Optimal Wind Conditions for the Survival of Larval 



Northern Anchovy, Engraulis mordax: 



A Modeling Investigation 



J. S. Wroblewski, James G. Richman, and George L. Mellor 



ABSTRACT: How the frequency of storm events 

 can influence the survival of larval northern an- 

 chovy, Engraulis mordax, was investigated by com- 

 puter modeling. The hypothesis was as follows. 

 While wind events dissipate layers of planktonic 

 food, a total absence of wind mixing would reduce 

 upward nutrient flux and retard plankton produc- 

 tion. Therefore, there must be optimal conditions 

 of wind speed, duration, and frequency of wind 

 events for maximum survival of northern anchovy 

 larvae. From numerical experimentation, all wind 

 events were detrimental to post-yolk-sac larvae pre- 

 sent in the water column at the time of the storm. 

 However, if initial prey concentrations are insuffi- 

 cient for optimal growth of larvae, then a wind 

 event which increases primary and secondary pro- 

 duction may be beneficial to larvae emerging from 

 the yolk-sac stage after the storm. The conclu- 

 sion was that optimal conditions for larvae 

 survival occur when a wind event strong enough to 

 deepen the mixed layer into the nutricline is 

 followed by a period of calm. This period between 

 storms must be long enough for larvae to develop 

 into a stage where short-term starvation can be 

 endured. 



In 1975, Lasker hypothesized that larval an- 

 chovy survival was dependent on the stabihty of 

 the water column. Laboratory work (see Blaxter 

 and Hunter 1982; and references therein) 

 showed that first-feeding anchovy require very 

 high concentrations of plankton food. These high 

 concentrations are found in the upper water 

 column, but only during periods of low winds and 

 reduced turbulence (Lasker 1975; Owen 1989). 

 Recently Peterman and Bradford (1987) per- 

 formed a statistical analysis of wind and larva 



J. S. Wroblewski, Bigelow Laboratory for Ocean Sciences, 



W. Boothbay Harbor, ME 04575; present address: Ocean 



Sciences Centre, Memorial University of Newfoundland, St. 



John's, Newfoundland, Canada AlB 3X7. 



James G. Richman, College of Oceanography, Oregon State 



University, Corvallis, OR 97331. 



George L. Mellor, Princeton University, Princeton, NJ 



08540. 



Manuscript accepted April 1989. 

 Fishery Bulletin, U.S. 87:387-395. 



mortality data from the field that confirms 

 Lasker's hypothesis. There is a statistically sig- 

 nificant relation between larva mortality rate 

 and the frequency of calm, low wind speed peri- 

 ods which permit the maintenance of concen- 

 trated patches of food. 



However, completely calm wind conditions 

 cannot be ideal, because a stratified water 

 column reduces the vertical flux of nutrients into 

 the euphotic zone, reducing production of plank- 

 ton (Lewis et al. 1986). Eppley and Renger 

 (1988) recently measured the slight increase in 

 nitrate in the surface layer owing to a moderate 

 wind event in California coastal waters, and 

 found the data consistent with the wind driven, 

 nutrient flux dynamics of Klein and Coste (1984). 



Consider the fact that northern anchovy, 

 Engraulis mordax, populations off central and 

 southern California spawn mostly during the 

 winter and spring months (Smith 1972; Smith 

 and Richardson 1977), not during the summer 

 when winds are most calm and the water column 

 most stratified by solar heating (Husby and Nel- 

 son 1982). The timing of the spawning of these 

 populations may be an adaptation to maximize 

 the survival of larvae. 



This is the third in a series of modeling efforts 

 (Wroblewski 1984; Wroblewski and Richman 

 1987) to simulate the environmental conditions 

 which influence the survival of northern an- 

 chovy. Each successive model builds on the pre- 

 vious model by increasing complexity (and real- 

 ism) in the biological and physical dynamics. 

 Here we investigate by numerical experimenta- 

 tion the manner in which the frequency of storms 

 during the spawning season of northern anchovy 

 could influence survival of larvae. We find there 

 is indeed a theoretical optimum condition of wind 

 speed, duration of event, and frequency of 

 events for maximum survival. 



METHODS 



Our model investigates the mortality of north- 



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