Dorsey et a\ Cohort abundances and mortality of Anchoa mitchilli 



261 



tality was calculated from collections made between 

 0000 and 2400 h. Each of the duplicate samples was 

 treated as a separate data point in regression analy- 

 sis. An SNK test was applied following the regres- 

 sion analyses to determine which of the instantaneous 

 mortality rates of eggs and yolk-sac larvae differed sig- 

 nificantly from each other. In several cases the esti- 

 mated mortality coefficients did not differ significantly 

 from zero at the P = 0.05 level. These coefficients were 

 retained in our overall analysis of mortality rates and 

 comparisons among sites because we assumed that low, 

 near-zero rates may occur and that such estimates did 

 characterize low rates of loss at some sites. 



Pearson product-moment correlation coefficients 

 were calculated for mortality rates and selected bi- 

 otic variables. Possible correlations between egg or 

 yolk-sac larval mortality rates and initial abun- 

 dances, zooplankton biomasses, and gelatinous 

 predator biovolumes, were examined. 



Results 



Hydrography 



Mean values of temperature, salinity, oxygen, and 

 chlorophyll a fluorescence at 3-m depth are summa- 

 rized in Table 1. Chlorophyll a fluorescence tended 



to increase in a south-to-north direction. Both tem- 

 peratures and chlorophyll a fluorescence peaked at 

 site 5 (Fig. 1). Depth profiles of salinity and oxygen, 

 although not illustrated, indicated increasing strati- 

 fication in a south-to-north direction and nearly an- 

 oxic conditions near-bottom at sites 5, 6, and 7. 



Initial egg abundance and adult biomass 



Initial egg abundances of daily cohorts during each of 

 the 12 experiments were high and variable, ranging from 

 935 to 18,277 eggs/m 2 (Table 2). The mean initial abun- 

 dance was 6,630 eggs/m 2 (427.0/m 3 ). The initial abun- 

 dances of eggs differed significantly among experiment 

 dates (ANOVA, P<0.0002) and probably among sites, 

 although most sites were sampled on only one day. The 

 highest initial egg abundance, 18,277 eggs/m 2 , was 

 observed at site 7, the northernmost site (Fig. 1 ». 



The mean relative adult biomass (±2 standard er- 

 ror [SE]), estimated from the initial cohort abun- 

 dances of eggs, was 18.0 g/m 2 (±8.0 SE). Biomass 

 estimates varied widely among experiments and 

 sites, ranging from 2.5 to 49.3 g/m 2 (Table 2). 



Initial yolk-sac larval abundance 



The mean initial abundance of cohorts of yolk-sac 

 larvae was approximately 17 times less than initial 



