Weisberg and Burton Ichthyoplankton abundance and distribution in the Delaware River 



793 



delphia, PA and Wilmington, DE) was only about one- 

 fourth of that in the upstream area. Average larval 

 density in the lower region (downstream of Wilmington) 

 was less than 4/100 m 3 and never exceeded 10/100 m' 

 during any week of the study (Table 3). 



Species composition was fairly consistent between 

 the mid- and upper regions but differed substantially 

 in the lower river region (Table 1). M. americana, A. 

 sapidissima, and Alosa spp. were dominant in the up- 

 per river, occurring at densities of an order of magni- 

 tude greater than any other taxa. In the region fur- 

 thest downstream, where salt intrusion begins to occur, 

 species typical of the estuarine environment, such as 

 Leiostomus xanthurus (spot) and Anchoa mitchilli (bay 



anchovies), were at least as abundant as A. sapidissima 

 and Alosa spp. Freshwater species that occurred in 

 the upper river, such as Etheostoma sp. (darters) and 

 Ictaluridae (catfish), were not found in the more saline 

 region. 



With the exception of the Schuylkill River and the 

 C&D Canal, density and taxonomic composition in the 

 tributaries was similar to that in the most upstream 

 region of the tidal mainstem (Table 2). The Schuylkill 

 River is the largest tributary to the Delaware River 

 and flows through a heavily industrialized portion of 

 Philadelphia. Only four taxa were collected there, and 

 they generally occurred at densities 10- to 100-fold 

 lower than in any other tributary. No species occurred 



