4 
Fishery Bulletin 109(1 ) 
R j a (relative importance) is a measure of the importance 
of prey type a relative to other prey types. R : can 
range from 0 to 100, and high values indicate greater 
importance. Zooplankton length-weight relationships 
(Heinle, 1969) were used to estimate weights of prey in 
larval guts. 
To evaluate possible competition, diet overlaps be- 
tween larval taxa and length classes were evaluated 
from gut contents with Czechanowski’s index: 0 12 = 0 21 = 
1-0.5 (X|P (1 -P ; - 2 1 ) (Feinsinger et ah, 1981). 0 12 is the 
overlap index value of species-length class 1 on species- 
length class 2, P n is the proportion of food type i eaten 
by species-length class 1, and P l2 is the proportion of 
food type i eaten by species-length class 2. Index val- 
ues may range from 0.0 to 1.0. Values approaching 1.0 
indicate strong overlap. Index values were tested for 
statistical significance by comparing them with null 
models following procedures of Albrecht and Gotelli 
(2001) and Gotelli and Entsminger (2003). 
Results 
Hydrographic conditions 
There were strong gradients in hydrographic variables 
along the 31-rkm estuarine transition zone of the 
Patuxent River (Fig. 2). Highest temperatures were 
down-estuary. Salinities ranged from 0.0 to 10.0 and 
were highest down-estuary. Dissolved oxygen (DO) 
tended to be higher in the cooler and fresher up- 
estuary waters. 
In 2000, precipitation and river flow were below the 
historical average and relatively constant. And, except 
for an unusual cooling episode in late May, river tem- 
peratures generally were >20°C during surveys and 
approached 29°C in early July (Fig. 3). In May, the 
salt-front location was stationary near river kilometer 
(rkm) 56 before shifting 5 rkm down-estuary in mid- 
June, coincident with small increases in precipitation 
and river flow. DO levels were >8.0 mg/L in May, but 
declined to approximately 6.0 mg/L as temperatures 
increased in June. 
In 2001, hydrographic conditions were more variable 
(Fig. 3). Precipitation and river flow were below his- 
torical averages and more variable than in year 2000. 
River temperatures were <20°C in late April, fluctu- 
ated in early May, and gradually increased to >25°C 
by early summer. The salt front shifted 10 rkm up- 
estuary in early May, coincident with low precipitation 
and river flow, before moving 12 rkm down-estuary 
after high-flow events in late May-early June 2001. 
The mean salt front locations were at rkm 56 in 2001 
and rkm 54 in 2000. DO and pH levels were more vari- 
able in 2001. Although DO at mid-depth declined to 
4.0 mg/L at down-estuary stations in early May 2001, 
DO generally ranged from 5 to 10 mg/L during the 
two years. The pH ranged from 7.0 to >9.0 and was 
consistently >7.0 in both years; on average, pH was 
slightly higher in 2001. 
Species composition, diversity, and abundance 
of ichthyoplankton 
In the two survey years, a total of 198,161 fish larvae 
were collected, representing 28 taxa (Appendix Tables 
1 and 2). Twenty of the taxa occurred in both years. 
Species richness over the entire survey area was simi- 
lar between years (n spp 200 o= 24 > « spp ., 2 ooi= 23 ^ Species 
diversity was higher in the freshwater and salt-front 
regions (LT=0.72-0.86) than in down-estuary oligohaline 
waters {H'~ 0.25-0.50). Larval white perch, alewife, 
gizzard shad (Dorosoma cepedianum ), and striped bass 
were the most abundant taxa in freshwater. Naked goby, 
white perch, and striped bass larvae were the most 
abundant taxa in salt front and oligohaline regions. 
In both years, ichthyoplankton abundance was domi- 
nated by larvae of naked goby and white perch, which 
together comprised 94% and 89% of the catches, re- 
spectively, in 2000 and 2001. In 2000, catch per unit 
of effort (CPUE, no. of larvae/tow) was highest in the 
oligohaline region (625.1) where naked goby larvae 
dominated (Appendix Table 1). CPUEs were lower 
and similar in the salt front (278.9) and in freshwater 
(212.2). The coefficient of variation (CV) in CPUE, an 
indicator of spatial heterogeneity, indicated that catches 
were most variable in freshwater (CV FW =315%) and 
least variable at the salt front (154%). In 2001, CPUE 
was highest at the salt front (842.1) (Appendix Table 2) 
and lowest in the oligohaline region (189.6), primarily 
because larvae of naked goby were concentrated at the 
salt front in 2001. Relative variability in CPUE was 
similar across the freshwater, salt-front, and oligohaline 
regions in 2001 (CV range 217-254%). 
Larvae of several freshwater fishes (spottail shiner 
[Notropis hudsonius]; darters [Etheostoma spp.]; suck- 
ers [ Erimyzon spp.]; yellow perch [Perea flavescens ]) 
were far more abundant in 2001 than in 2000 (Appen- 
dix Tables 1 and 2). For example, the concentration of 
yellow perch larvae was 15 times higher in 2001. In 
contrast, larvae of gizzard shad, a freshwater species, 
were 4 times more abundant in 2000 than in 2001. 
Additionally, in the freshwater region in 2001, concen- 
trations of white perch larvae were >2 times higher and 
concentrations of striped bass larvae were >10 times 
higher than in 2000 (Fig. 4). Larvae of anadromous 
striped bass also were relatively abundant at the salt 
front in 2001 than in 2000. Larvae of naked goby, an 
estuarine species, although of comparable abundance 
in the two years, were concentrated in the oligohaline 
region in 2000, but at the salt front in 2001 (Appendix 
Tables 1 and 2, Fig. 4). The regional difference in goby 
abundance between years may be related to differences 
in the timing of production of these larvae in the two 
years (Fig. 5). Goby larvae occurred first on 2 May in 
2001 and had time to disperse into the salt front region 
during June. In 2000, goby larvae did not occur until 24 
May and then were mostly collected in the oligohaline 
region, downriver from the salt front. 
Differing regional patterns in overall larval abun- 
dance between years were largely attributable to the 
