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Fishery Bulletin 109(1) 
in the mainstem of Chesapeake Bay has shown 
above-mean concentrations of zooplankton and 
larvae of anadromous striped bass and white 
perch near the salt front and associated ETM, 
where physical processes act to trap and con- 
centrate plankton (North and Houde, 2001; 
Roman et al., 2001). Here, we identify ichthyo- 
plankton assemblages and analyze the feeding 
ecology of larval fishes in the Patuxent River 
subestuary. We tested two hypotheses: 1) envi- 
ronmental gradients define spatiotemporal dis- 
tributions and assemblages of ichthyoplankton 
in the estuarine transition zone; and 2) the salt 
front plays a key role in controlling assemblage 
structure and trophic interactions of larval fish 
communities. 
In analyzing trophic relationships of fish lar- 
vae co-occurring in the estuarine transition 
zone, we addressed the following questions: 
• Are diets controlled by the types of zooplank- 
ton prey available? 
• Is there significant dietary overlap among 
co-occurring larval fishes? 
• Do the salt front and estuarine transition 
zone control diets? 
• Does selection of prey types and sizes shift 
during ontogeny? 
Figure 1 
Patuxent River study area with sampling stations and regions 
for the years 2000 and 2001. The area between river kilometers 
(rkm) 46-77 defines the estuarine transition zone. Dashed lines 
in the enlarged map are conceptual partitions of freshwater, salt- 
front, and oligohaline regions. For individual surveys, the regions 
and numbers of stations within each region varied depending 
on the location of the salt front. 
Materials and methods 
Study area and sampling procedures 
Ichthyoplankton surveys on the tidal Patuxent River 
were conducted at 3-7 day intervals between 24 April 
and 5 July (13 surveys in 2000 and 17 surveys in 2001) 
to identify larval assemblages and shifts in seasonal 
abundances. Sampling was conducted from late spring 
to early summer to coincide with the peak seasonal 
spawning and larval production periods of anadromous 
and estuarine fishes. Samples were taken at 10 des- 
ignated stations along the channel in the estuarine 
transition zone (Fig. 1). Stations were located at 2-7 
river-kilometer intervals where depths ranged from 2 
to 10 m. Surveys were conducted during daylight and 
were of 8-10 hours duration. 
Sampling stations were located up-estuary, within, 
and down-estuary of the salt front in the estuarine 
transition zone (Fig. 1). The location of the salt front, 
defined by conductivities of 800-1000 pS (approximate 
salinities 0.4-0. 5), was determined in each survey. 
Ichthyoplankton distributions, concentrations, and 
sizes were evaluated with respect to the salt front 
and site of intersection of the 2.0 isohaline with the 
bottom (referred to as X 2 by Jassby et al., 1995). Most 
surveys were conducted from an 8-m boat (11 surveys 
in 2000; 14 in 2001). Ichthyoplankton was sampled 
by towing a 60-cm diameter, paired bongo net with 
333-pm meshes at 1 m/s in 5-min oblique tows from 
the surface to the bottom. Flow meters in net mouths 
measured volumes of water sampled for use in calcu- 
lating larval concentrations (no./m 3 ). The mean volume 
filtered in each tow (combined paired-net samples) was 
137 m 3 (±9 m 3 standard deviation [SD] ). In the final 
weeks of the survey during each year, juveniles and 
large larvae were sampled from a 16-m (2000) or a 
19-m vessel (2001) with a 2-m 2 mouth-opening Tucker- 
trawl with 700-pm meshes (23 June and 5 July 2000, 
and 31 May, 27 June, and 3 July 2001). Oblique tows 
were of 5-min duration from near-bottom to surface 
at approximately 1 m/s and filtered a mean volume 
of 575 m 3 (±71 m 3 SD). Ichthyoplankton samples were 
preserved in ethanol. 
Zooplankton (potential prey for fish larvae) was col- 
lected at each station by pumping 20 liters of water 
from surface, middle, and bottom depths (60 liters to- 
tal). Water from the three depths was combined and 
filtered onto a 35-pm sieve to concentrate zooplankton 
before preserving samples in 5% formalin. In the labo- 
ratory, zooplankters were identified, enumerated, and 
measured. 
At each station, measurements of temperature, sa- 
linity, conductivity, and dissolved oxygen were made 
at surface, mid-water, and near-bottom for analysis of 
ichthyoplankton occurrence and abundance. Surface 
pH was measured at alternate stations. River-flow data 
were obtained from a U.S. Geological Survey gauge 
at river kilometer 130, near Bowie, Maryland (USGS, 
http://waterdata.usgs.gov, accessed April 2010). 
