributions of larvae begun at locations A 

 and D (Fig. 9). The overall pattern of alongshore 

 distribution is similar to that produced by the nor- 

 mal mean conditions, but the larvae were general- 

 ly farther downshore. 



DISCUSSION 



Models have inherent assumptions and simplifica- 



597 



