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Fishery Bulletin 94(2). 1996 



mean egg abundance. In the 12 experiments, initial 

 yolk-sac larval abundances ranged from 56 to 781/ 

 m 2 with a mean of 385/m 2 (24.6/m 3 SE) (Table 2). 

 Mean abundances differed significantly among ex- 



periments (ANOVA, P<0.002). The highest abun- 

 dance of yolk-sac larvae was observed at site 7, where 

 eggs also were most abundant (Table 2). 



Gelatinous predator biovolumes 



The above-pycnocline biovolumes of gelatinous zoop- 

 lankters were highly variable among experiments, 

 ranging from to 612.8 mL/m 2 (Table 1). Ctenophore 

 M. leidyi biovolumes ranged from to 484.4 mL/m 2 

 and scyphomedusan C. quinquecirrha biovolumes 

 ranged from to 128.4 mL/m 2 . The mean biovolume 

 of M. leidyi was 4 times higher than that of C. 

 quinquecirrha. Biovolumes of each species were high- 

 est at site 4 (ANOVA, P<0.0001), near the mouth of 

 the Potomac River (Table 1; Fig. 1). 



Zooplankton biomass 



Zooplankton biovolumes from the 280-,um net ranged 

 from 6.8 to 20.3 mL/m 2 , mean = 13.6 mL/m 2 . The 

 two lowest zooplankton biovolumes (ANOVA, P 

 <0.02) were observed at site 4 (Table 1; Fig. 1 ), where 

 gelatinous zooplankton biovolumes were highest. The 

 highest zooplankton biovolume occurred at site 7, 

 coincident with highest anchovy egg and yolk-sac 

 larvae abundances. 



Egg and yolk-sac larvae mortality rates 



The overall mean instantaneous mortality rate for 

 eggs was 0.066/h. The estimates, which ranged from 

 0.001 to 0.185/h, represent stage-specific (20-h) mor- 

 talities of 2 ( /( to 98 ( 7r . The highest egg mortality rates 

 ( >0. 10/h ) were estimated on 3, 22, and 24 July (Figs. 

 3 and 4; Table 2). If only the seven rates judged to be 

 significantly different from zero (P<0.05) are included 

 in the analysis, the mean mortality was 0.094/h 

 (SE=0.017). 



The overall mean mortality rate of yolk-sac larvae 

 was 0.053 larva/h. The yolk-sac larval rates, which 

 ranged from 0.017 to 0.177/h, represent daily mor- 

 tality rates of 33.5 to 98.6%. The highest yolk-sac 

 larval mortality rates (>0.07/h) were estimated on 5, 

 7, and 22 July. If only the four rates judged to be 

 significantly different from zero (P<0.05 ) are included 

 in the analysis, the mean mortality rate was 0.077/h 

 (SE=0.035/h). 



The mortality estimates differed among the daily 

 surveys. Rates were estimated with low precision, 

 possibly because the eggs and larvae were distrib- 

 uted in patches within each sampling site. The stan- 

 dard errors of yolk-sac larval mortality rates were 

 relatively high at sites between the mouth of the 

 Chesapeake Bay and the Potomac River (sites 1—4) 



