WROBLEWSKI ET AL.: SURVIVAL OF NORTHERN ANCHOVY LARVAE 



10 11 12 13 14 15 



TIME ( days ) 



Figure 3. — Mortality rate predicted for larval northern anchovy 

 positioned at 3 m depth in the water column (the depth of max- 

 imum growth and survival in the model water column). Curve a 

 shows the predicted mortality rate for an event where the wind 

 blows at 16 m s"' for 24 hours. Curves b, c, and d show the 

 mortality rate for larvae experiencing multiple wind events. Curve 

 e gives the mortality rate where winds are calm during the sim- 

 ulated 15 d period. Day zero is the day of first feeding of a cohort of 

 larvae. Arrows indicate the beginning of wind events. 



The mortality rate also increases with the 

 strength of the storms. For larvae enduring 5 

 wind events of 20 m s~^ wind speed, each 

 lasting 24 hours, the mortality rate at day 13 is 

 35% d"^ and 45% at day 15 (curve d in Figure 

 3). Five storms of this magnitude occurring 

 within a 2 wk period are not hkely. However, 

 our predicted mortality rates of between 10 and 

 20% d"^ for more common wind conditions 

 (Fig. 4) are in the same range as rates calculated 

 from field data (see figure 1 of Peterman and 

 Bradford 1987). 



Food Limiting Conditions 



The next model experimentation was con- 

 ducted to determine the circumstances under 

 which wind events might prove beneficial. Up to 

 this point we have assumed that initial prey con- 

 centrations near the surface are sufficient to sup- 

 port optimal grovi^h of northern anchovy larvae 

 before any wind mixing disperses these concen- 



trations. As we have seen above, under these 

 circumstances, all wind events are detrimental 

 to developing larvae. However, we shall now 

 demonstrate that if initial food levels are not 

 sufficient for rapid growth of larvae, a storm 

 may actually enhance the survival of larvae 

 emerging from the yolk-sac stage after the 

 storm. 



The initial condition profiles of phytoplankton, 

 zooplankton, and nutrients for the case of insuffi- 

 cient, prestorm prey concentrations are shown 

 in Figure 5a. Notice that the concentration of 

 zooplankton near the surface is only 2.7 p.g atom 

 N €~^ where the initial concentration of zoo- 

 plankton in the previous simulations was 3.7 p.g 

 atom N €'^. As before, it is assumed that 

 only 25% of this total zooplankton biomass is 

 suitable as food for larval anchovy (estimated 

 from data of MuUin et al. 1985). With this food 

 hmiting condition, the growth rate of northern 

 anchovy larvae feeding near the surface is only 

 15% d"^ The mortality rate of the larvae 



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