FISHERY BULLETIN: VOL. 87, NO. 3, 1989 



also reducing the concentration of prey required 

 for the basehne growth rate of the anchovy lar- 

 vae. The crux of the model is the lowering of 

 prey concentration due to wind mixing, a corre- 

 sponding decrease in ingestion and growth rate 

 of anchovy larvae, and an increase over time of 

 the larval mortality rate with suppressed 

 grovd;h. 



Owen (1989) showed that microzooplankton do 

 occur in concentrations greater than 100 organ- 

 isms €~^ on the microscale (centimeters) in 

 patches of fine scale (meter) size. Owen found 

 that concentrations of organisms within the 

 patches were greater at low wind speeds. Motile 

 organisms showed higher patch concentrations 

 than would be expected solely from reproduction 

 of the organisms. Rothschild and Osborne (1988) 

 discovered mathematically that a beneficial ef- 

 fect of moderate turbulence is to increase the 

 encounter rate of prey and predator on these 

 microscales. These recent observations and 

 theory emphasize the need to examine the feed- 

 ing and survival of larval northern anchovy on 

 microscale and fine scales. Our model with a 2.5 

 m vertical grid spacing does not resolve bio- 

 logical and physical processes on these scales. 

 Future modehng research should compare the 

 beneficial (e.g., productivity enhancement) and 

 detrimental (e.g., prey dispersion) effects of 

 storm-induced mixing on the microscale feeding 

 environment of larval anchovy (see Vlymen 

 1977; Lasker and Zweifel 1978). 



We conclude that wind conditions during the 

 spawning period which are optimal for the sur- 

 vival of northern anchovy larvae encompass 1) 

 wind speeds high enough (>10 m s~\ de- 

 pending on the water column stratification) to 

 deepen the mixed layer into the nutricline; 2) 

 wind event durations long enough (greater than 

 half an inertial period, or about 8 hours at lat. 

 35°N) to maximally deepen the mixed layer, but 

 short enough to maximize calm periods between 

 storms; and 3) wind event frequency low enough 

 (one major storm every two weeks) to allow de- 

 velopment of larvae to a stage where carbo- 

 hydrate, protein, and lipid reserves can mitigate 

 subsequent starvation periods (about 15 days of 

 development, depending on water column tem- 

 perature). 



Our conclusions are supported by the observa- 

 tion that northern anchovy populations off cen- 

 tral and southern California spawn during the 

 winter and spring months, when wind conditions 

 can be optimal by our theoretical calculations. 

 They do not spawn during the summer when 



winds are most calm and the water column is 

 highly stratified, as one would expect consider- 

 ing Lasker's hypothesis alone. 



ACKNOWLEDGMENTS 



This research was supported by U.S. National 

 Science Foundation Grant OCE-8608786 to JSW 

 and National Aeronautics and Space Administra- 

 tion Grant NAGW-1451 to JGR. George L. Mel- 

 lor was supported by the Institute of Naval 

 Oceanography under Contract N0014-84-K-0640. 

 The Oceans Group at the Geophysical Fluid 

 Dynamics Laboratory in Princeton, NJ provided 

 computing time on a CYBER 205 computer. We 

 thank David Checkley, Jr. , Gail Theilacker, and 

 an anonymous reviewer for comments which im- 

 proved the manuscript. We dedicate this paper 

 to the memory of Dr. Reuben Lasker. 



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