1 
Abstract — Surveys were conducted 
to evaluate and compare assemblage 
structure and trophodynamics of 
ichthyoplankton, and their variabil- 
ity, in an estuarine transition zone. 
Environmental gradients in the salt- 
front region of the Patuxent River 
subestuary, Chesapeake Bay, were 
hypothesized to define spatiotempo- 
ral distributions and assemblages of 
ichthyoplankton. Larval fishes, zoo- 
plankton, and hydrographic data were 
collected during spring through early 
summer 2000 and 2001. Larvae of 28 
fish species were collected and species 
richness was similar each year. Total 
larval abundance was highest in the 
oligohaline region down-estuary of the 
salt front in 2000, but highest at the 
salt front in 2001. Larvae of anadro- 
mous fishes were most abundant at 
or up-estuary of the salt front in both 
years. Two ichthyoplankton assem- 
blages were distinguished: 1) river- 
ine — characterized predominantly by 
anadromous species (Moronidae and 
Alosinae); and 2) estuarine — charac- 
terized predominantly by naked goby 
( Gobiosoma bosc ) (Gobiidae). Tem- 
perature, dissolved oxygen, salinity- 
associated variables (e.g., salt-front 
location), and concentrations of larval 
prey, specifically the calanoid copepod 
Eurytemora affinis and the cladoceran 
Bosmina longirostris, were important 
indicators of larval fish abundance. In 
the tidal freshwater region up-estuary 
of the salt front, there was substan- 
tial diet overlap between congeneric 
striped bass ( Morone saxatilis) and 
white perch (M. americana ) larvae, 
and also larvae of alewife ( Alosa 
pseudoharengus) (overlap= 0.71-0.93). 
Larval abundance, taxonomic diver- 
sity, and dietary overlap were high- 
est within and up-estuary of the salt 
front, which serves to both structure 
the ichthyoplankton community and 
control trophic relationships in the 
estuarine transition zone. 
Manuscript submitted 30 January 2010. 
Manuscript accepted 30 August 2010. 
Fish. Bull. 109:1-19 (2011). 
The views and opinions expressed 
or implied in this article are those 
of the author (or authors) and do not 
necessarily reflect the position of the 
National Marine Fisheries Service, 
NOAA. 
Ichthyoplankton community structure 
and comparative trophodynamics 
in an estuarine transition zone 
Patrick A. Campfield (contact author)* 
Edward D. Houde 
Email address for contact author: pcampfield@asmfc.org 
University of Maryland Center for Environmental Science 
Chesapeake Biological Laboratory 
1 Williams Street 
Solomons, Maryland 20688, USA 
‘Present address: Atlantic States Marine Fisheries Commission 
1050 N. Highland Street 
Arlington, Virginia 22201 
Anadromous and estuarine fishes 
migrate in the spring months to oli- 
gohaline and freshwater regions of 
tidal tributaries in Chesapeake Bay to 
spawn, potentially leading to overlap- 
ping distributions and diets of their 
larvae. Spawning seasonality over- 
laps broadly for anadromous alosines 
(shads and herrings [Alosa spp.]) and 
moronids (striped bass [Morone saxa- 
tilis], white perch [M. americana ]). 
Estuarine species such as naked goby 
( Gobiosoma bosc ) and bay anchovy 
(Anchoa mitchilli) also spawn during 
late spring and early summer in tidal 
tributaries, and their larvae occur in 
oligohaline regions, often near the salt 
front (Shenker et ah, 1983; Loos and 
Perry, 1991). 
There is limited information on for- 
aging by larval fishes in Chesapeake 
Bay and the influences hydrographic 
features may have on foraging. Suc- 
cessful larval foraging improves with 
ontogeny as swimming ability and 
mouth gape increase, leading to shifts 
in types and sizes of zooplankton 
prey selected (Shirota, 1970; Arthur, 
1976; Pepin and Penney, 1997). With 
some exceptions, larvae of closely re- 
lated estuarine fishes in Chesapeake 
Bay often have similar diets (Setzler- 
Hamilton et al. 1 ) that could result in 
resource competition within larval 
1 Setzler-Hamilton, E. M., P. W. Jones, G. 
E. Drewry, F. D. Martin, K. L. Ripple, M. 
Beaven, and J. A. Mihursky. 1982. A 
comparison of larval feeding habits among 
striped bass, white perch and Clupeidae 
in the Potomac Estuary. Rep. Md. Univ. 
Chesapeake Biol. Lab., no. 81-87, p. 1-127. 
fish assemblages. Feeding habits of 
alewife (Alosa pseudoharengus) larvae 
in freshwater lakes (Norden, 1968), 
in laboratory experiments (Miller et 
al., 1990), and of congeneric larval 
alosines (American shad [A. sapidissi- 
ma] and blueback herring [A. aestiva- 
lis ]) in estuaries have been described 
(Crecco and Blake, 1983). Laborato- 
ry experiments on prey selection by 
naked goby larvae (Harding, 1999) 
and feeding by naked goby larvae in 
the Patuxent River (Breitburg, 1991) 
also are described. Foods and feed- 
ing by striped bass and white perch 
larvae have been reported (Setzler et 
al., 1981; Limburg et al., 1997; Chick 
and Van den Avyle, 1999; Shoji et al., 
2005; Martino and Houde, 2010). 
We refer to the frontal region where 
fresh and oligohaline waters merge 
as the estuarine transition zone, a 
region potentially important in con- 
trolling occurrences and feeding by 
fish larvae. This frontal region of- 
ten contains an estuarine turbidity 
maximum (ETM) and is an important 
nursery area for young fish (North 
and Houde, 2001, 2003; Winkler et 
ah, 2003; Shoji and Tanaka, 2006; 
Boynton et al. 2 ). Previous research 
2 Boynton, W. R., W. Boicourt, S. Brant, J. 
Hagy, L. Harding, E. D. Houde, D. V. Hol- 
liday, M. Jech, W. M. Kemp, C. Lascara, 
S. D. Leach, A. P. Madden, M. R. Roman, 
L. P. Sanford, and E. M. Smith. 1997. 
Interactions between physics and biol- 
ogy in the estuarine turbidity maximum 
(ETM) of Chesapeake Bay, USA. ICES 
Council Meeting Documents 1997/S: 1 1. 
(www.ices.dk/products/cmdocsindex.asp). 
