WROBLEWSKI ET AL.: SURVIVAL OF NORTHERN ANCHOVY LARVAE 



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Figure 5. — Temporal evolution of the plankton model in response to a wind event of 20 m s"' 

 blowing for 48 hours. The initial conditions shown in (a) are different from those in Figure la, as 

 the nitrate concentration continues to increase with depth. This allows more nutrients to be added 

 to the euphotic zone by wind mixing. The initial zooplankton concentration is less than sufficient 

 for optimal growth of northern anchovy larvae. 



increases over time even in the absence of any 

 wind event (Fig. 6, curve a). 



With these initial conditions, we now deter- 

 mine the influence of a storm with a wind speed 

 of 20 m s"' blowing for 48 hours. The mixed 

 layer deepens to 46 m, entraining nutrients into 

 the euphotic zone while dissipating the existing 

 vertical structure in the upper plankton and 

 nutrient profiles (Fig. 5b). Curve b in Figure 6 

 shows the decrease in the zooplankton concen- 

 tration at 3 m depth because of this mixing. 

 However, after the winds cease and turbulence 

 in the water column dissipates, the zooplankton 

 concentration begins to increase. Zooplankton 

 biomass increases as the grazers utilize in- 

 creased phytoplankton biomass (Fig. 5d). By day 

 6 (four days after the storm) the zooplankton 

 concentration has surpassed the initial concen- 

 tration, and by day 8 it has reached concentra- 

 tions high enough to support optimal growth of 

 anchovy larvae. 



However only larvae beginning to feed after 

 the storm truly benefit from poststorm increases 

 in zooplankton biomass. Larvae which existed 

 before and during the storm are adversely af- 



fected by the initial decrease in prey concentra- 

 tions owing to wind mixing. Their growth rate 

 slows and their mortality rate increases to 30% 

 per day by model day 6 (Fig. 6, curve c). But 

 larvae entering the first-feeding stage two days 

 after the storm when zooplankton concentrations 

 are increasing have only a slight initial increase 

 in mortality rate, and then a decrease (Fig. 6, 

 curve d). Larvae emerging from the yolk-sac 

 stage four days after the storm has passed, ex- 

 perience a reduction in mortaUty even below the 

 initial rate of 6% per day (Fig. 6, curve e) be- 

 cause there is sufficient prey concentrations to 

 support optimal growth. 



DISCUSSION 



It would appear advantageous for adult north- 

 em anchovy to spawn after a storm, so that by 

 the time the eggs hatch and the larvae emerge 

 fi"om the yolk-sac stage, the first-feeding larvae 

 will have high concentrations of prey (induced by 

 the storm) upon which to feed. There is recent 

 evidence that Atlantic menhaden, Brevoortia 

 tyrannus, are stimulated to spawn by the pas- 



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