10 
Fishery Bulletin 109(1 ) 
the copepod Eurytemora affinis, in the salt-front-ETM 
feature (Roman et al., 2001; North and Houde, 2006). 
Interannual differences in the timing and level of fresh- 
water runoff control spatiotemporal variability in the 
salt front-ETM feature, and the occurrences, abun- 
dances, and distributions of striped bass larvae and 
their zooplankton prey (Martino and Houde, in press). 
The potential importance of the salt front as a reten- 
tion feature has been recognized in previous research 
in Chesapeake Bay and its tributaries in which striped 
bass eggs or larvae, if advected below the salt front and 
ETM, apparently were lost to down-estuary dispersal 
(Secor et al., 1995; North and Houde, 2003; North et ah, 
2005; Secor and Houde 4 ). Our survey results support 
these findings and, additionally, we have documented 
distributions and overlaps of larval taxa and zooplank- 
ton prey in this estuarine transition zone. 
Larvae of striped bass and white perch were abun- 
dant above the salt front and in close proximity to 
4 Secor, D. H., and E. D. Houde. 1996. Episodic water quality 
events and striped bass recruitment: larval mark-recapture 
experiments in the Nanticoke River. Final Report to Mary- 
land Department of Natural Resources, 271 p. Center for 
Environmental Science, Univ. Maryland, Solomons, MD. 
it. Their yolk-sac larvae were virtually absent down- 
estuary of the 2.0 isohaline and abundance levels of 
all moronid larval stages declined to near-zero levels 
down-estuary of the 2.0 isohaline. Declines and ab- 
sences do not conclusively demonstrate failed retention 
or advective loss. However, Secor et al. (1995) released 
millions of marked, hatchery-source striped bass lar- 
vae in the Patuxent River and recaptured larvae from 
all release groups except those released below the salt 
front, providing strong circumstantial evidence that the 
front serves as a retention feature. 
We documented patterns in ichthyoplankton taxa 
occurrences, distributions, and peak abundances in 
the estuarine transition zone that indicate a predict- 
able seasonal progression in assemblage structure and 
distribution. A PCA identified riverine and estuarine 
ichthyoplankton assemblages. The two assemblages 
were characterized best by their occurrences with 
respect to salinity and, secondarily, to temperature 
and dissolved oxygen. Some taxa could not be assigned 
unambiguously to either the riverine or estuarine as- 
semblage because of extensive overlap of taxa in the 
transition zone. This ambiguity differs from reported 
ichthyoplankton assemblage structures often observed 
