Campfield and Houde: Ichthyoplankton community structure and comparative trophodynamics 
7 
Alosa pseudoharengus (alewife) 
Morone saxatilis (striped bass) 
6.0 
4.0 
2.0 - 
0.0 
Gobiosoma bosc (naked goby) 
0.92 0.92 0.81 0.72 0.14 0.01 
c be b I c 
Oligohaline Salt front 
Freshwater 
Figure 4 
Larval concentrations (no./m 3 ) by region. Student t-tests with 
unequal variances were applied. Significant differences are 
indicated by different letters (P<0.05). Vertical lines are stan- 
dard errors of the mean. Note differences in scale of y axes. 
Values in italics are frequencies of positive tows. 
significantly between years (Eurytemora, t=-1.35, 
P=0.18; Acartia, t- 0.71, P=0.48). 
Cyclopoid copepods (primarily Oithona spp.) 
were more abundant (PcO.Ol) in 2000 than in 
2001. Cyclopoid abundance was highest near or 
up-estuary of the salt front. In 2001, rotifers were 
the most abundant zooplankters and occurred in 
highest concentrations within or up-estuary of 
the salt front. Rotifer mean concentration was 12 
times higher in 2001 than in 2000 (P<0.01). 
Environmental factors and larval abundances 
The descriptive analyses and PCA indicated that 
hydrographic factors and larval prey controlled 
ichthyoplankton distributions in the two years. 
Multiple regression models identified some envi- 
ronmental factors that explained variability in 
ichthyoplankton abundance (Table 1). 
The model results indicated that alewife larvae 
were most abundant up-estuary of the salt front, 
where DO levels and Bosmina concentrations were 
highest (year 2000) and where temperatures were 
relatively cool (year 2001). For alewife, tempera- 
ture, dissolved oxygen (DO), salt front location, 
and concentration of Bosmina were significant 
descriptors of larvae abundance. 
For larval striped bass, concentrations of po- 
tential prey explained a significant proportion of 
variability in larvae abundance. In year 2000, 
salinity and concentrations of Bosmina and cala- 
noid copepods were significant factors (Table 1). 
In 2001, potential prey, salt front location, DO, 
and temperature were positively related to striped 
bass abundance, which was highest near the salt 
front and coincident with high prey densities. 
For white perch larvae, levels of salinity, DO, 
and temperature were positively related to white 
perch abundance in 2000, and highest concentra- 
tions were near the salt front, coincident with 
high prey concentrations (Table 1). The white 
perch regression model differed markedly in 2001 
when only temperature and salinity explained a 
significant portion of variability in white perch 
larvae abundance. 
For larvae of naked goby, concentration of co- 
pepod nauplii was a significant factor explain- 
ing goby abundance in year 2000 (Table 1) when 
concentrations were highest in oligohaline waters 
below the salt front that had relatively low DO levels, 
low Bosmina densities, and high copepod nauplii den- 
sities. In 2001, concentration of goby larvae was not 
significantly related to any prey-density variable, but 
larvae were most abundant in the relatively warm wa- 
ter within the salt-front region. 
Diets and prey 
Feeding incidence in the four taxa of fish larvae that we 
examined ranged from 72% to 97%. Feeding incidence 
did not differ significantly among the freshwater, salt- 
front, and oligohaline regions. 
The diet of larval alewife was diverse (Fig. 8), dif- 
fered between years, and shifted with ontogeny and 
growth. Copepod nauplii and rotifers represented >50% 
of diets in <11 mm alewife larvae. Invertebrate eggs 
and larger prey (i.e., Eurytemora and Bosmina) were 
more common in > 11 mm alewife larvae. Bosmina and 
cyclopoid copepods were more frequent in alewife diets 
in 2000, whereas rotifers and Eurytemora were more 
common in 2001. 
