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Fishery Bulletin 109(1 ) 
region in May and were dispersed up-estuary where 
large individuals (>10 mm) occurred in the salt-front 
and freshwater regions in June and July. 
Assemblage analysis 
A principal components analysis (PCA) defined riverine 
and oligohaline ichthyoplankton assemblages in the 
estuarine transition zone (Fig. 6). The first two prin- 
cipal components explained 60% and 59% of the vari- 
ance in assemblage structure in years 2000 and 2001, 
respectively: 
1 Riverine assemblage — comprised anadromous and 
freshwater taxa, predominantly white perch, striped 
bass, alewife and other alosines, and gizzard shad. 
Yolk-sac larvae were common in this assemblage, 
which tended to occur in fresher, cooler, more oxy- 
genated water. 
2 Oligohaline assemblage — estuarine-spawning spe- 
cies, such as naked goby, characterized this low- 
diversity assemblage. In late June, when water 
temperature reached 25 °C, juvenile Atlantic men- 
haden and bay anchovy joined this assemblage. 
Overall, assemblage structure and species associations 
were similar for the two years. Oligohaline and riverine 
assemblages were defined each year, but several con- 
stituent taxa (moronids, naked goby, alosines, and giz- 
zard shad) from each assemblage were found at the salt 
front. Overlap between the two assemblages occurred at 
the lower end of the salinity vector in each year’s PCA 
(Fig. 6), coinciding with salinities of 0. 2-1.0 that typi- 
cally contain the salt-front feature. 
Zooplankton distribution and abundance 
Highest concentrations of combined zooplankton taxa 
occurred near and up-estuary of the salt front. Mean 
concentration was significantly higher in 2001 (t=-4.59, 
PcO.Ol), primarily because rotifers peaked at 4000/L. 
In 2000, when rotifers were less abundant, the seasonal 
trend in zooplankton abundance was driven by the cla- 
doceran Bosmina longirostris. 
Zooplankters that are potential prey for larval fish 
differed in their spatiotemporal patterns of distribution 
and abundance. Naupliar stages of copepods, mostly 
Eurytemora affinis or Acartia tonsa, were common 
throughout the sampling area, but were often most 
abundant near and down-estuary of the salt front. 
Mean nauplii concentration was higher in 2001 (t=6.01, 
PcO.Ol). The cladoceran B. longirostris was most abun- 
dant in the freshwater region (Fig. 7) and was un- 
common at stations where salinity was >2. Its mean 
concentration did not differ significantly between years 
(#=-0.68, P=0.50). 
Adult stages of the two abundant calanoid copepods, 
Acartia tonsa and Eurytemora affinis, had different 
distributions. Acartia occurred down-estuary of the 
salt front and abundance was similar in each year. Eu- 
rytemora were present throughout the estuarine tran- 
sition zone, but usually were most abundant in the 
salt-front region, especially in 2001 (Fig. 7). The mean 
Eurytemora and Acartia concentrations did not differ 
